A new study claims that Tour de France cyclists add an average 8 years to their life span through extreme endurance training, and that people engaged in such activity should have no fear that they could be doing damage to their health. Hmm. Pretty big statements for an epidemiological study. Let’s dig in.
Here’s the abstract:
Increased Average Longevity among the “Tour de France” Cyclists.
It is widely held among the general population and even among health professionals that moderate exercise is a healthy practice but long term high intensity exercise is not. The specific amount of physical activity necessary for good health remains unclear. To date, longevity studies of elite athletes have been relatively sparse and the results are somewhat conflicting. The Tour de France is among the most gruelling sport events in the world, during which highly trained professional cyclists undertake high intensity exercise for a full 3 weeks. Consequently we set out to determine the longevity of the participants in the Tour de France, compared with that of the general population. We studied the longevity of 834 cyclists from France (n=465), Italy (n=196) and Belgium (n=173) who rode the Tour de France between the years 1930 and 1964. Dates of birth and death of the cyclists were obtained on December 31 (st) 2007. We calculated the percentage of survivors for each age and compared them with the values for the pooled general population of France, Italy and Belgium for the appropriate age cohorts. We found a very significant increase in average longevity (17%) of the cyclists when compared with the general population. The age at which 50% of the general population died was 73.5 vs. 81.5 years in Tour de France participants. Our major finding is that repeated very intense exercise prolongs life span in well trained practitioners. Our findings underpin the importance of exercising without the fear that becoming exhausted might be bad for one’s health.
And here’s why it’s wrong:
The study compared professional cyclists to the general population, only controlling for country of origin and age. So the pro’s were compared to a group in which some members suffered from known causes of reduced life span like: genetic disease, poverty, smoking, obesity, sedentary lifestyles, and oh yeah, WORLD WAR II, without controlling for any of them.
Were the pro cyclists conscripted at the same rate as the general population? Given the popularity of competitive cycling in France, Belgium, and Italy, I would suspect that even if they were, these men were kept from the front lines. Their data did not include cause of death.
Did they come from similar socio-economic backgrounds? Today’s pro’s usually come from fairly well-off families rather the poor families. You can’t say the same thing about the general population, and poverty is known to reduce life span.
I could go on, but you get the idea: The study was so poorly-controlled that its breathtakingly over-reaching conclusions are totally unsupported. Even if we assume their data is sound, they simply cannot conclude that pro cycling caused the increase in life span. Doing so is the cardinal sin of epidemiology. Correlation does not imply causation. It’s only a clue to look closer with more powerful studies that most often find no causal link, only a coincidence and one or more confounding variables.
They acknowledged some of these factors as limitations of their study (but not the deadliest war in human history). Yet, despite already published studies that clearly show heart damage in marathon runners they confidently concluded:
In our opinion, physicians, health professionals and general population should not hold the impression that strenuous exercise and/or high-level aerobic competitive sports have deleterious effects, are bad for one’s health, and shorten life.
I’d add that using poorly-controlled, observational data to recommend extreme endurance exercise–even beyond the point of exhaustion–is not only unsupported by the data, it’s irresponsible.
Other Longevity Studies
Several observational studies have looked at similar data and reached a wide range of conclusions. They have suggested several factors as possible causes of the increase in life span, including: the sports themselves, genetic selection of elite athletes, active and healthy lifestyles of athletes, lower rates of smoking, less alcohol consumption, lower body weight, lower overall blood pressure, and lower risk of ischemic heart disease caused by any of the previously listed factors.[3-7]
So which is it? Did the sports cause longer life span or was it another factor or combination of factors? Observational studies are simply incapable of determining this. They can only show association. That’s why a well done observational study concludes only that a correlation exists and modestly offers several possible causes, if any.
Evidence Against Endurance Sports
Endurance athletes, especially marathon runners, have been the subject of recent studies designed to observe the effects of endurance sports on the heart. Nearly all of them have shown acute damage, and some have shown long-term damage correlated to degree of participation in endurance sports.
