Don’t treat obesity as physiology or physics

I have a whole bunch of draft and outline blog posts from winter and spring that I was never able to find the time to finish off. To clear them out of the way, I’ve bashed out some half-hearted conclusions, and will post them this month.

Flicking through the pile of Natures that never got read properly, ready to be rid of them, I alighted on Gary Taubes’s opinion piece: Treat obesity as physiology, not physics. Bear with me while I appear to be completely off-topic talking science for a while.

Taubes argues that:

…obesity is a hormonal, regulatory defect… it is not excess calories that cause obesity, but the quantity and quality of carbohydrates consumed. The carbohydrate content of the diet must be rectified to restore health.

Taubes set out his case that it is not useful to think of obesity as a straightforward energy in/out imbalance that causes weight gain, but that it’s in understanding that specific forms of that energy — carbohydrates, and sugars doubly so — activate our body’s own fat accumulation systems (through the well understood insulin process) where solutions lie. You’ll be familiar with it from all that “Atkins diet” and “glycaemic index” stuff: energy in the form of carbs bad; energy from dietary fat not so much.

Taubes thinks this is important stuff because:

…the overeating hypothesis has failed. In the United States, and elsewhere, obesity and diabetes rates have climbed to crisis levels… despite the ubiquity of the advice that if we want to lose fat, we have to eat less and/or move more.

There is an obvious response to this, but Taubes pre-empts it:

Yet rather than blame the advice, we have taken to blaming the individuals for not following it ‘properly’.

Suggesting that Taubes thinks that if only we change the advice from “eat less and exercise more” to his “don’t eat high glycaemic index foods”, the advice will be followed and we will then succeed in defeating obesity.

I imagine any “advice” we give will be useless, whether it’s based on physics or physiology.

Because while obesity is about physics and physiology — and psychology and genetics and half a dozen other fields of science — none of those things explain what is important: why there is more obesity now than in the past, and how to make there be less of it in the future.

The laws of physics haven’t changed in fifty years. Physiology, and the genes that underlie it, can change — but only by evolution over the course of hundreds of generations, not a few decades. Sure, our bodies have a mechanism for turning carbohydrates into fat stores. But they always have.

The focus is on the quantity of energy in and the quantity out because that is what has changed during the rise of the obesity crisis. By all means refine that to a specific focus on an excess of high-glycaemic index foodstuffs and a deficit of burning off specific sugars, but the problem that really matters remains fundamentally not one of physics or physiology but of our environment.

Taubes is right to treat those who “blame individuals for not following the advice properly” with contempt. But not because the advice is wrong. Because any “advice” — right or wrong — is going to be useless. This is not a problem that individuals have created for themselves, and it’s not a problem that individuals can be “advised” to solve for themselves. This is a problem of the environment that we live in: the types of food that are available to us, and the opportunities for an active healthy lifestyle that have been taken away from us.

Taubes later uses an analogy with smoking and lung cancer, and the analogy perfectly describes what’s wrong with the idea that obesity should be treated as a physiology problem. We know a great deal about the physiology of smoking-related lung cancer. We know how all of the many different carcinogenic chemicals within cigarette smoke flow through the lungs and pass through membranes into the cells. We know the chemical reactions that they participate in and how those reactions cause damage to the cells’ DNA. We know exactly which pieces of DNA damage result in the harmful mutations that transform them into cancer cells, driven to grow and divide. We know exactly how those mutations — to genes with names like RAS and RAF, and EGFR and a dozen others — change the shape of the proteins that those genes encode, and why that change of shape causes those proteins to misbehave. We know how these things result in the tumour evading the body’s inbuilt defences, how they hijack the blood supply to allow their expansion, and how they go on to invade and destroy neighbouring tissue and eventually escape and metastasise.

And knowing these things about physiology makes not the slightest difference to solving the smoking problem. Smoking-related lung cancer, like obesity, is a process of physiology. But it’s a problem of environment. And the most important lesson from smoking for obesity is that you can’t solve a problem environment with advice alone. Bad lifestyle choices are not an individual failing. Good lifestyle choices need an infrastructure to support them.

All those helmets posts in one place

Last year I went into some detail about why existing research into the efficacy and safety of helmets for cycling does not come close to the standard of evidence that is normally required and which we would usually demand for a medical intervention (which is what they are). Basically an application to helmets of all those things that Ben Goldacre bangs on about. But I never added an easy to link to contents page when it was complete. So here it is.

Killer Cures: a very brief introduction to why it is so important to do proper thorough research on medical interventions.

So what’s the best evidence we have on bicycle helmets?: a brief review of the research that has been performed on helmet efficacy — how it was done, what it found, and what the limitations of the methods were.

Headline figures: putting the relative risk figures reported by research papers into context of absolute risk.

What is a bicyclist?: looking at one of the major flaws in existing research on helmet efficacy — ignoring the differences between transport, leisure and sport cycling.

Would a helmet help if hit by a car?: a brief diversion into one of the side-arguments, but an important one given that most transport and leisure cycling deaths and serious injuries involve a motor vehicle.

Risk compensation and bicycle helmets: why it’s important to test for potential side-effects of interventions, some of the proposed side-effects, and why the research in this area also has too many limitations.

The BMA, the BMJ, and bicycle helmet policy: introducing the BMA’s position on helmets. Just one of several organisations with a dodgy position, but one that I think is particularly important/interesting.

How did the BMA get bicycle helmets so wrong?: a sort of conclusion piece, reiterating the importance of doing proper research on both the efficacy and the side-effects of medical interventions, and how an organisation which should know better managed to abandon this principle in favour of anecdotes for helmets.

Followed by some frivolous posts:

• Appendix: Bad Science Bingo in the BMA’s “safe cycling” pages
• Won’t somebody please think of the children?
• Prevention and cure
• Who are all these self-harming Dutch helmet wearers?

Prevention and cure

While organising notes, I stumbled upon this quote I bookmarked years ago, from the great Harvard cancer biologist Judah Folkman:

A pediatric surgeon in Boston just finished a difficult operation. To relax, he went to the Charles River and sat down on a bench. Suddenly, he heard cries of ‘Help! Help!’ and saw a person drowning. The surgeon jumped into the river and pulled the person to safety. He lay exhausted on the banks of the river and again heard, ‘Help! Help! ’ He glanced at the river and saw another person drowning. Despite his exhaustion, he jumped into the river and pulled the second drowning person to safety. Now, he was truly exhausted and lay on the ground huffing and puffing and again heard, ‘Help! Help! ’ He raised his head to look toward the river and saw a third person drowning, but he also noticed two basic researchers walking by the river. The surgeon shouted, ‘Colleagues, you must help! This is the third drowning person in the river in one afternoon! ’ The researchers looked at the river and then at the surgeon and said, ‘Three people drowning in one afternoon? This is very interesting! We’ll walk upstream to see who’s throwing them in!’.’’

