1 NO EFFECT OF MENSTRUAL CYCLE PHASE ON GLUCOSE KINETICS AND SUBSTRATE METABOLISM AT REST OR DURING 90 MINUTES OF MODERATE INTENSITY EXERCISE

Resting and exercise fuel metabolism was assessed in three different phases of the menstrual cycle characterized by different levels of estrogen relative to progesterone: early follicular (EF, low estrogen and progesterone), mid-follicular (MF, elevated estrogen, low progesterone) and mid-luteal (ML, elevated estrogen and progesterone). It was hypothesized that exercise glucose utilization, and whole-body carbohydrate oxidation, would decrease sequentially from the EF to the MF to the ML phases. Normal weight, healthy females, experiencing a regular menstrual cycle, were recruited. Subjects were moderately active, but not highly trained. Testing occurred following 3 days of diet control, and after an overnight fast (12-13 hr). Resting (2hrs) and exercise (50 % VO2 max, 90 mins) measurements of whole-body substrate oxidation, tracer determined glucose flux, and substrate and hormone concentrations were made. No significant difference was observed in whole-body fuel oxidation during exercise in the 3 phases (non-protein respiratory exchange ratio: EF 0.84 + 0.01, MF 0.85 +0.01 ML 0.85 + 0.01), nor in rates of glucose appearance or disappearance. There was, however, a significantly higher glucose (p < 0.05) and insulin ( p < 0.001) concentration during the first 45 mins of exercise in the ML vs EF and M