Lipolytic response to glucose infusion in human subjects

We have determined the effect of various rates of glucose infusion on the rates of release of glycerol (Ra glycerol), free fatty acids (Ra FFA), and on energy metabolism in normal human volunteers. Plasma kinetics were determined with use of the stable isotopic tracers D-5-glycerol and [1-13C]palmitate, and energy metabolism was determined by indirect calorimetry. The effect of glucose infusion on Ra glycerol and Ra FFA was dose-dependent. At an infusion rate of 1 mg X kg-1 X min-1, no effect on either kinetic factor was noted. At 4 mg X kg-1 X min-1, both Ra glycerol and Ra FFA were suppressed; at 8 mg X kg-1 X min-1, Ra FFA was even more depressed, but Ra glycerol was similar to the value during the 4 mg X kg-1 X min-1 infusion. At all infusion rates tested, the amount of potential energy available from the sum of the glucose infusion and endogenously mobilized fat was always greater than when no glucose was infused. Glucose decreased fat mobilization by both inhibiting lipolysis and stimulating reesterification, thus causing a significant increase in triglyceride-fatty acid substrate cycling within the adipose tissue.