Resting metabolic rate and diet-induced thermogenesis : studies in humans on individual differences and on the impact of nutritional and non-nutritional factors

This thesis contains studies on resting metabolic rate (RMR) and diet-induced thermogenesis (DIT) of humans using indirect calorimetry (ventilated hood system) to assess energy expenditure. A literature survey of aspects of human energy exchange and of problems of energy balance is included. At first, methodological studies on the measurement of RMR, DIT and fuel utilization are presented. Measurements of RMR and respiratory quotient showed good reproducibility, in contrast to those of DIT and fuel utilization rates. Secondly, the nature and magnitude of inter-individual differences in RMR and DIT were assessed. No significant difference in DIT was found between non-obese women and obese women with a wide range in body fat distribution. Men had a significantly higher DIT compared to women. obese women with a non-abdominal type of body fat distribution had reduced RMR's in comparison with non-obese men, non- obese women and obese women with an abdominal type of body fat distribution. Thirdly, the impact of nutritional and non-nutritional factors on RMR and DIT was studied. No significant diurnal variation in RMR and DIT was found. Psychological stress did not affect RMR, but potentiated DIT. Unduly heavy exercise was found to have a significant after-effect on RMR and DIT, whereas moderate to heavy exercise did not have a systematic after-effect on RMR. Ethanol (20 g) induced a significant thermic effect, but did not affect DIT. Differences in palatability among otherwise identical mixed meals or sucrose solutions did not produce differences in DIT. Short-term carbohydrate overfeeding increased DIT, but not RMR. Carbohydrate overfeeding attenuated the after-effect on RMR of unduly heavy exercise. This thesis shows that inter-individual differences in RMR and DIT exist, even after statistically accounting for differences among individuals in age, body weight and body composition. This thesis shows also that, in addition to nutritional factors, non-nutritional factors affect the postprandial rise in energy expenditure.