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.