A study of the thermic responses to a meal and to a sympathomimetic drug (ephedrine) in relation to energy balance in man.

1. Sixteen adult male volunteers were selected on the basis of body size and customary food intake: half could be described as "lean' and habitually consuming large amounts of food (group mean +/- SEM: 15.03 +/- 1.13 MJ/d), the high-energy-intake group (HEI group), and half though "lean' admitted to a weight problem and regularly consumed a lower than average food intake (group mean +/- SEM: 6.90 +/- 0.39 MJ/d), the low-energy-intake group (LEI group). 2. Energy expenditure was measured by open-circuit indirect calorimetry. Resting metabolic rate (RMR) was recorded. A meal (Complan, either 2.1 MJ or 4.2 MJ), ephedrine hydrochloride (0.25 mg and 0.50 mg/kg body-weight) or a water control were then administered and metabolic rate (MR) was measured for 4 h. Blood was collected before and 1 h after the meal or drug, and the serum analysed for various hormones and blood metabolites. 3. The size of the thermic response to feeding but not the time-course was related to meal size in both groups. MR increased by 21.6 and 28.6% in the HEI group and by 8.2 and 20.0% in the LEI group in response to the 2.1 and 4.2 MJ Complan meals respectively. Fasting insulin levels were similar in both groups but showed a significantly higher level in the LEI than HEI group after the Complan meals. 4. The mean RMR increased by 5.2 and 10.3% in the LEI in response to ephedrine and by 15.7 and 11.2% in the HEI groups after 0.25 mg and 0.50 mg ephedrine/kg respectively. The rise in serum-free fatty acids in response to ephedrine was significantly higher in the HEI group than in the LEI group. 5. These results suggest (1) the meal size required to promote a maximum thermic effect is smaller in energetically-inefficient individuals (2) the sensitivity to a sympathomimetic drug is also increased in energetically-inefficient individuals. 6. We conclude that in energetically-efficient individuals both the thermic response to a meal and the sympathetic-mediated thermogenesis are lower than in energetically-inefficient ones.