(I think actually that it would work better if cast with public health researchers in place of basic researchers. The basic researchers would be too busy describing in obscure detail of the currents of the river, while translational researchers designed a better buoyancy aid for those currents.)

Folkman was applying the metaphor to his own field, cancer, but it works equally well for death and injury on our streets. The “road safety” approach to the problem has people studying the currents and advocating hi-viz vests and bicycle helmets, while spending billions on air ambulances and major trauma units. The “road danger reduction” approach goes upstream and asks why we are allowing large volumes of fast moving vehicles into the places where we live and work and play and learn. And it’s notable that in medicine, it’s the surgeons who think that preventing injury means bicycle helmets, and the public health researchers who think that preventing injury means calming and removing cars and trucks.

Here are a few of them: Danny Dorling talking about the open sewers of the 21st century; Harry Rutter’s Street Talk on moving towards a healthier city; and Ian Roberts, acting badly, on The Energy Glut. And you can hear Robert Davis talking about “road danger reduction” at London South Bank University on thursday next week.

During the 20th century, life expectancy lengthened by 30 years in the developed world. 25 of those years are attributable to public health intervention — to prevention rather than cure. But prevention disproportionately helps the poor and frequently hinders the rich. Guess which branch of medicine gets all the money.

Cycling abuse

I stumbled upon an article in The Lancet, volume 138, issue 3554, of the 10th October 1891, which it seems has been overlooked by the internet so far. It celebrates the rise of the bicycle, but warns against its abuse — addiction, even. It has a message that James Cracknell might like to ponder before getting too carried away with the fabulous medicinal properties of bicycle helmets: cycling isn’t dangerous, it’s those sick addicts who like to race themselves to exhaustion who are dangerous.

Those who believe in the necessity of physical exercise, and we belong to their number, have need also to remember that even so good a thing as this is in excess an evil. The use of the cycle is a form of bodily recreation in itself doubtless wholesome; none the less is it open to the mischievous effects of undue indulgence. Tempted by the ease of movement, combined as a rule with attractive scenery, everyone tries it. Everyone too, finds he can do something with it, and considerations of weather, constitution, age, and health are apt to be dismissed with summary imprudence. One fruitful source of injury is competition. In this matter not even the strongest rider can afford to ignore his limit of endurance. The record-breaker, who sinks exhausted at his journey’s end, has gone a point beyond this. The septuagenarian who tries to rival his juniors by doing and repeating his twenty or thirty miles, perhaps against time, is even less wise. Lady cyclists, too, may bear in mind that their sex is somewhat the weaker. So likewise amongst men the power of endurance varies greatly, and is is better for some to admit this and be moderate than to labour after the achievements of far more muscular neighbours. In short, whenever prostration beyond mere transient fatigue follows the exercise, or when digestion suffers and weight is markedly lessened, and a pastime which ought to exhilarate becomes an anxious labour, we may be sure that it is being overdone. He that would reap its best results must content himself with much less that this; but unless he can observe such moderation, he had better abstain from it altogether.

From here, The Lancet moves on to a fascinating case series of three patients with gastric ulcers cured by a diet of ice cream. (Perhaps the placebo effect of ice cream will turn out to be more powerful than four sugar pills.)

Won’t somebody please think of the children?

In December 2005, an article of massive importance was published in the British Medical Journal. Doctors counted up the number of children being admitted to A&E with musculoskeletal injuries (breaks and sprains — many of which would have been caused by bicycle-related incidents) on summer weekends  and discovered a startling pattern. A new preventative intervention was discovered.  They authors say:

The figure shows the weekend attendance to our emergency department in June and July between 2003 and 2005. The mean attendance rate for children aged 7-15 years during the control weekends was 67.4 (SD 10.4). For the two intervention weekends the attendance rates were 36 and 37 (mean 36.5, SD 0.7). This represents a significant decrease in attendances on the intervention weekends, as both are greater than two SD from the mean control attendance rate and an unpaired t test gives a t value of 14.2 (P < 0.0001). At no other point during the three year surveillance period was attendance that low. MetOffice data suggested no confounding effect of weather conditions.

From this data on the effect of Harry Potter books on injury rate it should be blindingly obvious that countless lives would be saved if legislation made Harry Potter books compulsory — for children at the very least (we can perhaps allow adults the freedom to choose to turn themselves into dribbling brain damaged wrecks by not reading Harry Potter).

Anybody who cycles while not reading Harry Potter clearly deserves to have their brains smeared across the road. They lack any credibility.

Gwilym, S. (2005). Harry Potter casts a spell on accident prone children. BMJ, 331:1505-1506 doi:10.1136/bmj.331.7531.1505

Further reading: ‘Tis the season, from Language Log.

Appendix: Bad Science Bingo in the BMA’s “safe cycling” pages

This is just a crude brain dump of a post that comes after the serious series — posts one, two, three, four, five, six, seven and eight.

Sorry, I just can’t get over these extraordinary pages on the BMA’s website. Here’s a very quick run through some of the Bad Science Bingo points that leaped out.

There were the canards, fallacies, and methods of misdirection:

1. Obviously there’s the emphasis on anecdotes and cases, the lowest form of evidence, which are essentially appeals to emotion.
2. Coupled with that the description of the “beliefs” of a few doctors, designed to nudge readers into conformity (acting as a subtle argument from authority for readers who are not doctors, and an argumentum ad populum for those who are).
3. Specifically in a couple of the anecdotes the selective recall of serious injuries in non-helmet wearers and minor injuries in helmet wearers (creating the illusion of control).
4. “Figures from New Zealand show that in 2006 there were 883 cyclists injured and nine killed. This corresponds to 20 people per 100,000 injured and 0.2 people per 100,000 killed. These figures are lower than those reported for 1994 when legislation was first introduced.” Fun factoids, but they don’t actually say anything about helmet efficacy. Lots of things changed between 1994 and 2006. (Post hoc, etc.) Perhaps there is evidence for NZ’s legislation improving safety but the 2006 crude injury statistics aren’t it.
5. Incidentally, while we’re on correlation and causation, the authors even get their statements on cycle tracks subtly wrong: “During the period of 1976 to 1995 Germany almost tripled their mass of cycle networks and this led to a 64 per cent drop in cyclist deaths.” While the evidence of a causative link is much stronger here, it’s a lot more complicated that a simple one “led to” the other. The reference does indeed state that Germany tripled their cycle network and that their death rate fell, but it notes that the later is in part the result — directly and indirectly — of the former.
6. I loved this statement, when discussing the side-effect of reduced rates of cycling: “If legislation were to reduce the rates of serious injury and promote increased public confidence in cycling, the effect might be to make cycling more popular. Clearly, there is a need for further research on this matter.” I don’t know where to begin. After dismissing all the side-effects of helmets as being based on too weak and preliminary evidence, the BMA counter it all with a speculation based on none at all — and tell us that there is a clear need for more research. Well quite.