Several studies have shown damage to the heart in lifelong endurance athletes, including myocardial fibrosis (thickening of the heart values) that can lead to heart failure.[8,9] An enlarged heart was once thought to be a temporary adaptation to endurance athletics, but at least one study has found the syndrome can persist decades after retirement from professional cycling. Marathon running is now well-correlated with an increased risk of heart attack caused by atherosclerosis. Even completing a single marathon can result in temporary heart damage. The evidence is steadily mounting that too much endurance exercise pushes the heart too far beyond it’s evolved capability and leads to disease.
Another thing to consider is the low bone mineral density most endurance athletes suffer from.[14-16] A frail skeleton in advancing age is a precursor to breaking bones from a fall, and that can lead to death among the elderly.
There is an interesting line of research, still in early stages, that shows that glucose may speed up the aging process in worms and yeasts. In one study, lifespan was reduced by 20%. This metabolic pathway may have been conserved through evolution in humans. The diet of a typical endurance athlete is loaded with sugar in order to sustain enormous efforts hour after hour. It’s too early to say, but it’s possible that this diet has a negative effect on life span. An author of one of the studies was convinced enough to have removed all sugar and starch from her diet.[18,19]
The prevailing notion in the Paleo community is that humans rarely pushed their cardiovascular systems to the extremes that professional cycling and other endurance sports demand. Hunting was a matter of stalking prey (mostly walking quietly), then sprinting in for the kill. Since these extreme endurance events are evolutionarily novel, human genes are likely poorly adapted to them, making them detrimental to human health. This consistent with the new clinical evidence that lifelong marathon running and competitive cycling results in heart damage.
On the the other hand, proponents of endurance running claim that persistence hunting observed in hunter gatherer populations and several anatomical features unique to humans suggest that we are well adapted to endurance exercise. Humans are the only mammals that sweat in the heat, are relatively hairless, and are bipedal. These factors allow us run more efficiently in hot weather than other species. Using these advantages, we can pursue prey over long distances during the hottest part of the day until it suffers from heat exhaustion and is killed. This idea is known as the Endurance Running Hypothesis, and is advocated by biologists Daniel E. Lieberman and Dennis M. Bramble.[21,22]
I think this point of view has some problems. It’s likely that hairlessness and sweating were adaptations to hot climates, but to claim they were adaptions to a specific behavior in such climates seems a bit overreaching. These traits gave the humans that had them an advantage in nearly every activity carried out in the heat, not just in persistence hunting. Also, running for miles in hot weather seems like a pretty inefficient way of hunting. The hunter(s) would have to expend an enormous amount of energy for each hunt, including those that were unsuccessful. Ambush hunting or scavenging would have been more calorically efficient, producing selection pressure against persistence hunting. Many anthropologists have challenged the Endurance Running Hypothesis, stating that archaeological and ethnographic data do not support it. Still, one cannot ignore the fact that no species has nearly the same capacity for endurance running as humans and that some hunter gatherer groups traditionally practiced persistence hunting in Africa and North America.
Below, David Attenborough narrates a modern day persistence hunt by the San, a hunter gatherer group in Southern Africa that carries the oldest existing human DNA.
An Evolutionarily Consistent Theory
On one hand we have some rather problematic observational studies that show higher life span among professional cyclists and endurance athletes. On the other hand, we have observational studies showing clear evidence of heart damage in lifelong marathon runners and professional cyclists. Evolutionary arguments seem to support the notion that extreme endurance exercise is novel to humans, but how can we explain the epidemiology? We can simply ignore them for being poorly-controlled studies, or we can suggest a confounding variable: genetics.
A study published March 2011 in PLos ONE suggests that genetics may be a factor in increased lifespans among endurance athletes. Here’s the abstract. The complete paper is available at PLoS ONE and it’s worth reading in its entirety.
