ENERGY-EXPENDITURE BY INDIRECT CALORIMETRY IN PREMENOPAUSAL WOMEN - VARIATION WITHIN ONE MENSTRUAL-CYCLE

Energy expenditure in relation to the menstrual cycle was determined b y indirect calorimetry in premenopausal women. For each subject, three measurements were made within a single menstrual cycle. Energy expend iture measurements coincided with the subject's expected hormonal flux es of estradiol and progesterone: menstrual phase-both hormones at bas al levels; follicular phase-elevated estradiol, and luteal phase-eleva ted progesterone. In experiment 1, resting energy expenditure of 14 wo men was determined for 1 hour using a canopy system for calorimetry; i n experiment 2, 24-hour energy expenditures of 12 subjects were measur ed in a room-size calorimeter. Blood from fasted (12 hours) subjects w as collected following measurements of energy expenditure and analyzed for serum estradiol-17B and progesterone by radioimmunoassay. In expe riment 1, resting energy expenditure did not differ within one menstru al cycle; neither estradiol nor progesterone affected resting energy e xpenditure. In experiment 2, 24-hour energy expenditure was significan tly lower (P < 0.013) during the follicular phase when compared with t he menstrual (- 3.8%) and luteal (- 4.9%) phases. Lowered 24-hour ener gy expenditure during the follicular phase may in part be due to a dec rease in spontaneous activity and exercise. Energy expenditure during sleep, an indicator of metabolic energy expenditure, was significantly greater (P < 0.0001) during the luteal phase than during the menstrua l (+ 6.7%) and follicular (+ 5.4%) phases; this was a reflection of in creased progesterone (P < 0.0001). Twenty-four hour energy expenditure (mean +/- SEM) during the menstrual, follicular, and luteal phases wa s 8.86 +/- 0.26, 8.52 +/- 0.22, and 8.96 +/- 0.21 MJ/d, respectively. Corresponding values for energy expenditure during sleep were 5.49 +/- 0.09, 5.56 +/- 0. 10, and 5.86 +/- 0. 11 MJ/d. The menstrual cycle is a significant contributor to variation in energy expenditure through progesterone-mediated increases in metabolic rate. Variation in metabo lic energy expenditure was detectable when the contributory components of 24-hour energy expenditure were measured.