Beta-hydroxy-beta-methyl butyrate increases fatty acid oxidation by muscle cells

The leucine metabolite beta-hydroxy-beta-methyl butyrate (HMB) enhances the effect of exercise in increasing fat-free muscle mass in humans (Nissen et al., J. Appl. Physiol. 81: 2095-2104, 1996). we examined its effect on fatty acid (FA) utilization by two muscle cell lines, rat heart H9C2 and mouse skeletal C2C12 cells. H9C2 and C2C12 myoblasts were differentiated in culture into myotubes and were exposed for 48 hours to 0, 6 and 12 mM HMB. The cells were then washed and assayed for uptake, lipid-incorporation and oxidation of ³H-oleate. Oxidation was quantitated from tritium released as water after medium treatment with trichloroacetic acid and passage on a Dowexresin. Uptake was estimated from cell-associated tritium and FA incorporation into lipids from chromatography of cell extracts on silica G (hexane:diethyl ether:acetic acid, 80:20:1). HMB treatment did not alter cell protein or cell DNA per dish. Similarly, it did not affect oleate uptake or its incorporation into phospholipids, triglycerides and cholesteryl esters. In contrast, HMB increased the tritium released into the medium by 25 and 30 percents for heart and skeletal muscle cells, respectively (p<0.005). The increase was observed following removal of HMB and may have reflected a change in capacity of FA-oxidative enzymes or in levels of critical cellular substrates or cofactors. This HMB-induced increase may provide the muscle cell with an oxidative advantage during exercise and could account for some of the decrease in muscle fat observed with HMB intake in humans.