Effects of insulin on human adipose tissue metabolism in vivo.

1. The metabolic effects of insulin on human adipose tissue were studied by combining the euglycaemic clamp technique with measurement of arteriovenous differences across the subcutaneous adipose tissue of the anterior abdominal wall. 2. Eight normal subjects were studied after an overnight fast, and for 120 min during infusion of insulin (mean arterialized plasma insulin 50-55 m-units/l). 3. During the insulin infusion, the arterialized and abdominal venous levels of both non-esterified fatty acids and glycerol fell, and the arteriovenous differences for the release of these substances narrowed. The fractional rate of re-esterification of fatty acids was around 20% in the fasting state and increased to almost 100% during hyperinsulinaemia. 4. In the fasting state the uptake of glucose and 3-hydroxybutyrate by adipose tissue could account for only 20% of the oxygen uptake. During insulin infusion, adipose tissue glucose uptake increased and could account for more than 100% of oxygen uptake, implying storage of glucose. 5. Net balances of different substrates across adipose tissue were examined by calculating fluxes in terms of microgram-atoms of carbon. In the fasting state adipose tissue was in marked negative carbon balance (because of the export of non-esterified fatty acids); during insulin infusion it just reached 'carbon balance'. These results were in contrast to those from a previous study of glucose ingestion, in which the adipose tissue showed marked positive carbon balance (net substrate deposition).