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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