GENDER DIFFERENCES IN LEUCINE KINETICS AND NITROGEN-BALANCE IN ENDURANCE ATHLETES

The current Canadian Recommended Nutrient Intake (RNI) for protein (0. 86 g.kg-1.day-1) makes no allowance for an effect of habitual physical activity. In addition, Tarnopolsky et al. (J. Appl. Physiol. 68: 302- 308, 1990) showed that males may catabolize more protein than females consequent to endurance exercise. We examined nitrogen (N) balance and leucine kinetics during submaximal endurance exercise to determine th e adequacy of the current Canadian RNI for protein for male and female endurance athletes. Athletes were matched for equal training volume, competitive status, and conditioning and were fed diets isoenergetic w ith their habitual intake, containing protein at the Canadian RNI. Sub jects were adapted to the diet for 10 days before completing a 3-day m easurement of N balance. N balance showed that the RNI was inadequate for females (-15.9 +/- 6.0 mg.kg-1.day-1) and males (-26.3 +/- 11.0 mg .kg-1.day-1). Leucine kinetics during exercise were determined for eac h subject on day 3 of the N balance experiment by use of a primed cont inuous infusion Of L-[1-C-13]leucine and the reciprocal pool model. Ex ercise resulted in a significant (P < 0.01) increase in leucine oxidat ion for both groups. Males oxidized a greater amount of leucine during the infusion than females (P < 0.01). Leucine flux also increased sig nificantly (P < 0.01) during exercise in both groups. We conclude that the current Canadian RNI for protein is inadequate for those who chro nically engage in endurance exercise. Moreover, higher leucine oxidati on in male athletes, at rest and during exercise, could result in thei r having an even higher protein requirement than female athletes. Futu re investigations of protein metabolism in athletes should consider ma les and females as distinct groups.