And there were specific claims or activities that run counter to the cited evidence, or subtly misrepresented it (I did not systematically check references, these are simply things that leaped out as contradicting what I recall of the literature):

1. On page 2 the BMA list the things they are doing in addition to promoting helmets. The first item is “publicity and education campaigns in order to raise drivers’ awareness of more vulnerable road-users, including cyclists”. We know that these don’t really work.
2. The “risk compensation” section on the fifth page cites just one source, the Spanish study described on Monday, whose study design we know can not answer the question that they are asking it to answer.
3. “As noted in Table 2 the Macpherson and Spinks 2007 Cochrane review found no evidence to either support or counter the possibility that legislation may lead to negative societal and health impacts such as reductions in cycling participation.” You would probably read this and think, “studies have been done and they found no evidence for X.” It actually means, “the studies didn’t bother looking at X.”

And there were fun inconsistencies:

1. Kirsty’s story on page 5, “Doctors believe that had she not been wearing a cycle helmet at the time of her crash, she would have died,” and on page 6, “They have been shown to reduce the risk of head injury and its severity should it occur. This does not apply to fatal crashes but in such instances the force of impact is considered to be so significant that most protection would fail.”

The resource is just generally bizarre. It has a very weird set of focusses. On one page it gives a seemingly arbitrary selection of factoids from cyclist demographics (notably absent is any acknowledgement that “cycling” is not a single activity); on another it notes the diversity of cycle helmet standards — but fails to discuss any of the important consequences of this, such as how few helmets these days meet the stricter standards that applied in the past, back when most of the evidence on helmet efficacy was collected. In a table on the fifth page they mention that a study found no evidence of helmets causing or exacerbating rotational injuries — yet this is the only mention they make of the rotational injuries problem. Their inclusion and omission criteria appears to be completely random.

Anyway, enough of this. I don’t want to hog the game — your turn.

How did the BMA get bicycle helmets so wrong?

In 1958, the UK licensed a drug for treating morning sickness. It worked very well. The studies all showed that pregnant women suffering from morning sickness received much relief with the drug. Three years later it was withdrawn, but not before 2,000 babies were born with birth defects — 20,000 worldwide — three quarters of whom would die in infancy. The drug was, of course, thalidomide. It managed to get licensed because too many of the people studying it were focused on very specific aspects of its activity on the disease states that it was thought to treat, and too few were stepping back and looking at the big picture. It prevented morning sickness, therefore it worked — the logic of the day.

Joe’s anecdatum: In 2003, Joe, an 18 year old male, slipped on some wet stairs in a block of flats. His head fell eight feet onto the concrete floor. He was not wearing a bicycle helmet. He had a headache for the rest of the evening. He has never been diagnosed with any long-term ill-effects.

A disaster on the scale of Thalidomide can’t happen these days because the path to drug licensing forces researchers to comprehensively check all effects and outcomes of a new drug. Individual researchers will know in extravagant detail very narrow aspects of how a new drug achieves its desired effect. Some of them will know the exact rate at which it crosses the various barriers into the blood and into organs; others will know the exact chain of activation of molecules and genes within cells, down the individual amino acid residues that are modified and the exact number of seconds after the drug is administered; others will know the exact schedule and mechanism by which the drug is broken down or expelled from the body. They’ll be really excited and enthusiastic about their new drug. But when somebody steps back and points out that the drug causes heart failure, it won’t get anywhere.

But the BMA seems to forget everything it knows about testing interventions when it comes to bicycle helmets. There are some superficial differences between helmets and what we normally think of as “medical intervention”. They are a physical intervention rather than a drug — but medicine deals with and properly tests physical interventions all the time. And it’s supposed to prevent rather than treat injuries — but medicine deals with and properly tests preventative measures, including conventional drugs, all the time. There is no intrinsic reason why bicycle helmets can not be tested properly, in line with the rules that were designed to prevent another thalidomide disaster. We have the methods and the expertise.

Joe’s anecdatum: In 2009, Joe, a 23 year old male, slid on the gravel on the Greenwich Peninsula Thames Path, hitting his head on the concrete path and writing off an £800 camera lens. He was not wearing a cycle helmet. He was unhappy and was bored for several hours waiting for Lewisham Hospital to glue his face back together. He stayed home all next day. He has never been diagnosed with any long-term effects.

And yet the evidence that we have on bicycle helmets is currently in a worse state than the evidence that got thalidomide licensed. There is some (limited) evidence that in people who have had crashes, helmets reduce the rate of specific types of head injury — just as there is undisputed evidence that thalidomide is effective in relieving morning sickness. But there is also (equally limited and disputed) evidence of several different side effects — an increase in other types of injury* and an increased rate of crashes (particularly crashes with vehicles, which are more likely to have negative outcomes). And there is also evidence that helmets discourage many people from cycling* — an activity that adds many quality years to people’s lives by preventing or delaying cardiovascular disease, cancers, diabetes, depression, dementia, and all those other diseases of sedentary lifestyles. Helmets might be an effective intervention for the types of injuries they are claimed to prevent, but that would be irrelevant if, like thalidomide, they cause more problems than they solve.

Joe’s anecdata: In 1991, Joe, a 6 year old male, on separate occasions smashed his head open a door, some concrete steps, and a glass coffee table. On no occasion was he wearing a cycle helmet. He has a scar on his forehead that is almost identical to James Murdoch’s. Unlike James Murdoch, he has never been diagnosed with any other long-term impairment or ill-effects.

I’m not saying that they do. The issue is not that there is overwhelming evidence against helmets. The evidence that they are the cause of crashes and other injuries is no stronger than the evidence that they prevent head injuries. The issue is that the evidence either way is nowhere near good enough to make a recommendation. If helmets were a drug, they would be nowhere close to getting licensed right now.

Which is why British doctors should be embarrassed that the British Medical Association currently lobbies for helmets to be compulsory when riding a bicycle. Imagine if a pharmaceutical company developed a drug which, if administered before receiving a specific kind of traumatic injury, makes that injury easier to treat. Imagine doctors and medical scientists lobbying for it to be compulsory for everybody to take this drug daily, without anybody ever having checked for side-effects.

How has this situation arisen?  The policy decision has largely been made on the insistence of A&E consultants and trauma surgeons.  Consider the anonymous quotations that are scattered through the BMA’s cycling pages:

‘I have seen – in my practice and when working in A/E – quite a number of serious head injuries from children falling off bicycles. I have also seen a number of children who wore helmets who only suffered minor injury. I am convinced that helmets reduce injury.’ — GP

’I would certainly support cycle helmet wearing for cyclists. I have seen far too many young lives ruined by head injuries.’  — Consultant in Emergency Medicine

’I am an Emergency Department Consultant and a keen cyclist. I wholly agree…that we need to move to an environment where cycle helmet wearing is the norm, rather than the exception’  — Emergency Department Consultant

’As a regular commuting cyclist through twelve miles of heavy London traffic and as a Consultant Emergency Physician I whole-heartedly support the BMA’s stance on the introduction of legislation to make the wearing of helmets mandatory.’  — Consultant and Honorary Senior Clinical Lecturer in Emergency Medicine

’Over the [last] 16 years I have worked in A/E. I have dealt with hundreds of head and facial injuries, particularly in children, that could have been avoided had a cycle helmet been worn. I have also had the misfortune to deal with a number of fatalities that I believe would have been avoided by simply wearing a helmet. I firmly believe that legislation making cycle helmet usage mandatory is essential.’  — Emergency Medicine Consultant and Clinical Director

‘I have worked in emergency medicine for the last twelve years. Personally I cycle around two and a half thousand miles each year and my family are rapidly becoming keen cyclists also. Prior to working in emergency medicine, I did not routinely wear a cycle helmet.

