Subcutaneous adipose tissue: a source of lactate production after glucose ingestion in humans

The in vivo kinetics of lactate and pyruvate in the extracellular space of subcutaneous adipose tissue after glucose ingestion were investigated in healthy volunteers by the use of a microdialysis sampling technique. Comparison was made with the metabolite levels in venous plasma. The absolute subcutaneous tissue concentrations of lactate and pyruvate were estimated in the fasting state by perfusion with varying lactate- and pyruvate-containing solutions. An equilibrium with the surrounding extracellular fluid was found for both lactate and pyruvate in concentrations similar to those in venous plasma. After glucose ingestion there was an increase in the circulating levels of glucose, lactate, and pyruvate, which returned to base-line values within 3 h. There was a more marked increase in lactate in subcutaneous adipose tissue than in venous blood, and the adipose tissue lactate remained elevated for at least 3 h. In contrast, pyruvate levels increased much less in subcutaneous fat than in venous blood. The addition of isoproterenol (which inhibits adipose tissue glucose metabolism) to the tissue perfusate lowered the subcutaneous tissue lactate levels significantly but did not affect the subcutaneous pyruvate levels. These data suggest that human subcutaneous adipose tissue is a source of in vivo lactate production after glucose ingestion. Since lactate is thought to be a major substrate for glycogen synthesis in the liver, the present findings may provide evidence of a new and important role of the adipose tissue metabolism in the regulation of whole body glucose homeostasis in humans.