Are ‘Endurance’ Alleles ‘Survival’ Alleles? Insights from theACTN3 R577X Polymorphism
Exercise phenotypes have played a key role for ensuring survival over human evolution. We speculated that some genetic variants that influence exercise phenotypes could be associated with exceptional survival (i.e. reaching ≥100 years of age). Owing to its effects on muscle structure/function, a potential candidate is the Arg(R)577Ter(X) polymorphism (rs1815739) in ACTN3, the structural gene encoding the skeletal muscle protein α-actinin-3. We compared the ACTN3 R577X genotype/allele frequencies between the following groups of ethnically-matched (Spanish) individuals: centenarians (cases, n = 64; 57 female; age range: 100-108 years), young healthy controls (n = 283, 67 females, 216 males; 21±2 years), and humans who are at the two end-points of exercise capacity phenotypes, i.e. muscle endurance (50 male professional road cyclists) and muscle power (63 male jumpers/sprinters). Although there were no differences in genotype/allele frequencies between centenarians (RR:28.8%; RX:47.5%; XX:23.7%), and controls (RR:31.8%; RX:49.8%; XX:18.4%) or endurance athletes (RR:28.0%; RX:46%; XX:26.0%), we observed a significantly higher frequency of the X allele (P = 0.019) and XX genotype (P = 0.011) in centenarians compared with power athletes (RR:47.6%; RX:36.5%;XX:15.9%). Notably, the frequency of the null XX (α-actinin-3 deficient) genotype in centenarians was the highest ever reported in non-athletic Caucasian populations. In conclusion, despite there were no significant differences with the younger, control population, overall the ACTN3 genotype of centenarians resembles that of world-class elite endurance athletes and differs from that of elite power athletes. Our preliminary data would suggest a certain ‘survival’ advantage brought about by α-actinin-3 deficiency and the ‘endurance’/oxidative muscle phenotype that is commonly associated with this condition.
In other words, elite endurance athletes may be endowed with genes that predispose them to incredible feats of endurance and also exceptionally high life span. Their genes are the primary factor in determining their life span. This unique genetic profile is also common in centenarians, so perhaps we should be wondering why endurance athletes rarely live past 80 years of life, and not decades longer. If that is the case, it would appear that endurance sports do in fact negatively impact life expectancy. This theory is consistent with the longevity studies, the evidence showing negative health outcomes, and the evolutionary perspective that humans may not be well-adapted to endurance exercise. A lot more study is needed before the theory is confirmed, but it does tie everything together nicely.
What should we do with all this conflicting data and theory? Observational studies show higher life span among endurance athletes, but it could be due to genetic or other factors, and new clinical studies show detrimental cardiovascular effects from endurance exercise. Still, we can’t wait for better studies or new insights to guide us, we live in the world now and must decide on a course of action now. I suggest we exercise a little epistemic modesty and admit that we can’t say for sure what the effect of extreme endurance sports are on human life expectancy. In this case, I think we should follow the examples we can observe in nature that hint at the evolutionary appropriateness of endurance exercise. While some hunter gatherers engage in persistence hunting, most do not, and evidence that it occurred in the past is has been contended.
The honest answer right now is that we don’t know how extreme endurance exercise affects life span, but there are not-yet-established scientific and evolutionary reasons to believe humans aren’t optimally adapted to it, and the observed increase in endurance athlete life span is due to genetic differences, not the effect of endurance exercise. Therefore, I think the best course of action is to avoid extreme endurance exercise, and if you do engage in it, limit it’s frequency to protect your heart. Also be aware that it may cause still unknown health problems due to its evolutionary discordance (a hint of unwelcome unknown unknowns).
Postscript on Cycling Folklore
You may have heard the claim that professional cyclists life spans are shortened on average by 15 years. Tim Moore made this claim in his book French Revolutions: Cycling the Tour de France. (Read the excerpt on Google Books) I could not find his source for that claim, he did not respond to my tweet (his only contact method) asking for evidence, nor could I find a study that supports it, which leads me to believe that this claim might be a bit of cycling folklore. It’s been repeated in The New Statesman and other media outlets. He might counter that new doping methods and drugs caused a more recent decline in pro cyclist life span that would not appear in studies with subjects from previous generations. Maybe it’s true, but studies available at the time didn’t support it.
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