I have seen numerous examples of patients sustaining severe head injuries from which they will never recover whilst cycling at low speed without a helmet. I have never seen this pattern of pathology in cyclists wearing helmets under these circumstances.

I am aware of the recent Cochrane review on the subject. I firmly believe that all cyclists should wear helmets. I also believe that the only way to ensure this happens is through legislation. I can see no justification for allowing this entirety predictable pattern of head injuries to persist. I strongly support the BMA position…’  — Consultant in Emergency Medicine

That’s five emergency medics and a GP, all reciting anecdotes from A&E. Nobody who specialises in, say, public health.

Emergency medics and trauma surgeons are obviously very enthusiastic about the potential to put an end to injuries, just as people who were very focused on the problem of morning sickness were excited by thalidomide. But ironically, most doctors and scientists are not very good with complexity. They are good with the intense detail of their own specialism, but when they have a problem to solve they fail to consider that there might be relevant things happening outside of their own field. When emergency medics want to solve the problem of head and brain injury, they look at those injuries in isolation from the rest of medicine. It’s not their job to think about the bigger the picture, or worry about things like side-effects.

Indeed, dare I suggest that for most working emergency medics and GPs, the science of evidence-based medicine is not their job or even a major part of their training: they only need to practice what the scientists amongst them tell them to practice; most working doctors don’t need to understand how we know their interventions work.

Which is fine. But that stuff is somebody‘s job, and somebody isn’t doing it right at the BMA.

This way of thinking about the issue — as an isolated problem of emergency medicine — is reflected all through the BMA’s bizarre “safe cycling” pages, which emphasise these individual anecdotes and opinions of doctors in that field (despite “expert opinion” being frequently out of line with the science and despite everything we know about the ability of anecdotes to lead readers astray), while failing to ever think of the issues around helmets in terms of effects and side-effects or the usual path of research that is demanded for medical interventions.

The authors of the Cochrane review on bicycle helmets say, in dismissing risk compensation, “the fundamental issue is whether or not when bicycle riders crash and hit their heads they are benefited by wearing a helmet.” This is exactly analogous to saying that “the fundamental issue is whether or not when a pregnant woman has morning sickness her symptoms are relieved by thalidomide.” That is not the fundamental issue at all. The fundamental issue with any medical intervention is whether it does more help than harm, whether it improves the length and quality of our lives, whether we are better with it or without. That the authors of a Cochrane review are allowed to get away with saying otherwise is a great failure for evidence-based medicine. That the BMA think there is sufficient grounds not merely to promote this intervention but to enforce it is an epic failure.

* I thought about posting separately on these sets of side-effects too, but those posts would have been much like the rest of this series: there’s a plausible hypothesis, there’s some evidence to support it, but the evidence has limitations. Ultimately the conclusions always are: the evidence base is nowhere near good enough to support helmet promotion, let alone legislation.

The BMA, the BMJ, and bicycle helmet policy

The reason I pick up the bicycle helmet theme again this week is that the BMJ is running a sidebar poll of their readers (or, more accurately, of cycling tweeters and recipients of Robert Davis’s emails ;-)), asking whether it should be compulsory for adult cyclists to wear helmets.

The BMJ is the journal of the British Medical Association, the professional association and trade union of British doctors. Part of the BMA’s remit it to lobby the government on issues that its members believe are important, and it has some clout in this area. These policies are decided by a representative democracy — a group of members elected by region and by field. In recent years, this body has decided that it is BMA policy to support legislation that would make helmets compulsory for cyclists.

Doctors might not even have noticed the adoption of this policy.  To most it is probably an irrelevance — most people will not cycle in the conditions that prevail in this country and doctors are no exception. And I imagine that very few have read the quite astonishing “promoting safe cycling” pages of the BMA website. Readers of Ben Goldacre should get their Bad Science Bingo cards out before clicking the link.

Tomorrow I’ll dissect those pages and ask how they came to be so bad. But there is a more basic issue here. Never mind whether helmets are effective or not, aren’t there more important policies that the BMA should be pursuing?

In 2002, the BMJ polled readers about issues of health and road danger — a slightly more scientific and insightful survey than the free-for-all yes/no question that they ask this week, and one much better targeted to British doctors rather than every joker on the internet.  They asked readers to judge the importance, on a scale of 1 to 6, of various interventions for promoting the health and safety of pedestrians and cyclists. Helmets came out bottom of the doctors’ priority list:

Average ranking	Response
3.25	More and better cycle safety training
2.87	Compulsory cycle helmet wearing
3.42	Separate lanes for bicycles in urban areas
4.04	Traffic calming to reduce vehicle speeds in urban areas
4.04	Reduce car use by better public transport and by encouraging walking and cycling
3.85	Banning motorised vehicles from towns and cities

Interestingly, helmets for cyclists was ranked as only a slightly more sensible solution than helmets for pedestrians. Indeed, the results for pedestrians look much like the results for cyclists.

It’s the most heartening thing I’ve read in a long time. Most doctors get it. They’re not ignoring the bull. Certainly all of the public health doctors and epidemiologists (the people with the most exposure to scientific methods, incidentally) that I know get it. The problem is not that cyclists are taking insufficient measures to protect themselves from danger, it is that they are put in danger by motorists and by the government policies and societal norms that support the mixing of fast-moving motor vehicles, including those driven by people known to be dangerous and incompetent, with cyclists and pedestrians in our towns and cities.

Alongside their policy of lobbying for legislation to compel the use of helmets, the BMA has drawn up a set of recommendations for motor-vehicle reduction. But while the former policy is actively being pursued in Westminster and in the nations, the latter looks to have fallen by the wayside, and is still stuck in 1997. Why?

Risk compensation and bicycle helmets

Some months ago I left a series on bicycle helmets hanging while I got distracted with other things. We had looked at what the best evidence for the efficacy of helmets in preventing injury in the event of a crash is, and some of the reasons why we should be cautious about that evidence. We found that if you’re unlucky enough to have been hospitalised while riding a bicycle, you’re less likely to be there with a head or brain injury if you were wearing a helmet at the time of the crash. We noted several ways in which this protective effect is exaggerated and used to mislead, we noted that reduction in injury is from a very low level anyway, and that the research so far done fails to provide any sub-analysis of very different riding styles, such as racing cyclists, mountain bikers, and utility cyclists.

