A few years ago, Swedish researchers published A study of dog ownership in 35,000 pairs of twins. By comparing identical and fraternal twins, they were able to estimate how much of the decision to get a dog is environmental (you grew up in a household with a dog, for example) compared to genetics. Overall, genes seemed to explain about half of the variation in a dog’s ownership, with increasing importance as we age. If you have a dog when you are 50 years old, that has almost nothing to do with if you had one when you were a child.
The researchers were interested in this question because some (but not all) previous investigations has suggested that dog owners live longer and have a lower risk of heart problems than non-dog owners. Maybe it’s because dogs provide social support; maybe it’s because you have to take them out for a walk every day. Genetic data suggest a third possibility: There may be “pleiotropic” effects, meaning that the same genes that predispose some people to dog ownership also predispose them to better health.
Those dog findings stood out to me because they reflect some of the open questions about exercise and health. There is overwhelming evidence that people who exercise more tend to be healthier and live longer. But how much of that reflects underlying predispositions to exercise? Y to be healthy? And to what extent is regular exercise a “choice” versus a reflection of our innate preferences?
As it happens, another study of Nordic twins has some insights into these questions. This one, published in Scandinavian Journal of Medicine and Science in Sport by a team in Finland led by Urho Kujala of the University of Jyväskylä looks at 17 pairs of identical twins with a highly unusual characteristic: despite their shared genetics, they do not have similar exercise habits.
The first thing to note is how unusual these pairs of twins are. The twins in the study were drawn from two previous Finnish twin studies that included thousands of pairs of identical twins. The vast majority of them had similar levels of physical activity. the top notch runner The line of mice often used in laboratory studies took mice that loved to run, crossed them with each other, and produced mice that loved to run even more. I’d like to think that human behavior (and mating patterns) are a bit more complex than that, but the twin data certainly suggests that our genes influence our predilection for movement.
Still, they found these 17 pairs whose paths had parted. There were two different subgroups: young twins in their thirties whose exercise habits had diverged for at least three years, and older twins in their fifties to seventies whose habits had diverged for at least 30 years. On average, the exercising twins engaged in about three times as much physical activity, including active commuting, as the non-exercising twins: 6.1 MET-hours per day compared to 2.0 MET-hours per day. For context, running at a pace of ten minutes per mile for half an hour consumes about 5 MET-hours.
All sets of twins underwent physical examinations and the results were more or less what one would expect. The exercising twins had a higher VO2 max (38.6 vs. 33.0 mL/kg/min), a smaller waist circumference (34.8 vs. 36.3 inches), less body fat (19 .7 vs. 22.6 percent), significantly less abdominal fat and liver fat, and soon. the study is free to read if you want to dig deeper into the details, but the results are not surprising. Exercise clearly improves a whole host of health parameters, and genes clearly matter too; after all, the differences are not that great.
How big could the differences be? A 2018 case to study of researchers at California State University Fullerton looked at a single pair of identical twins, then 52 years old. One was a marathon runner and triathlete who had covered almost 40,000 miles between 1993 and 2015. The other was a truck driver who did not exercise. In this case, the exercising twin weighed 22 pounds less and had a 30 percent lower resting heart rate. What’s most fascinating is that muscle biopsies showed that the marathon runner had 94 percent slow-twitch fibers, while the truck driver had only 40 percent slow-twitch fibers. No one before or since (to my knowledge) has shown such a dramatic change in muscle properties.
The burning question, especially for those of us who would like to challenge our apparent genetic destiny, is what set these pairs of twins on divergent paths. In Fullerton’s study, the sedentary twin suffered a minor ankle injury that derailed his participation in high school sports and he never exercised again.
In the Finnish study, there was no overwhelming pattern as to why one twin stopped exercising and the other did not. On questions about their motivations to exercise, the active twins reported more interest in dominance, fitness, and psychological well-being, but those differences may well be the result of different exercise habits rather than a cause. A key barrier for inactive twins was the pressure of family and work commitments when they were young. Interestingly, those barriers eventually evened out between sets of twins once their children were older and their careers more advanced, but by then, exercise patterns were established and the inactive twins never got into the habit again. The lesson: That maelstrom of madness at the beginning of the race and of young children is the hardest time to maintain the habit of exercise, but it is also the most crucial.
Like all discussions of nature and nurture, this one has to end somewhere in the middle. Clearly, genes matter, not only for health outcomes, but also for behaviors that we generally think of as purely voluntary. Equally clear, our paths are not set in stone. In the Swedish dog data, the influence of your childhood environment drops to almost zero by the time you’re 50, but half of the variation in dog ownership is still attributed to “unique, non-shared environmental effects,” in other words. , to the vicissitudes of his path through life. If he wants to exercise regularly in his 50s, choose his path carefully and try not to sprain his ankle.
For more sweat science, join me on Twitter Y Facebookregister in the electronic newsletterand check out my book Enduring: Mind, Body, and the Curiously Elastic Limits of Human Performance.
