Precursors of plasma triglyceride fatty acids in obesity

The contribution of plasma free fatty acids (FFA) and blood glucose to the production of endogenous plasma triglyceride fatty acid (TGFA) has been studies in 13 post-absorptive human subjets who were selected on the basis of obesity, hypertriglyceridemia or both. Tracer doses of palmitic acid-9,10^?3H were given as priming injections followed by constant infusions for 9 hr to ensure that the ratio of specific activities, VLDL-TGFA/plasma FFA, reached a constant level. Once constant, this ratio provides an estimate of the proprotion of the endogenous TGFA derived from ciruculating FFA. In five lean subjects this ratio was 80%–100%, while in eight obese subjects the range was 22%–64%. Overall there was a highly significant negative correlation between the equilibrated specific activity ratios and the degree of obesity. In six subjects, three lean and three overweight, infusions of ¹⁴C-U-glucose were also given. Glucose carbon was readily incorporated into triglyceride glycorol, but there was minimal incorporation of glucose into TGFA, confirming that lipogenesis is negligible in the postabsorptive state and suggesting that the additional source of fatty acids for plasma TGFA production in the overweight subjects is likely to be stored hepatic triglyceride.