Gender differences in leucine, but not lysine, kinetics

Abstract

There is a controversy in the literature as to the effects of gender on leucine kinetics. Two research groups found that men oxidize more leucine during exercise, whereas another group showed no gender effects. The purpose of our study was to examine the effects of gender on leucine and, for comparison purposes, lysine kinetics. Our subjects (n = 14) were seven matched pairs of men and women selected for their exercise habits and age. After 1 wk of a standardized diet, they exercised at 50% of maximal O(2) uptake for 1 h. There was an effect of exercise in both genders: an increased leucine oxidation and an attenuation in nonoxidative leucine disposal compared with rest (P < 0.05). Furthermore, our study confirms that there are gender differences in leucine, but not lysine, kinetics. Men had a higher rate of leucine oxidation and a lower rate of nonoxidative leucine disposal during exercise (P < 0.05). For women, a larger proportion of their exercise energy needs came from fat; for men, a greater fraction came from carbohydrate (P < 0.05). We conclude that female exercisers rely to a greater extent on fat as an energy source, thereby using less carbohydrate, amino acid, and protein as a fuel source.

Basal muscle intracellular amino acid kinetics in women and men

“…Our data are also consistent with a previous report in which forearm protein synthesis and breakdown, as estimated with the two-pool model, was greater in men that in women when the data were normalized by total forearm volume, but these differences were no longer detected when the data were expressed by unit of forearm muscle mass (9). Again, from these data, it was not possible to determine if women had a higher intracellular amino acid recycling into synthesis or lower turnover, which could have explained the lower amino acid oxidation rate previously observed (13,14,16,19,30), because the model could not account for intracellular amino acid kinetics. Our current study demonstrates for the first time that the muscle intracellular kinetics (including incorporation into proteins, release from breakdown, recycling from breakdown into synthesis, and transport) of three different amino acids are not different in women and men and provides strong evidence of a lack of sex differences in baseline muscle protein and amino acid kinetics.…”

“…In other words, the greater energy consumption per unit of active metabolic tissue observed in men would suggest that the turnover and/or oxidation of some substrates are higher in men. Several studies have reported that the whole body rate of leucine appearance (an index of whole body proteolysis) and nonoxidative rate of disappearance (an index of whole body protein synthesis) were not different between women and men (13,14,16,19,30). Nevertheless, the basal rate of leucine oxidation has been shown to be significantly lower in women even when the data are expressed per unit of lean mass (13,14,16,19,30).…”

“…Several studies have reported that the whole body rate of leucine appearance (an index of whole body proteolysis) and nonoxidative rate of disappearance (an index of whole body protein synthesis) were not different between women and men (13,14,16,19,30). Nevertheless, the basal rate of leucine oxidation has been shown to be significantly lower in women even when the data are expressed per unit of lean mass (13,14,16,19,30). Furthermore, it is well known that leucine oxidation is higher in men than women during aerobic exercise (13,14,16,19).…”

“…However, because adult women have an increased proportion of peripheral fat mass, when the kinetic data were expressed per amount of forearm muscle, rather than forearm volume, these differences disappeared (9). Nonetheless, several investigators have reported significant differences in whole body leucine kinetics, with men exhibiting higher rates of leucine oxidation both at rest and during aerobic exercise, even after correction of the data by lean body mass (13,14,16,19,30). Because leucine is mainly oxidized in the muscle, a higher leucine oxidation indicates a greater net muscle amino acid/protein catabolism in men, which contrasts with the muscle kinetic data previously reported (9).…”

Amino acid ingestion improves muscle protein synthesis in the young and elderly

“…Leucine oxidation at rest (39) and after exercise (24) has been shown to be lower in women than in men. However, we have been unable to demonstrate a sex difference in the metabolism of any other amino acid, including phenylalanine, despite obvious differences in musculature and hormonal regulation (36).…”