We also made careful note of the fact that a reduction in the rate of head injury in the event of a crash is a different finding to a reduction in the rate of injury and death of bicyclists. We briefly began the exploration of what this means by considering the fact that helmets are not much defence against a motor vehicle.

How could a reduction in head injury in cyclists who crash not mean a reduction in injury and death in bicyclists? Well, helmets could be causing other kinds of injury in crashes. Or they could be causing crashes — particularly the worst kinds of crashes.

The latter is a particularly interesting avenue. The idea is risk compensation or risk homeostasis, a phenomenon documented in fine detail by John Adams in the 1985 book Risk and Freedom. Adams showed that advances in road safety — seatbelts, motorcycle helmets, safer vehicle designs and wider, straighter, safer road designs — are never followed by quite the cut in injuries and deaths that is predicted; that while road “safety” has improved crashes are no less frequent, and that bystanders — pedestrians and cyclists — are butchered at an ever increasing rate.  There is a set level of danger that people are willing to tolerate, and so motorists were compensating for the new safety features by driving faster and taking more risks. To put it in Adams’s technical terms, potential “safety benefits” were instead absorbed as “performance benefits”.

James Hedlund reviewed the evidence on risk compensation and came up with a set of rules for when people are likely to compensate for a safety intervention:

1. They know it’s there.
2. They know it’s a safety feature.
3. There is a potential performance benefit to be had.
4. There is freedom to realise that performance benefit.

Well cyclists know whether or not they’re wearing a helmet, they know that helmets are meant for safety, there are potential performance benefits — riding faster, through smaller gaps, in more hostile traffic, or with less caution in conditions that would otherwise advise it — and cyclists are generally free to ride more furiously if they want to. (Indeed, you may be wanting to cycle faster, in which case go ahead and use a safety feature as a performance benefit if that works for you.)

But that’s only a hypothetical reason to expect risk compensation by cyclists wearing helmets, not evidence that it actually happens. And very little effort seems to have been put into researching that — perhaps because it’s difficult to devise a properly controlled test. A study of cyclists in Spain attempted to test the idea by comparing the rate of helmet wearing in traffic law violators to the rate in non-violators, finding that law breakers were less likely to be helmet wearers, the opposite to what they say should be expected if there is risk compensation. However, this study could not control for all possible differences between the populations (“confounding variables”) — for example, helmet wearers are already a population of safety-conscious conformists, less likely to commit traffic violations, and so a better question to ask would be whether those helmet wearers acted even more cautiously when their helmets were taken away from them, and whether the non-wearers behaved even more recklessly when given a helmet. (This study is, embarrassingly, the British Medical Association’s sole reference for their dismissal of risk compensation.) A more recent study observed a set of participants behaviour with and without a helmet, using speed as an indicator of risk taking and heart rate variability as a proxy for risk perception. This study found that when helmet users had their helmet taken away, the risk taking (i.e. speed) reduced to keep the risk perception stable. However, the study only looked at 35 people, and only looked at proxy variables. Neither study is very convincing — the limitations I describe here are just the tips of the icebergs — and certainly nowhere near strong enough or specific enough to guide policy. We still have a mere plausible hypothesis with no good evidence as to whether or not it’s true.

The authors of the Cochrane review acknowledge the suggestion that risk compensation by cyclists could affect their crash rate, but believe that is unlikely. It’s interesting to see a hypothesis dismissed with the argument from personal incredulity in a Cochrane review.

What is not touched on in the review, and which is potentially far more important (given the fact that crashes with motor vehicles are more likely to kill or seriously injure), is the risk compensation effect not of cyclists themselves but of the other road users around them — i.e., of the motorists. Look again at Hedlund’s rules. Motorists can see whether a cyclist is wearing a helmet; they know that helmets are supposed to be a safety feature; they can potentially find performance benefits — they think they can squeeze through tighter gaps when overtaking against oncoming traffic, or pass more quickly, or shoot in front while turning, because if they hit the cyclist then no harm is done; and there is nothing to stop them realising that performance benefit, since the police, if there even are any, are rarely even aware of the relevant traffic rules, let alone bothered with enforcing them. There is therefore a plausible hypothesis that motorists will take more risks around cyclists who wear helmets than around cyclists who do not.

This hypothesis is made all the more plausible by the fact that, in addition to potentially making cyclists seem less vulnerable, helmets make cyclists look more competent: in surveys of motorists’ beliefs, most assume that cyclists who wear helmets are more experienced and more “responsible“, meaning that they may be driving more carefully around non-helmeted cyclists who they expect to do something silly. And motorists overwhelmingly think that cyclists should be forced to wear helmets — presumably so that the motorists can get the performance benefits of driving more dangerously around them.

The motorist risk compensation theory has famously been tested by @IanWalker in one of the most delightful experiments in the field. Walker rode around Salisbury and Bristol on a bicycle fitted with an ultrasonic distance sensor measuring the effect of a number of variables on passing distance, including rider position in road, type of motor vehicle, and whether he was wearing a helmet. Analysis of over 2,000 passes showed that motorists tended to give on average around 5-10 cm less space when the rider wore a helmet. It’s not much difference, and the effect of motor vehicle type, perceived rider gender, and rider’s distance from the edge of the road were all stronger.

But it’s important to note that there is always a distribution of passing distances — a bell curve. There are a few motorists who give a lot of room, a few who scrape past, and a lot clustered in the middle, giving a little over a metre distance. When wearing a helmet, the bell curve shifts in a little bit. The cautious drivers give a little less space, the average drivers give a little less space, and the dangerous drivers give a little less space.  It’s the latter who are now more likely to drive into you.

Walker’s research, delightful as it is, is itself not without limitations. Most important amongst them is that, when it comes to answering questions of cyclist safety, it suffers the same limitation of measuring only proxy variables: passing distances rather than actual risk of crashes and injuries. But it tells us that there is a very important reason to study more than just the isolated risk of head and brain injury in the event of a crash.

Helmets are a medical intervention, exactly like a drug or surgical procedure. They are a preventative intervention and they are a physical intervention, but neither of those are alien to medicine and to the modern methods of evidence-based medical science. And risk compensation is just a side-effect of this medical intervention, like the side-effects of drugs. The side-effects of drugs that make it to market are by definition outweighed by the beneficial effects; but ten times as many drugs are discarded during development because the research finds that either the side-effects are so big or the beneficial effects are so small that the harm outweighs the help.

The authors of the Cochrane review defend their dismissal of risk compensation by saying “the fundamental issue is whether or not when bicycle riders crash and hit their heads they are benefited by wearing a helmet.” And that’s fine if you’re in the preliminary stages of developing an intervention and you are so far only concerned with whether it has beneficial effects. But the authors go far beyond that early stage in their conclusions, recommending that this intervention be compulsory — despite there being very good reasons to suspect that there are potentially major side-effects of this intervention. They can’t have it both ways. If you haven’t bothered studying the side-effects you can’t license the drug. It might kill people.

