24-hour energy expenditure and the menstrual cycle.

To determine whether 24-h energy expenditure changes during the menstrual cycle, 10 normally menstruating women in their 20s and 40s were measured repeatedly for periods of 36 and 46 h by simultaneous direct and indirect calorimetry. A standardized sedentary daily schedule included three meals to provide food intake equal to expenditure. Eight of the 10 women showed increases of 8-16% during the 14-day luteal phase following ovulation, a significant increase (p less than 0.00002 by direct calorimetry, p less than 0.001 by indirect calorimetry). One subjects, whose increase was 14% following ovulation, showed no significant change during a month when she took an oral contraceptive. Progesterone from the corpus luteum is the likely cause of a 9% increase in 24-h energy expenditure in normally menstruating women.