INTRAINDIVIDUAL VARIATION IN THE BASAL METABOLIC-RATE OF WOMEN - EFFECT OF THE MENSTRUAL-CYCLE

The validity of using the basal metabolic rate (BMR) to calculate an i ndividual's energy requirements is based upon the assumption that the intraindividual variation in BMR is small. Early studies (pre-1940) on BMR in women had shown that the menstrual cycle may have a profound e ffect, contributing to high levels of intraindividual variation, To in vestigate this issue further, and to explore whether BMR is indeed a b iological constant in women, sequential measurements of BMR were made in women over one menstrual cycle, Two independent studies were undert aken in which the BMR of 12 weight-stable women (not taking the contra ceptive pill) was measured every day (excluding weekends) for a period of 5 weeks. The six women participating in the first study were measu red using a Douglas bag, while the six subjects in the second study we re measured with a Deltatrac (Datex, Helsinki), Nine of the 12 subject s demonstrated a peak in BMR during the late luteal phase of the menst rual cycle, while 8 of the 12 subjects exhibited a fall in BMR after t he onset of menstruation, Group analysis of the results indicated that the BMR during the early follicular phase was significantly lower tha n the BMR during the late luteal phase (Wilcoxon's signed rank test: P <0.01). The level of intraindividual variation was assessed by calcul ating the coefficient of variation (CV) for the measurement period, Th e CV in six of the women exhibited a level of variation comparable to men (2-4%). However, intraindividual variation in the BMR of the other six women was considerably higher (up to 12%). In these six women, th erefore, BMR may not be considered a biological constant, All availabl e data on sequential measurements of BMR in fertile women were collate d from the literature for the first time, and statistically analyzed. BMR during the early follicular phase was found to be significantly lo wer (P <0.05) than during the late luteal phase, Further work on the b iological constancy of BMR in women and the impact of the menstrual cy cle on BMR is required, This area is of particular importance given th e fact that the energy requirements of both individuals and population s are now calculated using the BMR. (C) 1996 Wiley-Liss, Inc.