1. Introduction
In recent years, an enormous growth of interest has been placed on nutritional supplements for enhancing training-induced adaptations. This interest has been motivated by this theory that nutritional supplements may improve performance by increasing anabolic and decreasing catabolic effects, thereby improving body composition and decreasing exercise-induced muscle damage (1-3).
One potential nutrient is the leucine metabolite β-hydroxy β-methylbutryate (HMB)supplementation, that it may result in several ergogenic advantages, including anti-catabolic (4), anabolic (5) and lipolytic impacts (6). HMB a leucine and 2-ketoisocaproic acid metabolite has been suggested to enhance recovery and improve skeletal muscles during high-volume and high-intensity exercises (7-10). It has been reported that HMB increased strength (11), power (12) and lean body mass (5, 6) through muscle breakdown inhibition during exercise and protein synthesis increase after exercise. Recent data suggested that the advantages of HMB supplementation may be attributed to the some mechanisms including; 1) upregulation of IGF-I (Insulin-like growth factor 1) expression in skeletal muscle, 2) stimulating synthesis of protein via the mammalian/mechanistic target of rapamycin complex 1 (mTOR), 3) membrane stabilization of muscle cells, 4) decreasing proteolysis by inhibition of the ubiquitin-proteasome pathway (8, 13).
Previous studies also have found that supplementation with HMB resulted in beneficial effects on performance by increasing anabolic hormones, including IGF-1, growth hormone and testosterone and decreasing catabolic stress hormones such as cortisol (7, 14). HMB also may increase androgen concentration through increasing cholesterol, drawing on this hypothesis that, HMB is metabolized to β -hydroxy β -methylglutaryl CoA (HMG-CoA) which is a main source of cholesterol synthesis and precursor of androgens such as testosterone (15).
While several studies have supported the HMB beneficial effect in physical activity and clinical condition, some other studies have shown conflicting results about the HMB supplementation effect on testosterone and cortisol concentration in athletes (16-18). For example, Durkalec-Michalski et al. reported an increase in testosterone after 12 weeks supplementation with HMB (3x1g HMB/ day) in 84 highly-trained males (19). However, in another study, testosterone and cortisol did not change in 28 male trained athletes after 6 weeks HMB supplementation (3g/day) (20).
From our point of view, no systematic review and meta-analysis has been conducted over the HMB supplementation effect on testosterone and cortisol in trained athletes. Moreover, due to the contradiction results between studies and common HMB consumption as an ergogenic aid, particularly among athletes, we aimed to conduct this systematic review and meta-analysis to assess the effects of HMB supplementation on testosterone and cortisol in trained athletes.