3.1. Search outcomes and overview of included studies
Our initial search led to 184 relevant studies. After removing duplications, a wide range of screening of the titles and abstracts, the careful assessment was performed on 142 related articles. Of these, 11 articles remained after considering the inclusion and exclusion criteria for the eligibility. Eventually, 7 articles, including, 8 effect sizes, which studied a total of 235 participants aged 16.1 –24.9 years, were identified in the current systematic review and meta-analysis.Figure 1 represents the selection process and reasons for excluding the studies. The data in Table 1 illustrates the main characteristics of the articles in our systematic review and meta-analysis.
In brief, the studies (by 235 participants) were published between 2001 and 2017. The duration of these RCTs ranged between 1.5 to 12 weeks. 2 studies used a randomized crossover design (19, 29), one of them was also single blind (30) and the other studies had the design of double-blind. Two studies were conducted in Poland (19, 29), 1 in the United States (30), 1 in Iran (31), 2 in Australia (20, 32) and 1 in Israel (7). The effect of exercise on testosterone and cortisol concentration was examined in 7 studies (7, 19, 20, 29-32); and 8 effect sizes were extracted from these studies for testosterone and cortisol concentration. The total number of subjects who completed the studies in inclusion criteria was 121 subjects in the intervention and 107 in the placebo groups. The dose of HMB supplementation was 3 g/day in all studies.