If you've ever wondered why those trips to the gym -- and all that exercise -- haven't yielded the benefits you expected, don't beat yourself up about it. For some of us wanting to get stronger and more fit but not having much luck, it may be determined in our genes.

Based on a genomewide association study, scientists discovered 29 genes that significantly predicted the body's ability to improve its aerobic response (how much oxygen your muscles burn or the amount of blood pumped by your heart) to exercise, specifically endurance training. Out of those, differences in 11 DNA sequences provided a predictable snapshot of a patient's true fitness potential.

First, researchers examined the DNA taken from muscle biopsies on nearly 500 sedentary patients, then assigned volunteers to customized, thrice-weekly endurance training sessions for 20 weeks. After the exercise period ended and new muscle biopsies were taken, as much as 20 percent of the participants experienced far smaller improvements than scientists expected.

Having this kind of information at hand, experts say, will allow physicians to more accurately determine a more beneficial personalized exercise program down the road. Until then, scientists warn folks not to stop exercising as their discovery only encompasses the genes that may affect endurance, and not cholesterol, heart rates or the body's regulation of insulin.

Journal of Applied Physiology February 4, 2010 Free Full Text PDF

USA Today February 4, 2010

MSNBC February 4, 2010