J. Clin. Biochem. Nutr., 25, 97-107, 1998 Resting Metabolic Rate and Diet-Induced Thermogenesis during Each Phase of the Menstrual Cycle in Healthy Young Women Tatsuhiro MATSUO,1 * Shinichi SAITOH,2 and Masashige SUZUKI,2 1 Division of Nutrition and Biochemistry, Sanyo Women's College, Hatsukaichi 738-8504, Japan 2 Institute of Health and Sport Sciences, University of Tsukuba, Tsukuba 305-8576, Japan (Received June 20, 1998) Summary The effects of the menstrual cycle on resting metabolic rate (RMR) and diet-induced thermogenesis (DIT) were studied in nine healthy young women aged 18-19 years. All subjects were eumenorrheic, with regular menstrual cycles ranging from 28 to 32 days. RMR and DIT were measured in the mid follicular phase and in the mid luteal phase. On the experimental days, subjects fasted overnight; then the RMR was measured by indirect calorimetry. For the measurement of DIT, subjects were fed a meal containing a uniform amount of energy (2.53 MJ) eaten within 15 min, and then indirect calorimetry was performed during rest for 180 min. The RMR was significantly higher in the luteal phase than in the follicular phase (67.0 vs. 62.5 J/kg/min, p<0.01). DIT was also significantly higher in the luteal phase (4.0 vs. 3.2 kJ/kg/3 h, p<0.01). The postprandial respiratory exchange ratio was slightly lower in the luteal phase than in the follicular phase (0.78 vs. 0.81). These results suggest that the menstrual cycle phase affects both the RMR and DIT. Higher postprandial energy expenditure and fat utilization in the luteal phase may be related to sympathetic and endocrinal actions. Key Words: resting metabolic rate, diet-induced thermogenesis, fat utilization, menstrual cycle, women Interest in the possible variability of energy expenditure during the menstrual cycle has increased in recent years. Although studies to assess the effect of the menstrual cycle on the basal metabolic rate were conducted as early as the 1920's [1-10], the findings were not conclusive. An increased basal metabolic rate during * To whom correspondence should be addressed . 97 98 T. MATSUO, S. SAITOH, and M. SUZUKI the luteal phase was found by some researchers, whereas others did not observe such a rise. More recent studies by Tai et al. [11] and Piers et al. [12] found no significant variations in the resting metabolic rate (RMR) during the different phases of the cycle. Diet-induced thermogenesis (DIT) has not been studied as extensively as the influence of the menstrual cycle on the RMR. Weststrate [l0] measured both RMR and DIT in 23 women during the follicular and luteal phases over the course of three menstrual cycles and reported no significant differences between the two phases for either variable. Metha and Pande [13] compared DIT during the premenstrual and postmenstrual phases of the cycle in subjects who drank 200 ml of milk and found no significant difference between the two phases. However, Piers et al. [12] reported a significant increase in DIT during the luteal phase of the cycle; whereas Tai et al. [11] suggested that DIT decreased significantly during postovulation phases (average of luteal and late luteal) compared with preovula- tion phases (average of early follicular and follicular). Because of the difficulty of studying women at a particular phase of the menstrual cycle, most studies on DIT in women have not taken potential periodic- ity into account. However, any variation in DIT during the different phases of the menstrual cycle could be important in energy balance. Such a difference could have considerable implications for the validity of previous studies as well as an effect on future methods. This study was thus carried out to establish whether there are consistent changes in the RMR and DIT over a single menstrual cycle in healthy young women. SUBJECTSAND METHODS Subjects. Nine young Japanese women (aged 18-19 years) who did not have a habit of daily exercise were recruited from Sanyo Women's College (Hiroshima, Japan) to participate in this study. All procedures were approved in advance by the Institutional Review Board of Sanyo Women's College and were in accordance with the Helsinki Declaration of 1975, as revised in 1983. After a detailed explana- tion of this study, each subject gave her informed written consent. The subjects were found to be free of disease by a medical examination before the study. Subject characteristics are shown in Table 1. The subjects' height and weight, from which the body mass index (BMI) was calculated, were measured by conventional methods. Percentage of body fat, fat mass, and fat free mass (FFM) were measured with a bioelectrical impedance analyzer (Model TBF-102, Tanita Co., Ltd., Tokyo). All subjects had normal menstrual cycles that ranged from 28 to 32 days. The phase of the menstrual cycle was determined as described previously [11] (follicular phase, days b-10; luteal phase, days 21-25). Experimental design. During the period of the study, each subject maintain- ed a normal life style and ate ad libitum except for the day before the experimental period. That evening each subject ate the same supper (50 kJ kg' body weight) at J. Clin. Biochem. Nutr. MENSTRUAL CYCLE AND ENERGY METABOLISM 99 Table 1. Subject characteristics measured during a single menstrual cycle.a aValues are means±SE for nine subjects . 