The effect of exercise and improved physical fitness on basal metabolic rate.

1. The suggestion that there is a sustained enhancement in metabolic rate after exercise was investigated during the course of a study in which six normal-weight volunteers (three men, three women) took part in a 9-week training programme. Baseline values were assessed in a 3-5 week control period of minimal activity before training. At the end of the study the subjects were capable of running for 1 h/d, 5 d/week. 2. Throughout the entire study the subjects were maintained on a constant diet. Measurement of energy expenditure by the doubly-labelled water (2H2(18)O) method showed that the subjects had an energy imbalance of +3% in the control and -20% at the end of the training period. The subjects were in positive (1.1 (SE 0.2) g) nitrogen balance in the second week of the control, and in negative (-0.6 (SE 0.3) g) N balance in the last week of the exercise period. 3. Over the course of the study maximum oxygen consumption (VO2max) and high-density-lipoprotein-cholesterol levels increased by 30%. Heart rate at rest and when performing a standard step test fell significantly. 4. Body composition was assessed weekly by 40K counting and skinfold thickness measurements, in addition to 2H2 dilution at the beginning and end of the study. Fat-free mass was apparently gained in the early phases of the study, but there was lack of agreement between the different methods of assessing body composition. Changes in body-weight were not significant. 5. Basal metabolic rate (BMR), overnight metabolic rate (OMR) and sleeping metabolic rate (SMR) were measured on three occasions: in the control period, and the beginning and end of the training periods. Average BMR in the control period was 5.91 (SE 0.39) MJ/24 h and was not changed with activity. There were no changes in OMR (5.71 (SE 0.27) MJ/24 h in the control) nor in SMR (5.18 (SE 0.27) MJ/24 h in the control), nor in BMR, OMR or SMR when expressed per kg body-weight, or per kg fat-free mass. 6. These results do not support the suggestions that there is a sustained increase in BMR following exercise that can usefully assist in weight-loss programmes.