One Twin Exercises, the Other Doesn't
by Gretchen Reynolds
It is hard to determine the exact long-term effects of exercise on individuals. Most large-scale current studies focus on questionnaires, interviews, or medical records to study the role of exercise but the only thing they were able to find was that individuals who exercise are healthier than individuals that don't.
If twins have the same DNA, were raised in the same environment and had a similar upbringing they can provide insight into the effects of exercise and lifestyle. "Some past studies had found that older identical twins whose workout habits had diverged over the years tended to age differently, with greater risks of poor health and early death among the sedentary twin." However this was the first study done on young twins where they were asked questionnaires about their health starting at age 16 and continued every few years.
The idea was to find twins that had changed their exercise habits after leaving their childhood homes and they found a total of ten pairs of identical male twins. They measured their endurance capacity, body composition, insulin sensitivity, and even their brains to determine their overall health.
They found that the sedentary twin had lower endurance capacities, higher body fat, and signs of insulin resistance. Their brains were also unalike, the active twin had more grey matter especially in motor control and coordination.
Although the study was small and not a randomized trial, it did involve identical twins and strongly suggests that the results imply that the differences in their exercise habits, caused the differences in their bodies.
related article: https://www.ncbi.nlm.nih.gov/pubmed/25003773