The influence of nutrient administration on energy expenditure in man

Recent data on the thermic effect of nutrients are presented. When given intravenously (i.v.), glucose (+ insulin) induces an increase of energy expenditure (EE) of 7% of energy infused, whereas lipid (Intralipid) infusion stimulates EE by 3% of energy infused. The stimulation of EE due to amino acid infusion in depleted patients is 30-40% of the energy infused as amino acids. Glucose induced thermogenesis includes an 'obligatory thermogenesis' which accounts for the energy cost of storing the nutrient and a 'facultative thermogenesis' which is mainly due to a stimulation of sympathetic activity; the latter is suppressed by propranolol infusion. Comparison of nutrient induced thermogenesis during continuous administration of nutrients between the enteral and parenteral routes reveal similar responses. This shows that the energy costs of digestion and absorption of nutrients is low in comparison with the cost of nutrient storage. However, when given by the enteral route, the net efficiency of energy utilisation is also dependent on the co-efficient of nutrients absorption. Oral administration of nutrients (bolus) induces a larger thermogenic response than continuous enteral administration, due to a larger nutrient storage for the former. It is therefore advisable to allow for the thermic effect of nutrients when assessing the efficiency of energy utilisation in patients.