Effects of leucine and its metabolite β‐hydroxy‐β‐methylbutyrate on human skeletal muscle protein metabolism

Key points The branched‐chain amino acid (BCAA) leucine acts as both a ‘trigger’ for the initiation of protein synthesis, and as a substrate for newly synthesized protein. As a BCAA, leucine can be metabolized within skeletal muscle, leaving open the possibility that leucine metabolites might possess anabolic properties. One metabolite in particular, β‐hydroxy‐β‐methylbutyrate (HMB), has shown positive effects on lean body mass and strength following exercise, and in disease‐related muscle wasting, yet its impact on acute human muscle protein turnover is undefined. We report here that HMB stimulates muscle protein synthesis to a similar extent to leucine. HMB was also found to decrease muscle protein breakdown. Our observation that HMB enhances muscle protein anabolism may partly (or wholly) underlie its pre‐defined anabolic/anti‐catabolic supplemental efficacy in humans. Abstract Maintenance of skeletal muscle mass is contingent upon the dynamic equilibrium (fasted losses–fed gains) in protein turnover. Of all nutrients, the single amino acid leucine (Leu) possesses the most marked anabolic characteristics in acting as a trigger element for the initiation of protein synthesis. While the mechanisms by which Leu is ‘sensed’ have been the subject of great scrutiny, as a branched‐chain amino acid, Leu can be catabolized within muscle, thus posing the possibility that metabolites of Leu could be involved in mediating the anabolic effect(s) of Leu. Our objective was to measure muscle protein anabolism in response to Leu and its metabolite HMB. Using [1,2‐ 13 C 2 ]Leu and [ 2 H 5 ]phenylalanine tracers, and GC‐MS/GC‐C‐IRMS we studied the effect of HMB or Leu alone on MPS (by tracer incorporation into myofibrils), and for HMB we also measured muscle proteolysis (by arteriovenous (A–V) dilution). Orally consumed 3.42 g free‐acid (FA‐HMB) HMB (providing 2.42 g of pure HMB) exhibited rapid bioavailability in plasma and muscle and, similarly to 3.42 g Leu, stimulated muscle protein synthesis (MPS; HMB +70% vs . Leu +110%). While HMB and Leu both increased anabolic signalling (mechanistic target of rapamycin; mTOR), this was more pronounced with Leu (i.e. p70S6K1 signalling ≤90 min vs . ≤30 min for HMB). HMB consumption also attenuated muscle protein breakdown (MPB; −57%) in an insulin‐independent manner. We conclude that exogenous HMB induces acute muscle anabolism (increased MPS and reduced MPB) albeit perhaps via distinct, and/or additional mechanism(s) to Leu.

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