Many scientific studies show that people who exercise vigorously are leaner and have better cholesterol profiles than those who do not. However, unless the studies take nonexercising subjects and train them, they do not show whether the exercise causes better cholesterol profiles and weight loss, or whether having better cholesterol levels and being leaner makes exercise easier and leads to its vigorous pursuit. There are, of course, training studies that show that sedentary men, who after becoming runners became leaner and increase their so-called "good cholesterol", high-density lipoprotein cholesterol (HDL-cholesterol.) Therefore, at least some of the healthy characteristics of the runners are due to their exercise regime. The complexity here, though, is that we also know that men who begin an exercise program with high HDL-cholesterol will be better runners after training. This may be due to the types of muscle with which they were born. How then do we distinguish the benefits of exercise training from those of genes? One approach is to train sedentary men and women to run and to subsequently monitor changes in their HDL-cholesterol levels and weight. However, it is difficult to take people who are not exercising and persuade them to run long distances, and almost impossible to track them for longer than one year. Identical twins have exactly the same genes, and, by studying those who exercise differently, we have been able to show there are long-term benefits of vigorous exercise. For example, if one twin runs fifty miles per week, and the other does not exercise engage in any exercise, then the differences in their HDL-cholesterol levels and body weights are not due to genes, but are likely due to the exercise regime of the active twin. The active twin may also choose to eat differently, for example avoid desserts. However, the adoption of other healthy living behaviors may be considered an additional beneficial result of exercising. We studied 35 pairs of identical twins, 25 male and 10 female, who were discordant for exercise. On average, the active twins ran 39 miles per week, and the sedentary twins averaged a little over four.