I recently stumbled across a blog post from April 2015 in which NY Times writer Gretchen Reynolds was discussing the “right dose” of exercise to increase longevity. Her article was based on two new “impressively large-scale” studies that she thought would provide “clarity” on the topic.
It’s a principal of toxicology that “the dose makes the poison.” In other words, drugs that are beneficial in small does may be toxic if you take more than the prescribed amount. For many people, exercise might as well be a toxic chemical: they want to know what is the minimum dose necessary to stay healthy. Even better if they could take a pill and not have to break a sweat.
My attitude is different. It seems sad to me that a natural activity like exercise would be “dosed” like a pharmaceutical. And in any case, I’m skeptical about scientists who claim they can measure such things.
My faith in humanity was restored, however, when I read the post’s top-ranked comments: readers had raised interesting questions about the studies’ logic, disputed the purpose of the article, and taken Reynolds to task for naivete.
In fact, after reflecting on the comments, I’ve concluded that the “right dose” of exercise is the maximum amount possible…
Reynolds starts her post by reminding people that exercise, like medicine, is generally thought to reduce the risk of many diseases and premature death. Unfortunately, as she points out, exercise doesn’t come with “dosing instructions,” but new studies might help us bracket a range. Reynolds summarizes their findings as follows:
the ideal dose of exercise for a long life is a bit more than many of us currently believe we should get, but less than many of us might expect. The studies also found that prolonged or intense exercise is unlikely to be harmful and could add years to people’s lives
These conclusions are pretty vague, but the bigger question is to what extent the findings are reliable. Towards the end of the post, Reynolds acknowledges that the studies can’t “prove that any exercise dose caused changes in mortality risk, only that exercise and death risk were associated.” That’s because the studies measured correlations in large pools of survey data but didn’t include randomized, controlled trials that could establish causation.
That critical weakness didn’t stop her from selling the story– but her readers weren’t so easily fooled. One of them asked the following question:
This article reports associations. Isn’t it possible–or likely–that the unhealthiest people from the beginning chose not to exercise at all? And the group of people who chose to exercise the most (and the most vigorously) were those who were healthiest?
Correlation (another word for association) isn’t the same as causation. Another reader put it this way:
Perhaps the reason they don’t feel well is the same reason they die earlier? The causation may go the other way.
These readers are referring to the problem of “confounding,” that is, the risk that the real causative factor is some underlying variable that was missed in the study. In this case, the underlying variable might be a person’s health, which could be a cause of both longevity and a propensity to exercise. In other words, if you’re lucky enough to be healthy, then you live long and have the energy to engage in physical exercise. If you’re unhealthy, then you don’t have the energy to drag yourself to the gym, and even if you did, it might not do anything for you.
A doctor posted a lengthy comment on this topic, explaining that confounding is “extremely common in biological systems.” The doctor warned that “glib summaries” by “lay journalists” like Reynolds might mislead the “unwary reader.” Another reader was more blunt:
Like most of Gretchen’s posts, what the above is really saying is that the article constitutes primarily anecdotal recountings. Spectacularly unscientific.
Readers also objected to the purpose of the studies. “Surely mortality is the wrong issue,” one of Reynolds’ readers opined. Another wrote that preventing premature death is “such a negative reason” to exercise when the real purpose is to make”life fuller and more enjoyable.” Someone else explained that exercise makes him feel “good in the present and ready for my day,” and another remarked, “It’s about so much more than living longer. Exercising gives you the ability to ENJOY your life.”
One commenter made a very simple point: “Keep moving. No scientific justification needed.”
Notwithstanding the reactions noted above, people are going to keep asking the question, what is the “right dose” of exercise for a longer life?
You can accept the studies’ conclusion, that exercise contributes to health and longevity, which is the consensus view. Or you can go with the confounding hypothesis raised by Reynolds’ readers, according to which people who are healthier live longer and exercise more because they enjoy it. Take your pick.
As for me, while I’m skeptical of associational studies, I do feel better when I’m active, and that’s one reason I exercise as much as possible. The other reason for exercise, in my opinion, is that it teaches discipline, focus, and determination. A pill that increased longevity without requiring exercise wouldn’t provide these benefits. And for those people who can’t bring themselves to go for a jog — maybe they’re unhealthy and jogging wouldn’t help — or maybe more discipline, focus, and determination are precisely what they need.
Returning to Reynolds’ blog, I found she is now touting a study in which researchers compared neural growth in the brains of one group of rats, who were allowed to run freely, to another group of rats who climbed walls with tiny weights attached to their tails. Judging from the comments, I’m not the only one who finds this a little bit of a stretch.
I think I’ll go for a run.
Note: I tried to read the studies cited in the blog post but they aren’t available without access to content on the JAMA network.