How can Ballin and Nordström justify such a radical conclusion, that exercise does not extend lifespan? To explain that requires some background. The first thing to point out is that this publication does not contain original data, rather it is what is called a meta-analysis. Instead of doing experiments, the authors combined the results of multiple published studies. An example of that is shown in the figure at the top of this post. This figure displays results from twelve published studies on the effect of exercise on either cardiovascular disease (e.g. number of heart attacks) and all cause mortality (i.e. how long the subjects of the studies lived.) Let's focus just on all cause mortality for which the authors looked at ten studies. In eight of those studies, the subjects who exercised lived longer, on average, than those who did not. In one study, the subjects who did not exercise lived longer than those who did. In one study, there was no difference between the two groups. However, every study has uncertainty, and in nine of the studies, due to that uncertainty, it was not possible to be 95% certain that there was any difference between the two groups. Further, when all ten studies were combined, the combined probability indicated that from these ten studies considered together, it was not possible to be 95% certain that exercise had any effect on all cause mortality. Thus, the conclusion of the authors of this meta-analysis is that there is no statistically significant difference between the longevity of those subjects who exercised and those subjects who did not.
But what about all the other studies that conclude that exercise does decrease all cause mortality? How did none of them end up in this meta-analysis? To answer that question I have to explain the difference between interventional studies and observational studies. The gold standard for scientific studies are randomized interventional studies. If you wanted to know if exercise extends lifespan, you would enroll a bunch of people in your study, randomize them into two groups (e.g. have them flip a coin, heads means exercise, tails means no exercise), have one group follow the exercise program you think will lead to a long life, have the other group not do that, and then keep track of them for decades until all of them have died. You then compare the average lifespans of the two groups. There are a lot of obvious problems with this approach. What exercise program do you test? There is almost an infinite number of possibilities. What does it mean not to follow that program? Supposing a member of the control group enjoys walking around their neighborhood. Are they forbidden to do that? How would you ever get anyone to agree to participate in such an invasive study? How would you ever get the review board at your university agree to such an immoral study? How would you ever guarantee continuous funding over the long period of this study? And what is the general public going to do during the decades of waiting before the last subject dies?
There are standard workarounds for many of these obvious problems but none of these workarounds are perfect or even close to perfect. As a result, an alternative approach to answering these questions has arisen, the observational study. The very beginnings of this field can be traced back to one such observational study done in the mid-1900s by Jeremy Morris and Ralph Paffenberger. They noticed that bus conductors, who walked around the bus during their work day, lived longer than the drivers of those buses who sat all the time. Today, such studies are usually based on a questionnaire. A list of participants is gathered, they are asked to fill out a questionnaire about how much they exercise, and then they are followed to see how long they live. This shares some of the same problems as the hypothetical interventional study I described above but eliminates many of the most serious problems and thus the vast majority of studies linking exercise to longevity are observational. This is important because observational studies have one major disadvantage compared to interventional studies; they cannot prove that one thing caused another, only that the two are linked. In the case of exercise and longevity, an observational study can show that exercise is linked to longevity, but not that the exercise caused the longevity. But what other explanation can there be? Ballin and Nordström would suggest that maybe both exercise and longevity are caused by a common factor, inherited good health for example. The idea is that a person who is born healthy will feel good and therefore want to exercise. They will also live a long time, not because they exercised but because of their inherited good health. If this contrarian opinion seems implausible to you, you can take comfort from the fact that the bulk of the medical community agrees with you. That is why my doctor recommends that I exercise and yours probably does too. That said, this disadvantage of observational studies is why Ballin and Nordström limited their meta-analysis to interventional studies. Because these studies are randomized, they are immune from this alternative explanation. If everybody's intuition is correct about the observational studies, such studies should be confirmed by the interventional studies, and according to Ballin and Nordström, they are not.
So is that it? Was all my Cycling for Health a waste of time? Not necessarily. Perhaps it is the interventional studies that are misleading rather than the observational studies. Those who would criticize the observational approach would use a catch-phrase popular in the scientific community "Correlation does not prove causation", that is, just because subjects who exercise live longer does not prove that the exercise caused the longevity. However, those who would criticize the interventional studies have their own catch-phrase: "Absence of proof is not proof of absence." Just because the interventional study failed to provide proof that exercise causes longevity doesn't prove that exercise does not cause longevity. Perhaps the wrong exercise program was tested or perhaps too few subjects were included to get the statistical significance needed for such a proof. To their credit, Ballin and Nordstöm noted these concerns and my guess is that they would be among the first to argue that the jury is still out on this important question.
So how does this publication affect the cycling I am doing? Not at all, as it happens. To me, someone with over 30 years experience as a working scientist, I am used to the eb and flow of scientific research. I believe that it is very difficult to study the effect of exercise on longevity for the reasons mentioned above and for many other reasons as well and so I am not at all surprised that definitive evidence for the value of exercise is still lacking. That being the case, I simply have to go with my gut and gamble on the answer that seems most plausible to me, and that answer is that exercise is good for my health. That said, I applaud Ballin and Nordstöm and wish them and all their fellow exercise the best of luck in the many years of exercise studies that will be necessary to provide a more convincing answer to this question. Meanwhile, I will continue biking.