G R Goldberg

Total free living energy expenditure was compared in lean and obese women by the new doubly labelled water method and partitioned into basal metabolism and thermogenesis plus activity by whole body calorimetry. Average energy expenditure was significantly higher in the obese group (10.22 versus 7.99 MJ/day (2445 versus 1911 kcal/day); p less than 0.001) resulting from an increase in the energy cost of both basal metabolism and physical activity. Self recorded energy intakes were accurate in the lean subjects but underestimated expenditure by 3.5 MJ/day (837 kcal/day) in the obese group. Basal metabolic rate and energy expenditure on thermogenesis plus activity were identical in the two groups when corrected for differences in fat free mass and total body mass. In the obese women in this series there was no evidence that their obesity was caused by a metabolic or behavioural defect resulting in reduced energy expenditure.

The fundamental principles of energy physiology were used to evaluate the validity of reported energy intake (EI) in 37 published dietary studies of adults providing 68 subgroups when classified according to sex and dietary method. EI was expressed as a multiple of BMR estimated using the reported heights and weights of the study populations (EI:BMR(est)). This ratio was compared with a study-specific cut-off value representing the lowest value for EI:BMR(est) that could, within defined bounds of statistical probability, reflect the habitual energy expenditure of a sedentary life-style. Mean EI:BMR(est) was 1.43 (0.19) compared with an expected requirement of 1.55. In 46 out of the 68 groups (68%), EI:BMR(est) was below the study-specific cut-off value. EI:BMR(est) was 1.37 (SD = 0.13) for women and 1.50 (SD = 0.16) for men (P less than 0.001). This could reflect either better reporting by men or a more active life-style. When categorized according to dietary assessment method, 64%, 88% and 25% of results fell below the acceptable cut-off value for studies by diet records, diet recall and diet history, respectively. These data indicate that dietary assessment methods have a strong bias towards underestimation of habitual energy intake.

The hypothesis that the energy cost of human pregnancy can be minimized by energy-sparing metabolic adaptations was tested using serial 24 h whole-body calorimetry. Eight healthy, well-nourished women were studied prepregnant and at 6, 12, 18, 24, 30 and 36 weeks gestation. Basal metabolic rate (BMR) showed highly characteristic changes within each subject and large inter-individual differences (F 3.5, P less than 0.01). Some subjects showed a highly significant depression of metabolism up to 24 weeks gestation in support of the initial hypothesis. At 36 weeks BMR ranged from +8.6 to +35.4% relative to the prepregnant baseline. This wide variability was not explained by differences in the amount of lean tissue gained. Women displaying the energy-sparing suppression of BMR tended to be thin, suggesting that changes in metabolism may be responsive to initial energy status (delta BMR v. prepregnant body fat: r 0.84, P less than 0.005). Changes in 24 h energy expenditure closely paralleled changes in BMR (r 0.98, P less than 0.001), since the energy cost of minor voluntary activity and thermogenesis remained very constant within each individual. Pregnancy decreased the net cost of weight-dependent and weight-independent standard exercises when expressed per kg body-weight: stepping -10 (SD 2)%, P less than 0.001 at 18-36 weeks, cycling -26 (SD 7)%, P less than 0.01 at 12-36 weeks. The average integrated maintenance costs of pregnancy matched previous group estimates from well-nourished women, but individual estimates ranged from -16 to +276 MJ (coefficient of variation 93%). This high level of variability has important implications for the prescription of incremental energy intakes during pregnancy. It may also have had evolutionary significance.