Metabolic changes during the menstrual cycle.

1. Eight women maintained on a constant diet throughout a single menstrual cycle were studied for cyclical changes in body-weight, carbohydrate, fat and protein metabolism. 2. A cyclical variation in weight occurred with two phases corresponding to the time of ovulation and menstruation. 3. Creatinine and nitrogen excretion in the urine also tended to be higher in the luteal phase. 4. Analysis of gas exchange in a respiratory chamber for 24 h periods at intervals during the cycle suggested that carbohydrate utilization tended to be less in the early follicular phase. 5. Feeding with a mixed diet produced marked increases in carbohydrate utilization during the day with little change in fat utilization from the fasting state. Carbohydrate utilization was less when fasting at night and tended to be less when fed during the day in the early follicular phase of the menstrual cycle. 6. Weight changes were not readily ascribed to changes in colonic function, or in glycogen or body protein storage.