The aim of this study was to determine the role of the autonomic nervous system (ANS) in obligatory and facultative components of the thermogenic response to food (TRF). Nineteen lean, healthy subjects participated in this study, which comprised two protocols, each exploring one component of the ANS. In the first experimental group, propranolol (prime: 80 micrograms/kg; continuous: 1 microgram.kg-1.min-1) was infused intravenously to inhibit sympathetic nervous activity (SNA), whereas in the second group atropine (prime: 5 micrograms/kg; continuous: 5 micrograms.kg-1.min-1) was used to inhibit parasympathetic nervous activity (PNA). The TRF was measured on four occasions: 1) after oral ingestion of a breakfast, during 0.9% NaCl perfusion, 2) after oral ingestion of the same breakfast, during the perfusion of one of the drugs, 3) after intragastric injection of a pureed form of the same meal as in part 1, during 0.9% NaCl perfusion, and 4) after intragastric feeding, during the administration of one of the drugs. Energy expenditure was measured by indirect calorimetry for 30 min before and 6 h after ingestion of the meal. Facultative TRF was defined as the difference between oral and intragastric TRF. Intragastric feeding significantly reduced TRF in both studies: 6.6 +/- 1.0 vs. 8.7 +/- 0.8% of the ingested energy in the SNA study and 5.5 +/- 1.6 vs. 7.4 +/- 3.1% in the PNA study. During propranolol infusion, TRF was significantly lower than it was during saline infusion after oral feeding (6.9 +/- 1.0% vs. 8.7 +/- 0.8% of ingested energy) but not after intragastric feeding. During atropine administration, TRF was reduced after both oral and intragastric feeding, although statistical significance was not reached in the latter. Atropine administration decreased gastric emptying (measured with an isotopic method) 2 h postingestion by 50%. These results show that the SNA is necessary for the facultative component of TRF to occur in humans. The role of the PNA appears to be related to its action on gastric emptying.
