Between-group differences in basal metabolic rates: an analysis of data collected in Scotland, the Gambia and Thailand.

Basal metabolic rate (BMR; using the Douglas bag method), body weight, and fat-free mass (FFM; using the skinfold method) were measured in 46 Scottish and 47 Gambian women in the non-pregnant non-lactating state and in 91 Scottish, 50 Gambian and 52 Thai women early in pregnancy. There were significant between-country differences in BMR (1) in absolute terms (MJ/d), (2) per kg body weight and (3) per kg FFM. However, these differences probably resulted from differences between the groups in average body weight and body composition. Differences per kg body weight were in part explained by differences in body fat content and the relatively low metabolic rate of adipose tissue. Differences in BMR/kg FFM were largely explained by between-group differences in the mass of the FFM, together with the finding that within each group, the BMR/kg FFM tended to decrease as weight increased. There were no significant differences in BMR between Scottish, Gambian and Thai women of similar FFM. There were, therefore, no apparent effects on BMR due to differences in race, climate, diet or nutritional status. Differences in the composition of the FFM could explain the 15 per cent lower BMR/kg FFM in the heaviest compared to the lightest subjects. This brings into question the widespread practice of using FFM as a metabolic reference standard. In contrast, BMR divided by the square root of FFM was similar in all three countries: for non-pregnant women, in MJ/kg0.5/d (mean and s.d.), Scotland 0.87 (0.07) and The Gambia 0.89 (0.07), and for pregnant women, Scotland 0.86 (0.07), The Gambia 0.87 (0.06) and Thailand 0.87 (0.08), and this may be a useful basis for comparing different groups in future. The accuracy of two equations for predicting BMR was assessed. The equation of FAO/WHO/UNU (1985) accurately predicted, from body weight, the average BMR of Scottish women, but under-estimated the average BMR of Gambian and Thai women by 5-10 per cent. The equation of Garby et al. (1988), which predicts BMR from FFM and fat mass, under-estimated the BMR of Scottish, Gambian and Thai women by 5, 10 and 14 per cent respectively, with the largest error within each group being for subjects with the smallest FFM.