title: Substrate utilization and enzymes in skeletal muscle of extremely endurance-trained men.
abstract: Substrate utilization during exercise at 65% of maximal O2 uptake (VO2 max) and biochemical characteristics of vastus lateralis were compared between five endurance-trained (T) and five untrained subjects (U). The oxidative enzyme activities were 100% greater in T than in U, and VO2 max was 50% higher. A greater proportion of ATP regeneration occurred through oxidative processes in T than in U (smaller leg lactate release and smaller muscle lactate accumulation). The respiratory exchange ratio together with the local leg respiratory quotient indicated a greater contribution of fat to oxidative metabolism in T than U (53 vs. 33%). No difference, however, in the ratio of plasma free fatty acid extraction to O2 extraction by the working legs was found between T and U. Thus it could be calculated that a greater fraction of fat oxidation would have been covered by intramuscular triglycerides in T than in U (34 vs. 15%, P less than 0.05). T in comparison to U were further characterized by a smaller glycogen breakdown and a smaller glucose uptake, which may have been one contributing factor that prevented the blood glucose level from falling in T. The greater leg muscle citrate concentration in T could have been one factor mediating a lower carbohydrate utilization as a response to an increase in the relative proportion of fat oxidation.
mesh_terms: D001642:Bicycling; D005230:Fatty Acids, Nonesterified / Q000097:blood; D006801:Humans; D007700:Kinetics; D007773:Lactates / Q000097:blood; D009132:Muscles / Q000502:physiology*; D010101:Oxygen Consumption; D019308:Palmitic Acid; D010169:Palmitic Acids / Q000378:metabolism; D005082:Physical Exertion; D012016:Reference Values
chemical_list: D005230:Fatty Acids, Nonesterified; D007773:Lactates; D010169:Palmitic Acids; D019308:Palmitic Acid