The studies presented in this thesis examine some of the factors responsible for variation in basal metabolic rate (BMR, defined as the energy expended by an individual lying quietly at rest in a thermoneutral environment, at least 12 hours postabsorptive). They are particularly concerned with the parts played by differences in body composition and cellular metabolic activity, under the influence of thyroid hormones and catechloamines, in explaining variability in BMR. In the great majority of people, and certainly most of those living in an industrialised society, BMR accounts for the largest part of daily energy expenditure, often making up more than two thirds of the total. Study of the factors that effect BMR is therefore central to our understanding of the causes of variation in daily energy needs. The first study undertaken sought to explore some of the general relationships between BMR and body composition in a group of 97 healthy women. BMR was measured using the Douglas bag technique, and body composition assessed by measurements of weight, height, body fat content (skinfold and densitometry estimates), circumferences and diameters. Differences in BMR between the women were found to be large (CV = 11.8%, standard deviation 159 kcal/day) and could be best explained by differences in FFM, accounting for 45 % of the total variance. The relationship between BMR and FFM was unaffected by body fatness or age. However, at a given FFM considerable variation in the BMR of individual women was evident. Moreover, for the purposes of predicting an individual's BMR, FFM was found to be no better than body weight. Simple differences in the weight of the FFM therefore, could only partially explain the variation in BMR between the women. A further observation from the study was that BMR expressed per kg body weight or per kg FFM tended to decline form light to heavy individuals. This finding has implications for the use of FFM as a metabolic reference standard, and it is suggested that it may relate to differences in the composition of the FFM. The role of the catecholamines was considered in a study which investigated the effect of B-adrenergic blockade on basal metabolic rate. The BMRs (measured using a ventilated hood system) of a group of 18 patients receiving beta blocker drugs in the treatment of cardiovascular disorders were compared to those of 28 healthy control subjects. In relation to the FFM (estimated from skinfold thicknesses) the BMRs of the B-blocker patients were found to 8% lower than that of the controls, equivalent to 136 kcal/day. The study revealed a potentially important side effect of this widely prescribed group of drugs and moreover, suggested that BMR has an adrenergically mediated component. A further study was undertaken with the aim of elucidating the causes of the marked variation in BMR relative to the FFM observed in the initial investigation. Two groups of women characterised by particularly high or low BMRs in relation to their FFM were selected for further study. Repeat measurements of BMR suggested that part of the differences between the groups, and by extension the initial study also, resulted from within-subject variation in BMR. Error in measurement of the FFM (estimated by skinfolds, total body water and densitometry) was found to be small and its potential contribution considered minor. It was estimated that genuine inter-individual variation in BMR in relation to the FFM was in the region of 100 kcal/day. Thyroid hormone levels were found to be significantly greater in the high BMR group than in the low and it was postulated that these differences were likely to be responsible for at least part of the variance in BMR relative to FFM. Thyroid status did not however, provide the complete explanation, a residual standard deviation approximately 70 kcal/day remained. It was considered likely that differences in the composition of the FFM were involved in explaining the remaining variance. Urinary catecholamine levels were comparable in both groups, however the possibility that differences in an adrenergically mediated process may have contributed to the differences in BMR could not be ruled out. Traditionally, differences in BMR have been ascribed to differences in body size, age, sex, race, climate and nutritional status. The studies presented in this thesis suggest however, that that these may have a common basis in that they relate to differences in one or both of the major determinants of BMR; to differences in body composition - primarily to the mass of fat-free tissue and to the relative proportions of its component parts - and to hormonally induced metabolic activity of the tissues
