4. Discussion
The efficacy of HMB has not been distinctly established regarding hormonal responses; thus, this systematic review and meta-analysis provided comprehension into the potential benefits conferred by HMB supplementation to help athletes to make aware decisions on its usage and impact. In this systematic review and meta-analysis of 7 studies (235 participants), we found that HMB consumption led to a significant decrease in cortisol and increase in testosterone concentrations after resistance exercises and aerobic and anaerobic combined activities, respectively. Several probable mechanisms have been suggested for the beneficial effects of HMB supplementation on testosterone and cortisol concentration in athletes.
Our results support those of past investigations showing that amino acid consumption may play a role in reducing exercise-induced increases in cortisol (33). Cortisol, is a catabolic hormone and basic glucocorticoid form in humans, secreted from the adrenal cortex in response to psychological and physical stress (34). During cortisol elevates in exercise, impacts of this hormone happen after exercise within the early recovery. Elevation in plasma cortisol concentration, within the physiological range in short-term, motivates proteolysis and augments plasma leucine utilization and concentration (35). The study of Knitter (36) showed that HMB operated as an anti-catabolic agent rather than an anabolic agent. The mechanism for this reaction is not exactly obvious. Some studies support the anti-catabolic hypothesis of HMB expressed by reductions in the muscle damage indices such as creatine phosphokinase and lactate dehydrogenase (15, 37) and subsequent lower cortisol levels. The major characteristics of skeletal muscle damage without cell necrosis is the muscle fibers disruption, exclusively the basal lamina sheath. Regarding mechanical stimuli, especially in resistance exercise, it is known that it can boost micro damage in muscle fibers imposed by contractions or overload and, depending on the volume, duration and intensity of training, the degree and severity of damage and discomfort may be combined over time (38, 39). As such, HMB can affect muscle damage and cortisol levels more efficiently after resistance exercise. However, more studies should be conducted to reveal physiological and molecular mechanisms by which muscle damage markers were decreased after HMB supplementation.
Moreover, cortisol shows an anticipatory reaction to exercise and physical activity (40), and Kraemer et al. previously suggested that this reaction was reduced using a herbal supplement with specifically designed (41). Because amino acids supplementation increases the circulating amino acid concentrations (42), possibly less cortisol was needed for gluconeogenesis before exercise to break down proteins into amino acids.
Reduced cortisol at rest enhances the anabolic-to-catabolic hormone ratio, that in theory, would increase testosterone chronically and muscle tissue protein balance (12). Also, the primitive destination of HMB is a conversion to HMG-CoA in the cytosol of liver and muscle, thus HMB is a precursor of cholesterol (43, 44), and HMB supplementation leads to an increase in synthesis of cholesterol and may act as a structural component of cell membranes (45). Given that damaged muscle cells after exercise (46) may lose their ability to make sufficient HMG-CoA for the synthesis of cholesterol, HMB supplement for more than 6 weeks may be a suitable source of HMG-CoA to synthesize testosterone (15, 47). The increment in testosterone concentration seems to be linked with an increase in the endogenous cholesterol that is a substrate in the steroidogenesis process. Additionally, our results in regard to the HMB and aerobic and anaerobic exercise effects on changes in the testosterone response propose that acute ingestion does not increase the testosterone response to exercise. Townsend et al. study carried out 3 exercise protocols throughout the 12 weeks to assay the supplementation acute response. At supplementation of weeks 6 and 9, the response of testosterone was increased in the intervention group (24). Therefore, chronic supplementation of HMB may lead to a more pronounced response of testosterone. The increase of testosterone concentration could be considered as a beneficial result and demonstrate a better anabolic status in athletes, which plays a key roles in training adaptation and recovery in well-trained athletes (48).
To the best of our knowledge, this study is the first systematic review and meta-analysis that attempt to summarize the role of HMB supplementation on hormonal changes (cortisol and testosterone) in athletes. All included studies administered the same doses of HMB for intervention and all participants were male (except in one study (49)) that resulted in a decrement in heterogeneity. Moreover, the absence of heterogeneity among the included studies enhanced the power of our outcomes. However, some limitations should be mentioned. Considering the above items, it is not possible to evaluate the effect of the HMB consumption in female athletes and dose-response relationship between HMB supplementation and hormonal changes. Lack of information about data on frequency and intensity of exercise, genetic background, lifestyle factors and lack of complete baseline cortisol and testosterone data for subgroup analysis make overall interpretation of the results difficult. Eventually, the type of exercise was varied among the included studies, which could affect the reported outcomes.
In conclusion, the current systematic review and meta-analysis outcomes revealed that HMB supplementation may be linked with a decrease in cortisol concentration after resistance exercise. In contrast to cortisol, testosterone concentration significantly increased in aerobic and anaerobic combined training. more investigations are recommended with a variety of doses and exercises, longer time in periods and in both sexes in athletes.
Conflicts of interest :
None
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