MODEL FOR MEASURING ABSOLUTE RATES OF HEPATIC DE-NOVO LIPOGENESIS ANDREESTERIFICATION OF FREE FATTY-ACIDS

We have previously presented a precursor-product stable isotopic techn ique for measuring in vivo the fraction of very low-density lipoprotei n-fatty acids (VLDL-FA) derived from de novo lipogenesis (fractional D NL). Here, we propose a technique for converting fractional DNL into a bsolute rates of DNL and describe its explicit underlying assumptions. The technique combines the fractional DNL method with a modification of the method of S. Klein, V. R. Young, G. L. A. Blackburn, B. R. Bist rian, and R. R. Wolfe (J. Clin. Invest. 78: 928-933, 1986), for estima ting hepatic reesterification of free fatty acids (FFA). Infusions of [1,2,3,4-C-13]palmitate and [1-C-13]acetate are performed concurrently with indirect calorimetry in human subjects. Fractional DNL (based on mass isotopomer distribution analysis of VLDL-FA), the rate of appear ance of plasma FFA (R(a) of FFA), and net fat oxidation in the whole b ody are measured. Equations from the hepatic reesterification model, m odified to include the contribution from DNL, allow calculation of abs olute DNL (= fractional DNL x [R(a) of FFA - net whole body fat oxidat ion], when respiratory quotient < 1.0). Sample results from human subj ects with different dietary energy intakes are presented, with calcula tions of absolute DNL, absolute reesterification, and absolute fat oxi dation rates. The assumptions of this technique (in particular, that a ll fat oxidized is derived at steady state from circulating FFA and th at DNL and reesterification of FFA both occur exclusively in liver) ar e discussed. In summary, by combining a fatty acid turnover-indirect c alorimetric technique (to estimate net oxidation and reesterification rates) with a precursor incorporation method (to measure fractional DN L), absolute rates of fat synthesis, fat oxidation, and fatty acid ree sterification can be quantified in humans. Validation of this approach depends on experimental testing of the assumptions noted.