Second hand; unused

Thinking about how the Cycling Embassy might go about trying to generate political will to progress cycling, I’ve been researching previous failed attempts to advance cycling in this country.  So on Amazon I snapped up a second-hand copy of an out-of-print British Medical Association book written in 1992: Cycling: towards health and safety.

People in Public Health are very interested in the bicycle because it keeps you fit — thus reducing incidence of obesity, cardiovascular disease, cancer, diabetes, dementia, depression, etc, etc — in a way that can be effortlessly integrated into everyday routines.  And because it provides an alternative to transport modes that cause thousands of hideous traumatic deaths and injuries, even more air pollution-related deaths, isolation-related mental ill-health, and so on.

From a quick flick through, I’m expecting all that to be covered, in addition to a section on “barriers to cycling” which looks like it might cause a cardiovascular event itself by chatting about the weather while ignoring the elephant in the room.

But also when I quickly flicked through, I noticed I was breaking the spine.  This second-hand book has never been read, never been opened except to stamp “date of cataloguing 14 May 1992” and “disposed of by authority” on the inside cover.  Where does this never-before-read book make its way to me from?

Perhaps it’s for the best.

Passive driving

“The ideal of the ethical man,” wrote the great Victorian scientist and liberal Thomas Henry Huxley, “is to limit his freedom of action to a sphere in which he does not interfere with the freedom of others.”

At Bath Skeptics in the Pub in April, Ian Walker talked about transport-related (ir)rational behaviour and policy.  One of the ideas he talked about was “passive driving”.  The analogy, of course, is to passive smoking.  Every time a smoker lights up in a restaurant or pub or club, the health and life expectancy of all the diners, punters, and staff around that smoker takes a tiny hit.  And those people get nothing positive in return.  In a liberal society, we defend the right of smokers to give themselves horrible slow fatal diseases.  But we expect them not to interfere with the rights of everyone else to their health.  And on the occasions when they can not show that restraint voluntarily, we have to resort to legislation banning smoking in restaurants and pubs and clubs.

Similarly, every time you get into your car and fire up the engine, my health and life expectancy takes a hit, and I get nothing in return.  You get to work or to the shops or to a day out, but I get nothing except a reduced life expectancy. Every time you get in the driving seat, you are making the decision that your journey is worth more than my and everybody else’s health and wellbeing. How big a problem is it?

Well, before the ban on smoking in enclosed public spaces and workplaces, estimates were that around 600 people in the UK were dying prematurely each year because of exposure to second-hand  tobacco smoke in those environments.

Exposure to driving in the UK annually causes:

• over 2,000 deaths in what the DfT describe as road “accidents”, of which less than half are of car users (stats for drivers and passengers are, sadly, all combined). Around 500 pedestrians, just over 100 cyclists, around 500 motorcyclists and a few bus and coach passengers are killed in “accidents”.  A few of those deaths will have nothing to do with cars — indeed, some genuinely will be “accidents” — but most are in some way the consequence of other people choosing to get in a car, a choice that would never bring any benefit to the person killed. As Harry Rutter pointed out at Street Talks, pedestrian deaths are particularly high in children, the elderly, and the lowest socio-economic groups: people to whom the benefits of car use are often out of reach, but who have to suffer the negative consequences regardless.  Motor vehicles are the biggest cause of death in teenagers, who should have a large proportion of their lives ahead of them, arguably making road “accidents” a more important issue than diseases which kill late in life and thus take away fewer quality life years.
• Air pollution is not a fashionable topic, yet estimates of UK deaths attributable to it are even higher than for crashes, ranging from 12,000 to 35,000. Motor vehicles are not the only contributor to air pollution, but they are the major one.  Air pollution is especially a problem in cities, paradoxically the places that usually have the highest proportion of non-car users.  People living happily in cities without a car — who have perhaps even made the conscious decision to live somewhere within walking or cycling distance of employment and shops and services — again have to deal with the negative consequences of people driving into and through their city.
• Diseases associated with obesity and sedentary lifestyles are amongst the biggest killers of our time: cardiovascular disease, cancers, diabetes, even dementia.  We know that these diseases can be prevented or delayed by regular exercise — cycling, for example — and that exercise is therefore one of the biggest predictors of life expectancy.  But while a great many people in the UK would like to be able to make their regular short journeys by bicycle (not so much because they worry about their health, but because it’s cheap and simple), very few do.  The overwhelming reason people give for not cycling is that the roads are far too uninviting: because they’re full of fast moving and badly driven motor vehicles.  Every time somebody chooses to drive a car, the rest of us get none of the benefit, but we do get dangerous, intimidating, noisy and smelly streets, in which normal people will never want to ride a bicycle.

That’s just to list the obvious ways that other people choosing to drive has negative health consequences for you and me.

I was reminded of all this because today the Association of British Nutters Drivers are back in the news demanding their freedoms.  Nurse turned Tory MP, and now parliamentary under-secretary of state for health, Anne Milton said last week that allowing residents to close their residential streets to motor vehicles on sundays so that their kids can go out and play might be a good thing.  The ABD are said to be amazed that their freedom to drive wherever and whenever they like might, for just one day a week, come second to other people’s freedom to choose how to use just a little bit of their own neighbourhood. Once again, the ABD behave like spoiled children, throwing their toys around when told it’s somebody else’s turn to play.

The ideal of the ethical man is to limit his freedom of action to a sphere in which he does not interfere with the freedom of others.  The Association of British Drivers fail at this most basic principle of ethics.

@bengoldacre on bicycles

Like my googlereader and my drafts folder, my podcast app has a frighteningly large backlog.  This evening while on random play it stumbled on an episode of Little Atoms from 2008, with Ben Goldacre, in which he talks about the media’s obsession with simple, glamorous and individualistic quick fixes to complex social problems:

The idea that food, and particularly very specific individual types of food or specific molecules in food, are the most important lifestyle risk factors for ill-health is itself a very political and I think bogus project. The reason that people in Hampstead live ten years longer than people in Kilburn isn’t because they’re careful to eat a handful of brazilnuts every day to make sure sure they’re not deficient in selenium, it’s because of a whole complex nexus of complex interlocking social and political issues involving, you know, work exposure, social support, stability and all of that stuff.

To profess to people that these commodifiable lifestyle interventions — because it’s always those, it’s never, you know, walking schoolbuses or cycle paths, it’s always, you know, cherry juice, something that a press agency can put a release out about — to pretend that diet is the biggest lifestyle risk factor for ill-health is to my mind a very right-wing individualist manifesto.  It’s the deserving sick in the way that people used to talk about the deserving poor.  You’ll be chatting to somebody and they’ll say, “well, I have to walk past this council estate every day on the way to the tube and you can say what you like but I think it’s got to be diet because you see some of these people and they’re feeding their kids crisps for breakfast and they’re only three and it’s got to be diet hasn’t it?”  Well, yeah, diet might be part of it, but I think you’ll find it’s a bit more complicated than that.

