A few years ago, Swedish researchers published a study of dog ownership among 35,000 pairs of twins. By comparing identical and fraternal twins, they were able to estimate how environmental the decision to have a dog is — you grew up in a home with a dog, for example — versus genetics. Overall, genes seemed to explain about half of the variance in dog ownership, with increasing importance as you get older. If you have a dog at 50, it has almost nothing to do with the fact that you had a child with it.
Researchers became interested in this question because some (but not all) previous research has suggested that dog owners live longer and have a lower risk of heart problems than non-dog owners. This may be because dogs provide social support; maybe it’s because they need to be walked every day. Genetic data suggests a third possibility: Perhaps there are “pleiotropic” effects, meaning that the same genes that predispose some people to dog ownership also predispose them to better health.
These dog findings caught my attention 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 to what extent does this reflect underlying predispositions to exercise and to be in a good health? And to what extent is regular exercise a “decision” as opposed to a reflection of our innate preferences?
It turns out that another study of Nordic twins has some insights into these questions. This one, published in the Scandinavian Journal of Medicine and Science in Sport by a Finnish team led by Urho Kujala of the University of Jyväskylä, examines 17 pairs of identical twins with a very unusual characteristic: despite their common genetics, they do not have the same exercise habits.
The first thing to notice is how unusual these sets of twins are. The twins in the study were drawn from two previous Finnish twin studies that included thousands of identical twin pairs. The vast majority of them had similar levels of physical activity. The High Runner strain of mice that is often used in laboratory studies took mice that liked to run, crossed them together, and produced mice that liked to run even more. I’d like to think that human behavior (and mating patterns) are a bit more complex than that, but the data on twins certainly suggests that our genes influence our predilection for movement.
Yet they found these 17 pairs whose paths had diverged. There were two different subgroups: young twins in their 30s whose exercise habits had diverged for at least three years, and older twins in their 50s to 60s whose habits had diverged for at least 30 years . On average, the twins who exercised had about three times more physical activity, including active movement, than those who did not exercise: 6.1 MET-hours per day versus 2.0 MET-hours per day -hours per day. For context, running at a ten-minute pace for half an hour consumes about 5 MET-hours.
All sets of twins came in for physical exams, and the results were pretty much what you’d expect. Twins who exercised had higher VO2 max (38.6 vs. 33.0 ml/kg/min), smaller waist circumference (34.8 vs. 36.3 inches), lower body fat (19.7 vs. 22.6%), significantly less belly fat and liver fat, and soon. The study is free to read if you want to dive deeper into the details, but the results aren’t surprising. Exercise clearly improves a bunch of health metrics, and genes matter too – after all, the differences aren’t this big.
How big could the differences be? A 2018 case study by 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 completed nearly 40,000 running miles between 1993 and 2015. The other was a truck driver who didn’t exercise. In this case, the exercising twin weighed 22 pounds less and his resting heart rate was 30% lower. Even more fascinatingly, muscle biopsies showed that the marathon runner had 94% slow-twitch fibers while the truck driver had only 40% slow-twitch fibers. No one before or since (as far as I know) 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 destinies, is what set these sets of twins on divergent paths. In the Fullerton study, the sedentary twin suffered a minor ankle injury that derailed his high school sports participation, and he never ended up getting back to exercising.
In the Finnish study, there was no overriding pattern as to why one twin stopped exercising and the other did not. When it comes to their motivations for exercising, active twins reported more interest in mastery, improved physical fitness, and improved psychological well-being, but these differences may well be the result of different exercise habits rather than a cause. One of the main obstacles for the inactive twins was the pressure of family and work commitments when they were young. Interestingly, these barriers eventually evened out between the twin pairs once their children were older and their careers were more advanced, but by then the exercise habits were set and the twins inactive. never got back into the habit. The lesson: This whirlwind of craziness in early careers and in young children is the hardest time to maintain an exercise habit, but it’s also the most crucial.
Like all nature-nurture discussions, this one has to end somewhere in the middle. Clearly, genes are important, not just for health outcomes, but also for behaviors that we typically think of as purely voluntary. Obviously, our paths are not carved in stone. In the Swedish dog data, the influence of your childhood environment drops to almost zero by the time you turn 50, but half of the variance in dog ownership is still attributed to “effects”. unshared unique environments” – in other words, to the vicissitudes of your path through life. If you want to exercise regularly when you turn 50, choose your path carefully and try not to roll your ankle.
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