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.
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