Differential effect of insulin on saturated and unsaturated fatty acids.

Fatty acids fulfill important roles in physiology. The plasma concentrations of fatty acids are principally regulated by insulin, which suppresses the release of fatty acids from lipid stores, and catecholamines, which increase their release from lipid stores. Although insulin regulates the concentration of plasma free fatty acids (FFAs), little is known about the relative effects of insulin on the saturated compared with the unsaturated plasma fatty acids. In the current study, we specifically measured the plasma concentration of 3 saturated and 4 unsaturated fatty acids along with an estimate of lipolytic activity using a stable isotope of glycerol during a 3-hour, 1-step, euglycemic clamp study in humans. The data showed the expected decline in plasma fatty acids from 0.26 +/- 0.02 to 0.06 +/- 0.01 micromol/mL. Saturated fatty acids were reduced from 0.12 +/- 0.01 to 0.05 +/- 0.005 micromol/mL and unsaturated fatty acids were reduced from 0.11 +/- 0.01 to 0.01 +/- 0.001 micromol/mL after 3 hours of insulin infusion. At baseline, the ratio of saturated to unsaturated fatty acid was 55:45, which increased to 82:18 by the end of study. The changes in fatty acids were evident within 1 hour. Whole body lipolytic rates were measured with deuterated glycerol and decreased from 1.48 +/- 0.56 micromol x kg(-1) x min(-1) to 0.75 +/- 0.34 micromol x kg(-1) x min(-1). Baseline postabsorptive plasma fatty acid concentrations were significantly correlated to insulin sensitivity (M value) as measured during the euglycemic clamp. There were no significant differences between the more insulin-sensitive subjects when compared with the more insulin-resistant subjects with respect to the relative decreases in saturated and unsaturated fatty acids during insulin infusion. These data demonstrate a sustained differential effect of insulin on the plasma fatty acid profile.