The purpose of this pilot investigation is to use 1 H Magnetic Resonance Spectroscopy (MRS) to 1) document the change in intra-muscular lipid stores (IML) before and after a prolonged bout of endurance running and, 2) determine the pattern (time course) of IML replenishment following an extremely low-fat diet (10% of energy from fat) and a moderate-fat diet (35% of energy from fat). Specifically, the study will evaluate the change in IML following a 2-hour training run and the recovery of IML in response to the post-exercise low-fat or moderate-fat diet in 10 endurance trained athletes who will consume both diets in a randomly assigned cross-over fashion. We hypothesize that IML will be depleted with prolonged endurance exercise, and that replenishment of IML will be impaired by an extremely low-fat diet compared to a moderate-fat diet. Results of this pilot study will be used to apply for extramural grant support from NIH or the US Armed Forces to investigate the effect of dietary fat on the health and performance of individuals performing heavy physical training. It is anticipated that this methodology could also be employed in obesity research to delineate, longitudinally, the reported cross-sectional relationships among IML stores, insulin resistance and obesity.