19:00 h. Subjects fasted overnight and entered the experimental laboratory room at 08:00 h where they rested until the start of experiment at 10:00 h. The experiment was performed on the subjects in the follicular phase and luteal phase. RMR and DIT tests were performed by indirect calorimetry at 10:00-13:30 h after the subject had rested comfortably, in a supine position, while trying not to move or fall asleep. The DIT test meal consisted of bread and butter, boiled egg, bone-less ham, orange juice, apple, and yogurt and provided energy as carbo- hydrate (58%), fat (24%), and protein (18%). The energy of the test meal was 2.53 MJ. The subjects were fed a test meal at 10:15-10:30 h. The subjects then rested for 3 h (10:30-13:30 h). During rest, oxygen consumption and respiratory exchange ratio (R) were measured. Blood samples were collected from the cephalic vein at the level of the forearm to obtain serum and plasma at 10:15, 10:45, 11:15, 11:45, 12:15, and 13:15 h. The voided urine volume was noted and acidified with concen- trated hydrochloric acid (0.01 liter acid/liter urine). All procedures were perform- ed in a laboratory under uniform conditions (temperature: 22± 1°C, humidity: 60%). Measurements. Indirect calorimetry was performed by the ventilated hood technique. For measurement of oxygen consumption and R, the subjects wore face masks (Takei Co., Ltd., Tokyo) continuously for 210 min, except for meal time (15 min). All expired gas was collected into a Douglas bag (Takei Co., Ltd., Tokyo), and the bag was changed every 15 min during rest. The concentrations of oxygen and carbon dioxide in the expired collected gas were immediately analyzed by a gas analyzer (Model RAS-30, RAS-40, AIC Co., Ltd., Tokyo). Urinary nitrogen excretion over the metabolic test was determined by the micro-Kjeldhal method [14]. RMR and postmeal total energy output (PTEO) were calculated from oxygen consumption and carbon dioxide production, corrected for urinary nitrogen loss as described by Consolazio et al. [15]. The R was also calculated from the ratio of the volume of carbon dioxide production to the volume of oxygen consumption per unit of time and used as an index of carbohydrate and fat utilization [15]. DIT was determined by the following formula: PTEOi8o min- RMRi8o min. Plasma glucose, serum immunoreactive insulin (IRI), triacylglycerol, and free fatty acids were determined by methods reported previously [16-20]. Evaluations Vol. 25, No. 2, 1998 100 T. MATSUO, S. SAITOH, and M. SUZUKI of serum triiodothyronine (T3), thyroxine (T4), luteinizing hormone (LH), follicle- stimulating hormone (FSH), estradiol, and progesterone levels in fasting serum from the subjects were requested from Scripps Reference Laboratory (SRL Co., Ltd., Tokyo). Urinary epinephrine and norepinephrine excretions were assayed by high-performance liquid chromatography with electrochemical detection as modi- fied by the method of Refshauge et al. [20]. Statistical analysis. The statistical analysis was conducted with a personal computer (Macintosh LC 575, Apple Japan, Inc., Tokyo) using a statistical package program (Stat View 4.02, Abacus Concepts, Inc., Berkeley, CA). The statistical significance of the difference between phases was tested by Student's t-test. Fig. 1. Oxygen consumption (top) and respiratory exchange ratio (R, bottom) before and after meal ingestion measured during a single menstrual cycle. Oxygen consumption and carbon dioxide production were measured, and the R was calculated from these values. Each point represents the mean value±SE for nine subjects. J. Clin. Biochem. Nutr. MENSTRUAL CYCLE AND ENERGY METABOLISM 101 RESULTS Oxygen consumption and R Oxygen consumption and R during rest were measured for 3.5 h to assess RMR and DIT (Fig. 1). Basal and postprandial metabolic test results calculated from the data in Fig. 1 are shown in Table 2. RMR, DIT, and PTEO were significantly higher in the luteal phase than in the follicular phase (p<0.01, p < 0.05). On the other hand, basal and postprandial R were lower in the luteal phase than in the follicular phase, but the differences were not statistically significant. Substrates and hormones in plasma, serum, and urine Fasting serum estradiol and progesterone were significantly higher in the luteal phase than in the follicular phase (p<0.01, p<0.05) (Table 3).