[…]

All the evidence shows that public health interventions that have an effect on the health of a whole population are things that you do or change at the level of the whole population.  You know, change what’s available in the supermarket.  People eat what’s available to them easily and people behave in ways that are easy for them to behave.  So, say, if you make London a really hostile environment to cycle in then people won’t cycle in it.  If you design stupid cycle paths, some of which — I could show you photographs — are shorter than the length of a bicycle; if you put bicycle paths right next to a set of parked cars, so that if you cycled in it then every time somebody opened a door you would just die, but if you cycle outside it then all the cars get really indignant and beep their horns because they think that you should be in the little green cycle path then, if you do all of that then people will cycle less and people will be less healthy in your capital.  Whereas if you make excercise something that can be integrated naturally into people’s everyday lives then people will become healthier.

But fascinatingly all over the country there will be people who are working as public health professionals who will be sending out press releases saying, oh, we’re trying to get a walking schoolbus scheme together locally so that people don’t have to drive their children into obesity by driving them to school, and they’ll maybe use a little pun like that and feel really pleased with themselves, but they’ll be ignored and local papers won’t cover it and national papers won’t cover it, because newspapers want the goji berries, they want the glamorous easy quick fix.

It seemed an appropriate thing to share ahead of tonight’s Street Talk at the Yorkshire Grey, where Harry Rutter will be talking about the relationship between urban design and public health.

Sadly we have a society — or a media, at least — that does not merely ignore these projects in favour of glamorous miracle cures: we actively cry “state intervention” at anything that might make the world a better or nicer place.

That’s not what I said, say scientists

According to SCIENTISTS, “pollution is not improved by c-charge.”  (“Improved”? These scientists are so sloppy with their language.)

Journalists all over the city are this week reporting that the congestion charge has not reduced air pollution problems in central London, and that’s a fact, proven by science.  (As far as I know, the CCharge was never about air pollution — the clue’s in the name. But it’s potentially an interesting thing to look at all the same.  I can invent in my head plausible hypotheses for why it would improve air quality, and why it wouldn’t, but both would be useless without evidence either way.)

Unfortunately, I’m having a little trouble finding out who these so-called scientists quoted as the source for the claim are.  I asked scientists on twitter, but they couldn’t remember making the statement.

What I can easily find is a set of documents (none of them making the claim) reviewing work that explores a potential link between the CCharge and air pollution.  The documents are not new research published as peer reviewed articles in a scientific journal.  They are a “research report” — a King’s College academic’s review of what we know about the CCharge and air pollution — coupled with commentary and a press release.  The documents are all commissioned and published by the “Health Effects Institute“,

a nonprofit corporation chartered in 1980 as an independent research organization to provide high-quality, impartial, and relevant science on the health effects of air pollution. Typically, HEI receives half of its core funds from the US Environmental Protection Agency and half from the worldwide motor vehicle industry.

And that’s fine.  If the content is good, it doesn’t matter who funded it or where it was published.  I’m merely establishing exactly who is saying what.  The exact people are:

• Professor Frank Kelly, an environmental health researcher specialising in air pollution, who (as leader of an independent group of scientists) wrote the comprehensive research report reviewing the evidence.
• HEI’s Health Review Committee, who wrote a short commentary on Kelly’s research report.
• HEI’s press office, who wrote the press release, which is the only thing that most journalists read.

The main line of research reviewed by Kelly looked at roadside and background levels of nitrogen oxides (NOx), carbon monoxide (CO) and small particulates (PM10).  The data compared the change (if any) in these pollutants at locations within the CCharge zone from a few years before implementation to a few years after implementation.  It did the same for control locations in London but outside of the CCharge zone, to account for any unrelated trends in air pollution.

Kelly’s report concluded that there was no evidence of a CCharge effect on roadside levels of NOx; a complicated effect on background levels of NOx (whereby one type was marginally reduced and another type increased, especially near the boundary of the zone); but a marginal reduction in carbon monoxide and a reduction in particulates becoming more pronounced the closer one gets to the CCharge zone.  So the overall conclusion is that there is a small amount of evidence to indicate that the CCharge has made a small reduction to air pollution (the exact opposite of the claim attributed to “scientists” in the headlines).  However, the data was extremely limited — in some cases to single data points — and Kelly’s report doesn’t put much weight on any of the conclusions.

Even where there is sufficient data, Kelly’s report indicates that there are limitations to what this kind of data can say about the CCharge effects.  The CCharge zone is very small, he points out, and our atmosphere somewhat fluid: the air in London blows around and mixes, so even with sufficient data, this study design is not an optimal way to answer questions about the CCharge.* **

All of these limitations in study design and data quantity are reflected in the Health Review Committee’s short commentary on the report:

Ultimately, the Review Committee concluded that the investigators, despite their considerable effort to study the impact of the London CCS, were unable to demonstrate a clear effect of the CCS either on individual air pollutant concentrations or on the oxidative potential of PM10. The investigators’ conclusion that the primary and exploratory analyses collectively indicate a weak effect of the CCS on air quality should be viewed cautiously. The results were not always consistent and the uncertainties surrounding them were not always clearly presented, making it difficult to reach definitive conclusions.

Which is to say: the research so far isn’t really capable of answering any questions satisfactorily.  While the evidence is for a small improvement in air quality thanks to the CCharge, none of the evidence is very good.  They go on to make the academic’s favourite conclusion: more research is necessary.

That’s right, this is a 121 page research review with associated commentary which simply concludes that the existing data is insufficient to tell us anything useful at all.  That’s no criticism of Kelly or the HEI.  They set out to review the evidence; the evidence just happens to be severely limited.

The Health Effects Institute decided to press release this.  “Study finds little evidence of air quality improvements from London congestion charging scheme,” the press release screams in bold caps.  “Pollution not improved by C-Charge,” says Londonist. Can you spot the difference between the HEI press release and the Londonist headline?

There is an old saying that absence of evidence is not evidence of absence.***  It’s a classic source of bad science and bad journalism, and in this case it nicely sums up what is wrong with the Londonist piece.  A review which actually found very weak evidence that the CCharge improved air quality is covered as a study which found hard proof of the exact opposite.

* Indeed, Boris Johnson would like to blame all of the city’s problems on clouds blowing in from the continent rather than the motor vehicles that account for most of it.

** I could add to this limitation the fact that the CCharge was not merely meant to cut car use within the zone: it was meant to fund a massive increase in bus frequencies, subsidise fares, and generally make buses and trains more inviting throughout London.  The effect of the CCharge on road traffic throughout the capital is complex, so it’s questionable whether the “control” sites can be said to be unaffected by the intervention.

*** Before someone points it out, yes I know it’s a bit more complicated than that, but in this case the saying applies nicely.

