It turned out that these genetically identical twins looked surprisingly different beneath the skin and skull. The sedentary twins had lower endurance capacities, higher body fat percentages, and signs of insulin resistance, signaling the onset of metabolic problems. (Interestingly, the twins tended to have very similar diets, whatever their workout routines, so food choices were unlikely to have contributed to health differences.)
The twins’ brains also were unalike. The active twins had significantly more grey matter than the sedentary twins, especially in areas of the brain involved in motor control and coordination.
Presumably, all of these differences in the young men’s bodies and brains had developed during their few, brief years of divergent workouts, underscoring how rapidly and robustly exercising — or not — can affect health, said Dr. Urho Kujala, a professor of sports and exercise medicine at the University of Jyvaskyla who oversaw the study.