Sensitivity to acute insulin-mediated suppression of plasma free fatty acids is not a determinant of fasting VLDL triglyceride secretion in healthy humans

One important mechanism whereby obesity-associated insulin resistance leads to VLDL overproduction is thought to be by the increased flux of free fatty acids (FFAs) from extrahepatic tissues to liver, which arises as a direct consequence of impaired insulin action in adipose tissue and skeletal muscle. The aim of the present study was to address whether direct measures of peripheral tissue insulin sensitivity with regard to FFAs and glucose in the fasting state are good predictors of postabsorptive VLDL triglyceride secretion rate (VLDL-TG ASR) in humans, independent of obesity. Eighteen healthy control subjects, after an overnight fast, underwent three studies 3 weeks apart, in random order. Study 1: VLDL-TG levels, fractional clearance rate (per h), and VLDL-TG ASR were determined after an intravenous bolus of [1,1,2,3,3-H-2(5)]glycerol. Study 2: Insulin sensitivity (SI), acute insulin response (AIR), and acute C-peptide response to glucose were assessed by frequently sampled intravenous glucose tolerance test using the minimal model approach. Study 3: Insulin-mediated suppression of plasma FFAs (k) and insulin clearance were assessed in response to a low-dose stepwise intravenous insulin infusion. BMI (R-2 = 0.54), AIR, and fasting insulin levels were positively and S, negatively correlated with VLDL-TG ASR, but there was no significant association with plasma FFAs or k. Only BMI remained significantly associated with VLDL-TG ASR in multivariate analysis. The best multivariate model for VLDL-TG ASR (R-2 = 0.61, P = 0.0008) included BMI (P = 0.0008) and S-I (P = 0.12, inversely correlated). VLDL-TG secretion is predicted by BMI, independently of direct measures of insulin sensitivity. The sensitivity to insulin's acute suppressive effect on plasma FFA levels during fasting is not an important determinant of postabsorptive VLDL-TG secretion in humans.