Would a helmet help if hit by a car?

This post is part of a series: it starts with the intro to the helmets issue, then the summary of the best evidence on helmets, then a quick diversion into how dangerous cycling is and an attempt to define terms. And there’s more…

Brake, the “Road Safety” charity, say yes:

Helmets are effective for cyclists of all ages, in crashes which do and do not involve another vehicle.

That matters, because if cycling safety is in the news, journalists will go to Brake for an easy quote.

The British Medical Association also say yes:

Helmets provide equal level of protection from cars (69%) compared to other causes (65%)

This is important, because the BMA is a highly trusted organisation with political influence, and their current policy is to endorse the criminalisation of riding a bicycle when not wearing a helmet.

Interestingly, president of the Automobile Association Edmund King, who was giving away free advertising bicycle helmets in London this week, disagrees with the nation’s medics on both issues:

We don’t think helmets should be compulsory but we think there are benefits… Our view is that helmets do not protect against cars but they may protect against some of the 2.2m potholes which often are the cause of smashes into the ground by cyclists.

Carlton Reid adds a little detail:

Most bicycle helmets are designed for falls to the ground from one metre at speeds of 12mph. They offer almost zero protection in collisions between bicycles and fast-moving cars.

The risk reduction provided by helmets in bicycle crashes that do and do not involve motor vehicles is one of the few sub-group analyses that was performed in the case-control studies that are covered by the Cochrane Review, and it’s no surprise that this is the source for the BMA’s claim. In bicycle hospitalisations that did not involve cars it reported nearly 70% fewer head injuries in the helmet wearers. In bicycle hospitalisations that did involve motor vehicles there were nearly 70% fewer head injuries in helmet wearers.  A helmet is equally effective at preventing head and brain injury in crashes with cars as in solo crashes.

What makes Edmund King and Carlton Reid think they know better than the nation’s medics and road safety campaigners?  Indeed, what makes them think that they can go around claiming the opposite of the cold hard corroborated stats of the Cochrane review?

Well actually, they’re not. Not quite. King and Reid are judging helmet efficacy by a slightly different metric to the Cochrane Review.  The Cochrane Review is the looking at the set of bicyclists who have had an accident of a severity that hospitalises but does not kill outright.  The review says nothing about deaths, for example, and as the Cochrane Review itself notes, more than 90% of cyclist deaths are caused by “collisions” involving moving motor vehicles (the same proportion is found again by a separate route in the TRL review and again in NYC).  But only 25% of hospitalisations were caused by motor vehicles.  And while Cochrane suggested a whopping 85% of head injury hospitalisations (which in turn account for around half of all cyclist hospitalisations) could be avoided by wearing a helmet, the TRL review of post-mortem reports found that only 10-16% of all cyclist deaths might have been avoided.  Hospitalisations, of the sort reviewed by Cochrane, are not representative of deaths.  Fall off your bicycle and you might get hurt.  Get hit by a car and you might die.

That’s because when you fall off your bicycle, chances are you are toppling over some way — precisely the sort of simple fall that a helmet is designed for, and the sort of fall that is least likely to cause life-threatening injury to any other part of the body.  When hit by a car the body might be crushed, or thrown up and around at speeds that helmets are not designed for, and so there are many more opportunities to suffer fatal trauma to other parts of the body.

(As an aside, Brake actually get this one the wrong way ’round:

Nearly 50% of cyclist admissions to hospital are for head and facial injuries, and the majority of cyclist deaths and injuries are a result of head injury.

TRL has the answer to this one: around three quarters of cyclist fatalities did indeed involve a serious head injury.  But only about a quarter involved only a serious head injury.  The rest also involved one or more additional life-threatening injury.  The Brake claim is at best misleading.)

This doesn’t mean that the BMA and Brake are all wrong* and King and Reid are completely correct.  A car at speed may be able to cause the sort of multiple trauma that merely falling over doesn’t, but that doesn’t mean that cars aren’t also capable of causing the sort of crashes that helmets are designed for, especially in low speed city traffic.

So Edmund King is wrong**.  But within the untruth he is communicating an important truth: cars are responsible for the most serious injuries and death, and helmets will rarely help in those cases.

Brake and the BMA are correct.  But their strictly truthful statements hide the crucial details, without which they are liable to seriously mislead.

* Indeed, they can’t be wrong.  You can provide a hypothesis for why helmets might be useless in crashes with cars, but no hypothesis can trump the real world stats that say helmets are useful in crashes with cars.

** Carlton Reid is not wrong, because he specified fast-moving cars.

What is a bicyclist?

This post is part of a series: it starts with the intro to the helmets issue, then the summary of the best evidence on helmets, then a quick diversion into how dangerous cycling is. And it won’t end here…

A good review of a medical intervention starts by explaining the population being studied.  The Cochrane review of helmets for preventing head injuries in bicyclists explains that its population is the set of bicyclists who sustained an injury that was sufficiently major to make them go to the ER for treatment (and not sufficient to kill them before they could seek treatment).

The review does not explain what they mean by a bicyclist.  (And since the original papers under review are closed-access, behind an extortionate paywall, we can’t know whether those do.)  Presumably they mean people riding a bicycle at the time that they sustained their injury.

Is that people riding their bicycle leisurely along a rail trail or towpath?

Is that people touring, head down into the wind in the deserted mountains?

Is that people racing in a peloton down the dual carriageway?

Is that kids in the BMX park?

Is that mountainbikers on the downhill courses?

Is it businessmen on their Bromptons riding through the stop-start city traffic?  Old ladies bouncing down cobbled streets on their step-through upright bikes?  Village vicars doing their rounds?

Mountainbikers, city commuters, and rail trail riders are very different people exposed to completely differently environments and risks — and who have very different helmet-wearing and hospitalisation rates.  Lumping them all together is like lumping mountain hikers, sprinters, traceurs, marathon runners, city pedestrians and country footpath strollers together under the heading “walkers”.  But lump them together is exactly what the studies in the Cochrane review do, comparing the rate of head injury (as a proportion of all injuries) in helmet wearers and non-helmet wearers, and applying the results to make the recommendation that everybody should be made to wear a helmet while riding a bicycle, whatever their style and situation.  You may as well recommend Formula 1 safety gear for the drive to the supermarket.

Perhaps helmets help prevent head injuries in all people who use bicycles.  Perhaps it helps mountainbikers more than tourists.  Perhaps it’s the other way around.  We don’t know.  We could know.  The researchers could have made sure to collect the data (perhaps the data is even already there, in the medical records) and then done sub-group analyses on their data to give individual results for separate groups of bicyclists.  But they didn’t.  Why not?  Did it just not occur to them that “bicycling” might not be a single pursuit?  Or did they just assume that it didn’t matter, or that nobody would notice?  Either way, it amounts to a pretty serious limitation when you’re asking “should we legislate to ban all kinds of bicycle use except where the bicycle user is wearing a helmet?”