Muscle structure with low- and high-fat diets in well-trained male runners

Endurance capacity, maximal oxygen uptake capacity ((V) over dot O(2)max) a nd quantitative muscle ultrastructural composition was analyzed in 7 well-t rained male runners (mean age 37.1 years, mean (V) over dot O(2)max 60 ml/m in/kg) after a one month period of a tow-fat diet (dietary fat intake 18.4% and a similar period of a high-fat diet (dietary fat intake 40.6%), Betwee n these two interventional periods a washout period of one month was inters persed in which the nutritional fat content was approx, 32%; close to the a verage American Diet. During all three periods protein content of the nutri tion was kept nearly constant at: 15%. After the high-fat diet time to exha ustion in the endurance test increased significantly by 21% while (V) over dot O(2)max remained unchanged. Muscle mitochondrial volume density remaine d unchanged while the intramyocellular fat content increased by 60%. Due to large interindividual differences in this variable this difference did not become statistically significant. While some 20% of the mitochondria are l ocated in a subsarcolemmal location, only 10% of the lipid stores are assoc iated with these mitochondria, Less than 2% of the mitochondrial outer surf ace are in contact with lipid droplets whereas 25-35% of the lipid surface is in contact with mitochondria. None of these variables is significantly a ltered after a high-fat: diet. It is concluded that the change in endurance capacity of the subjects cannot bit explained based on the structural chan ges observed in skeletal muscle tissue. This may be related to methodologic al problems associated with the determination of intramyocellular fat conte nt.