Journal Title (Medline/Pubmed accepted abbreviation): Nutr. & Metab.
Summary of Background and Research Design
Background:β-Hydroxy-β-methylbutyrate (HMB) is a metabolite of leucine, a branched chain amino acid. There is evidence that HMB can reduce muscle breakdown in response to exercise in addition to other muscle wasting situations. HMB likely increases muscle protein synthesis via the mammalian target of rapamycin (mTOR) biochemical signaling pathway.
Hypothesis: One month of HMB supplementation will increase the volume of skeletal muscle in sedentary rats. Supplementation will also alter expression of proteins such as those in the mTOR and insulin signaling pathways.
Research design: randomized, placebo-controlled
Subjects: 14 male rats divided evenly into the two treatment groups
Treatment: 320 mg HMB/kg body weight was diluted in 1.0 mL of water and administered via gavage once per day for 1 month. The control group underwent the same procedure with pure water.
Experimental protocol: : Rats lived freely for the one month of supplementation. They were euthanized the day after their last dose of HMB. Tissues collected and analyzed included: serum, liver, the extensor digitorum longus muscle (EDL, one of the lower leg muscles), soleus (another lower leg muscle), and retroperitoneal adipose tissue (fat tissue from the abdomen).
Summary of research findings:
- Both groups of rats increased in total body mass, but the difference between groups was not significant (p=0.163).
- After one month, the EDL and soleus muscles were larger in the rats that had HMB supplementation (p=0.022 and p=0.024, respectively).
- HMB supplementation increased fasting insulin levels significantly (1.94 ± 0.85 ng/mL for the control group and 4.75 ± 0.27 ng/mL for the HMB group). Expression of the insulin receptor in both the muscle and the liver was also elevated; the difference was statistically significant only in the liver.
- The ratio of fasting testosterone:corticosterone was significantly higher in the HMB group, which may have contributed to muscle growth.
- HMB supplementation increased the expression of mTOR and increased phosphorylation of p70S6K. Both events suggest activation of the mTOR pathway.
Interpretation of findings/Key practice applications:
HMB supplementation was capable of increasing muscle size in the absence of a mandatory training protocol. It was also interesting to see that mTOR pathway was still activated after a 12 hr fast. This shows that regular HMB consumption can elicit lasting effects on muscular signaling without constantly being in the blood stream.
The doses of HMB that the rats consumed were rather high [320 mg/kg body weight = 24 g HMB/day for a 75 kg (165 lb) human]. The typical dose of HMB in human studies is 3 g/day. Also, the control was only plain water. Choice of a different placebo, such as a roughly isocaloric amount of carbohydrate, might have been appropriate.