Researchers in Finland looked at 17 pairs of identical twins who didn’t have similar exercise habits:
The first thing to note is just how unusual such twin pairs 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 High Runner mouse line that’s often used in lab studies took mice that loved to run, bred them with each other, and produced mice that love to run even more. I’d like to think that human behavior (and mating patterns) are a little 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 diverged. 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 got about three times as much physical activity, including active commuting, as the non-exercising ones: 6.1 MET-hours per day compared to 2.0 MET-hours per day. For context, running at a ten-minute-mile pace for half an hour consumes about 5 MET-hours.
All the twin pairs came in for physical examinations, and the results were pretty much what you’d expect. The exercising twins 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 percent), significantly less abdominal fat and liver fat, and so on.
[…]
A 2018 case study from researchers at California State University Fullerton looked at a single identical twin pair, then aged 52. One was a marathoner and triathlete who had logged almost 40,000 miles of running 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 percent lower. Most fascinatingly, muscle biopsies showed that the marathoner had 94 percent slow-twitch fibers while the truck-driver had just 40 percent slow-twitch. No one before or since (as far as I know) has shown such a dramatic change in muscle properties.
“The twins in the study were drawn from two previous Finnish twin studies that included thousands of pairs of identical twins.”
Population of Finland is only about 5.5 Million people. Considering how seldom one comes across identical twins in daily life outside Finland, the rational observer has to say — What?
“Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population),” according to Wikipedia, so millions of Finns should yield thousands of twins.
Very interesting.
Scandinavian countries like Finland usually have a proper population register, so it is easy to find all the twins (fraternal or identical) and collect data. You don’t need to call them back in for stuff like income or medical records because you just look them up using the register, you get IQ and personality and some other things for the men from the draft examinations, and so on. That’s why they do such amazing public health and economic research. Their real limit is simply that they are so small, so for rare things like twinning, the sample size can still wind up unfortunately small (adequate for normal analyses like heritability of exercise behavior or BMI, but then when you look for discordant twin pairs specifically, you may be underpowered). You can accumulate cohorts over time, but of course that causes problems of its own if you aren’t interested in the temporal trends.
Isegoria wrote: “according to Wikipedia”
Wikipedia? Let’s be serious!
Gwern is truly a national treasure.