HEPATIC SECRETION OF VLDL FATTY-ACIDS DURING STIMULATED LIPOGENESIS IN MEN

Fatty acids (FA) that are utilized for triglyceride (TG) synthesis in the liver and principally from two sources: FA synthesized de novo in the liver and preformed FA. We have measured the contribution from the two sources to very low density lipoprotein (VLDL) TG synthesis indiv idually for palmitate, oleate, stearate, and linoleate (approximate to 98% of the total FA of VLDL TG (VLDL TGFA)) by isotopomer analysis. F ive healthy men were studied in the basal state, and 1 (day 1) and 4 d ays (day 4) after the start of a hypercaloric carbohydrate-enriched di et (approximate to 2.5 times energy expenditure). The secretion of de novo palmitate was increased 15- and 43-fold after 1 and 4 days of hyp eralimentation (2.6 +/- 1.2 (basal state), 40.8 +/- 20.0 (day 1), and 113.3 +/- 42.0 mu mol/kg per d (day 4)). Even though 4 days of hyperal imentation increased the secretion of de novo stearate 43-fold and de novo oleate 70-fold (stearate; 0.2 +/- 0.2 (basal), 8.6 +/- 3.3 mu mol /kg per d (day 4), oleate; 0.4 +/- 0.4 (basal), 28.2 +/- 12.7 mu mol/k g per d (day 4)), palmitate accounted for 75-85% of all the de novo VL DL TGFA. One day of carbohydrate hyperalimentation tended to decrease the secretion while 4 days increased the secretion of all preformed FA in VLDL TG. The rate of secretion of preformed palmitate and oleate w ere almost identical (palmitate; 80.2 +/- 22.2 (basal), 45.1 +/- 23.8 (day 1), and 256.2 +/- 74.1 mu mol/kg per d (day 4), oleate; 95.2 +/- 22.8 (basal), 46.2 +/- 24.2 (day 1), and 356.8 +/- 74.1 mu mol/kg per d (day 4)) and collectively these two FA accounted for 80-90% of the s ecretion from the preformed source. Palmitate is the predominant produ ct of acute and prolonged carbohydrate mediated lipogenesis in the hum an liver. The pathway of further elongation and subsequent desaturatio n of de novo synthesized palmitate to generate stearate and oleate is inducible but, quantitatively, of minor significance in hepatic lipoge nesis.