Testosterone is the main male sex hormone – it is testosterone that makes the man a man.
Most likely, testosterone is also what makes men more muscular than women and what makes some men (presumably those with higher testosterone levels) naturally more muscular than others (those with less testosterone; be it because of their age, disease or genetic set-up).
As testosterone is the single most important hormone influencing the muscle growth, it is important to understand the many forms of this steroid.
First, we are talking about a steroid hormone and as every other steroid, testosterone is derived from cholesterol. Not enough cholesterol in your diet? Your testosterone levels will likely sink.
In males, the testosterone production is regulated by HPTA (hypothalamus-pituitary-testicles axis) which means that the whole process of testosterone release starts in the brain (hypothalamus is part of the brain responsible for the connection between neural and endocrine systems).
Therefore it shouldn’t surprise anyone that certain mental stimuli can cause significant increase in testosterone levels (like being in presence of “sexually receptive” female – what a beautiful scientific term…). For those quick to draw some consequences – please consider that catabolic glucocorticosteroids are also released in such situations.
The cascade looks like this: Hypothalamus releases the GnRH (Gonadotropin-Releasing Hormone) which stimulates the pituitary gland to release FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone). Those two hormones in turn stimulate the testosterone release in testicles.
More testosterone in the system means more anabolism so it may be a good idea to increase the levels artificially or naturally. The simplest way to go is to inject some testosterone or related substance (this group of substances is called AAS – anabolic androgenic steroids) in the muscle.
Very well, but we just gave the body information that there is too much of testosterone in the blood and that its natural production should be stopped. Athletes using AAS must therefore live with periods of mental and sexual arousal as well as long and painful periods of depressions, loss of libido and impotence. In the worst case the natural testosterone production is not restored at all.
So we may try a different strategy – to boost the natural testosterone production. There are several herbs and foods with testosterone-boosting properties but first one should avoid the ones that actually lower the testosterone levels: all soy products, licorice, spearmint.
Unfortunately, not all testosterone in the blood is freely available where we would like to have it. Much is found tied to SHBG (Sex Hormone Binding Globulin) and part is converted into female sex hormone estradiol by enzyme aromataze.
What else can influence our testosterone levels? Obesity. There is a very, very clear link between excessive fat deposits in the body and lowered testosterone levels. Fat cells produce the enzyme aromataze which, as we mentioned, converts testosterone into estradiol. So not only male sex hormones are down but also female sex hormones are up in obese individuals.
High alcohol consumption, starving and very high protein intake is also associated with lower testosterone levels.
Resistance exercise has long been associated with increases in testosterone and growth hormone but this correlation is not very strong and scientists are not sure about its impact on anabolic processes.
Race, age, sex
Men have testosterone levels about 7 times higher than women and they start to decline after 35-40 years of age by about 1-2% a year.
Most people believe that persons of African origin have higher testosterone levels: African males tend to be more muscular, have deeper voices and are often more impulsive than Caucasians or Asians.
Americans of African descent have also much higher prevalence of prostate cancer, usually attributed to long-term testosterone exposure.
Theoretically, such higher testosterone levels or possibly more responsive androgen receptors in Afro-Americans can be explained by the fact that they are mostly descendants of slaves brought to New World under very hard conditions.
Only the fit were probably sold as slaves and only the fittest probably survived the inhumane conditions of the journey. (http://www.ncbi.nlm.nih.gov/pubmed/21968274 )
Interestingly, various measurements failed to confirm the theory. The 1988 study by Osegbe and Ogubkewe (University of Lagos, Nigeria) concluded that higher prostate cancer prevalence among Africans cannot be related to androgens as Africans have testosterone levels similar to Europeans.
The largest ever analysis of 1637 white Americans, Mexican-American and black American men concluded that there are no significant differences between whites and Mexican-Americans. African-American men had somewhat higher testosterone levels which were attributed to lower marriage rate in this group (as we know, married men have less testosterone than single ones). http://www.ncbi.nlm.nih.gov/pubmed/19639516
Of course, there can be many other and less understood reasons for better athletic performance of Afro-Americans: rate of conversion to DHT, aromatization, AR sensitivity, estrogen levels and many others.
On the other hand, some studies suggest that there actually are significant differences in testosterone levels between various races. They explain the higher prevalence of prostate cancer in some populations by much higher exposure to testosterone in their youth.
In utero testosterone exposure
There is no doubt that the level of testosterone exposure before the birth (in the womb) plays an important role in physical and psychological traits of an individual.
In utero testosterone exposure is different from adult testosterone levels and there may or may not be a link between the two.
People with higher in utero exposure are less prone to some cardiovascular problems, tend to have more masculine appearance and are more assertive (this is also true for females).
The advantages of lower exposure are less studied but there are almost certainly many of them too.
Quick check: the longer the ring finger comparing to the index finger, the higher the testosterone exposure in the womb.
Mechanisms of action
While it is a well established fact that testosterone does influence muscle growth, the mechanisms of this action are poorly understood.
Stimulation of AR receptors (androgen receptors) by both testosterone and DHT (which is a stronger stimulant) is responsible for at least some of the muscle growth. This is important from bodybuilders’ perspective because resistance training is known to increase the AR concentrations in skeletal muscle tissue.
Increases in testosterone are associated with increased production of growth factors and increased peripheral sensitivity to growth hormone and IGF’s (insulin-like growth factors). The opposite is true for estrogens, at least in some animals (http://www.ncbi.nlm.nih.gov/pubmed/21703268 ).
Some of the testosterone anabolic effects can be probably attributed to its anti-glucocorticoid action. Testosterone removes glucocorticoids from their receptors. This is important since glucocorticoids, mostly cortisol, are potent catabolists.