Role of human liver lipogenesis and reesterification in triglycerides secretion and in FFA reesterification.

To measure 1) the contribution of hepatic de novo lipogenesis (DNL) and plasma free fatty acid (FFA) reesterification to plasma triglyceride (TG) secretion, and 2) the role of oxidation and hepatic and extrahepatic reesterification in FFA utilization, five normal subjects drank deuterated water and were infused (postabsorptive state) with [1-13C]palmitate and [1,2,3-2H5]glycerol. Total lipid oxidation (Lox) was measured by indirect calorimetry. FFA oxidation (2.76 ± 0.65 μmol ⋅ kg-1 ⋅ min-1) accounted for 45% of FFA turnover rate (Rt) (1.04 μmol ⋅ kg-1 ⋅ min-1) and 91% of Lox; FFA reesterification was 3.27 ± 0.54 μmol ⋅ kg-1 ⋅ min-1. Fractional and absolute TG Rt were 0.21 ± 0.02 h-1 and 0.11 ± 0.05 μmol ⋅ kg-1 ⋅ min-1. DNL accounted for 3.9 ± 0.9% of TG secretion, and hepatic FFA reesterification accounted for 49.4 ± 5.7%; this last process represented a utilization of FFA of 0.16 ± 0.02 μmol ⋅ kg-1 ⋅ min-1. We conclude that, in the postabsorptive state, 1) DNL and FFA reesterification account for only 50-55% of TG secretion, the remaining presumably being provided by stored lipids or lipoproteins taken up by liver, 2) most reesterification occurs in extrahepatic tissues, and 3) oxidation and reesterification each contribute about one-half to FFA utilization; FFA oxidation accounts for almost all Lox.