1. Eight women were studied under metabolic-ward conditions while consuming a constant diet throughout a single menstrual cycle. Basal body temperature, salivary and urinary hormone concentrations were used in monitoring the cycle and designing the study so that whole-body calorimetry for 36 h was conducted at four phases of the cycle in relation to the time of ovulation.2. The metabolic rate during sleep showed cyclical changes, being lowest in the late follicular phase and highest in the late luteal phase. The increase amounted to 6 1 (SD 27) YO. Energy expenditure (24 h) also increased but the change was not statistically significant (P > 0.05). Exercise efficiency did not change during the cycle.3. There were no significant changes in plasma thyroxine, 3,5,3'-triiodothyronine or free 3,5,3'-triiodothyronine concentrations to explain the metabolic rate changes; nor did they relate to urinary luteinizing hormone, pregnanediol-3a-glucuronide or oestrone-3-glucuronide excretion rates. No link with salivary cortisol or progesterone concentrations was observed, but there was a small inverse relation between the individual increase in sleeping metabolic rate and the subjects' falling ratio of urinary oestrone-3-glucuronide : pregnanediol-3a-glucuronide.The variability in energy balance of women during the menstrual cycle has been of interest since research workers in the 1920s examined the problem. At first there was little appreciation that changes might occur at times other than menstruation itself and whether systematic changes did occur was disputed (Snell et al. Hitchcock & Wardwell, 1929). Some authors failed to find any effect, whereas others showed an increase in basal metabolic rate (BMR) during the premenstrual phase of the menstrual cycle. In all these early studies the dietary intake was uncontrolled so that any changes in food intake could have affected BMR. More recently, Solomon et al. (1982) overcame this problem by measuring BMR in the course of studies which involved changing the protein intake in sequential menstrual cycles. Thus, the diet was maintained constant during a single cycle but the protein intake varied from cycle to cycle. The BMR was found to increase significantly during the luteal phase of the cycle. M E T H O D SThe purpose of the present study was to maintain food intake constant throughout the cycles of eight women who remained under metabolic-ward conditions. Total faecal and urine collections were made to quantify energy and protein losses, and thyroidal, cortisol and sex hormones were also monitored throughout the cycles to assess whether they might influence energy expenditure. Energy expenditure itself was measured under a variety of * For reprintshttps://www.cambridge.org/core/terms. https://doi

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.