Thermogenesis induced by nutrient administration in man.

After ingestion or intravenous infusion of a nutrient, the energy expenditure increases. This nutrient-induced thermogenesis represents 6 to 8% of the energy infused when glucose is infused with insulin (glucose clamp technique) to healthy subjects. At physiological plasma insulin levels (i.e. below 200 microU/ml), glucose induced thermogenesis (GIT) was 6%, whereas at supraphysiological levels (i.e. greater than 400 microU/ml) GIT was 8% of the energy content of glucose infused. This thermogenic response to glucose infusion includes two components: 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 thermogenic response to lipid infusion (Intralipid 20%) was 2 to 3% of the energy infused. The stimulation of energy expenditure following amino acid infusion has been studied less in healthy subjects; in depleted patients, the thermogenic response to amino acids has been evaluated to correspond to about 30 to 40% of the amino acid energy infused. In addition to the concept of the thermogenic response to nutrients, it is of interest to know the cost of nutrient storage; for glucose, it amounted to 12%, for lipid to 4% of the energy stored. It is concluded, that lipid induces a lower thermogenic response and has a smaller cost for storage than glucose. Thus, lipid infusion allows giving energy more efficiently to the patient than large amounts of glucose. In total parenteral nutrition, it is common practice to supply non-protein energy roughly equally between glucose and fat; this is more economical than the infusion of large amounts of glucose.