ROLE OF THE AUTONOMIC NERVOUS-SYSTEM IN THE THERMOGENIC RESPONSE TO FOOD IN LEAN INDIVIDUALS

The aim of this study was to determine the role of the autonomic nervo us system (ANS) in obligatory and facultative components of the thermo genic response to food (TRF). Nineteen lean, healthy subjects particip ated in this study, which comprised two protocols, each exploring one component of the ANS. In the first experimental group, propranolol (pr ime: 80 mu g/kg; continuous: 1 mu g.kg(-1).min(-1)) was infused intrav enously to inhibit sympathetic nervous activity (SNA), whereas in the second group atropine (prime: 5 mu g/kg; continuous: 5 mu g.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 break fast, during 0.9% NaCl perfusion, 2) after oral ingestion of the same breakfast, during the perfusion of one of the drugs, 3) after intragas tric 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 adm inistration of one of the drugs. Energy expenditure was measured by in direct calorimetry for 30 min before and 6 h after ingestion of the me al. Facultative TRF was defined as the difference between oral and int ragastric TRF. Intragastric feeding significantly reduced TRF in both studies: 6.6 +/- 1.0 vs. 8.7 +/- 0.8% of the ingested energy in the SN A study and 5.5 +/- 1.6 vs. 7.4 +/- 3.1% in the PNA study. During prop ranolol infusion, TRF was significantly lower than it was during salin e infusion after oral feeding (6.9 +/- 1.0% vs. 8.7 +/- 0.8% of ingest ed energy) but not after intragastric feeding. During atropine adminis tration, TRF was reduced after both oral and intragastric feeding, alt hough 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 n ecessary for the facultative component of TRF to occur in humans. The role of the PNA appears to be related to its action on gastric emptyin g.