The rise in testosterone levels during competition predicted aggression in males but not in females.[86] Subjects who interacted with hand guns and an experimental game showed rise in testosterone and aggression.[87] Natural selection might have evolved males to be more sensitive to competitive and status challenge situations and that the interacting roles of testosterone are the essential ingredient for aggressive behaviour in these situations.[88] Testosterone produces aggression by activating subcortical areas in the brain, which may also be inhibited or suppressed by social norms or familial situations while still manifesting in diverse intensities and ways through thoughts, anger, verbal aggression, competition, dominance and physical violence.[89] Testosterone mediates attraction to cruel and violent cues in men by promoting extended viewing of violent stimuli.[90] Testosterone specific structural brain characteristic can predict aggressive behaviour in individuals.[91]
In summary it’s important to know that this topic is still hotly debated, and there are a lot of inconsistencies in the data. We do know that soy contains phytoestrogens and does seem to have a lot of affects on the body, including some studies that show decreased Testosterone levels. For that reason (and the fact that it tastes like ass) I avoid it, and I recommend you also avoid it (in particular soy isolates!) if you’re seeking higher testosterone.
When you’re under stress (be it from lack of sleep, workplace stress, emotional stress, stress from a bad diet, overtraining etc.), your body releases cortisol. Cortisol blunts the effects of testosterone (47), which makes sense from an evolutionary point of view – if we were stressed as cavemen chances are it was a life or death situation – not running late to a meeting - in this state (i.e. running from a lion) the body wouldn’t care if you couldn’t get it up, there was more to worry about!

Testosterone is necessary for normal sperm development. It activates genes in Sertoli cells, which promote differentiation of spermatogonia. It regulates acute HPA (hypothalamic–pituitary–adrenal axis) response under dominance challenge.[22] Androgen including testosterone enhances muscle growth. Testosterone also regulates the population of thromboxane A2 receptors on megakaryocytes and platelets and hence platelet aggregation in humans.[23][24]
Testosterone belongs to a class of male hormones called androgens, which are sometimes called steroids or anabolic steroids. In men, testosterone is produced mainly in the testes, with a small amount made in the adrenal glands. The brain's hypothalamus and pituitary gland control testosterone production. The hypothalamus instructs the pituitary gland on how much testosterone to produce, and the pituitary gland passes the message on to the testes. These communications happen through chemicals and hormones in the bloodstream.
To get a good dietary source of selenium, you can eat shellfish or Brazilian nuts. 1-2 Brazilian nuts is enough to get 200% of your daily intake. You don’t need any more than that. Read more about increasing testosterone with Brazilian nuts here. It’s a good idea to get your selenium levels checked before hand and then adjust your diet as needed to avoid selenium toxicity.
Autopsy studies have found histological prostate cancer to be very common, with one series showing a prevalence of greater than fifty percent in men over age sixty (Holund 1980). The majority of histological cancers go undetected so that the clinical incidence of the disease is much lower, but it is still the most prevalent non-skin cancer in men (Jemal et al 2003). Prostate cancer is also unusual in comparison to other adult cancers in that the majority of those with the disease will die of other causes. Treatment of prostate cancer with androgen deprivation is known to be successful and is widely practiced, indicating an important role for testosterone in modifying the behavior of prostate cancer. In view of this, testosterone treatment is absolutely contraindicated in any case of known or suspected prostate cancer. The question of whether testosterone treatment could cause new cases of prostate cancer, or more likely cause progression of undiagnosed histological prostate cancer that would otherwise have remained occult, is an important consideration when treating ageing males with testosterone.
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As already indicated previously, testosterone levels, particularly bioavailable testosterone, fall with advancing age. This decline in testosterone availability may start to occur early in the forth decade but it usually becomes clinically manifest in the 50s and 60s. Although there is continuing debate about the best way to diagnose hypogonadism in the aging male, there appears to be a general consensus that symptomatic men with reduced levels of testosterone should be given a trial of testosterone therapy if there is no contraindication to do so (Bain et al 2007).
The reasons for considering such therapy become evident from the many associations, indicated above, that reduced testosterone has with a variety of both physiological functions (bone metabolism, muscle mass, cognitive function, libido, erectile function) and pathophysiological states (metabolic syndrome, diabetes mellitus, obesity, insulin resistance, autoimmune disease). Although a definitive long-term, large scale placebo-controlled double-blind study of testosterone therapy in the aging male has not yet been carried out, multiple shorter-term trials have suggested improvement by testosterone with a resultant enhancement of muscle mass, bone density, libido, erectile function, mood, motivation and general sense of well-being.
Testosterone levels generally peak during adolescence and early adulthood. As you get older, your testosterone level gradually declines — typically about 1 percent a year after age 30 or 40. It is important to determine in older men if a low testosterone level is simply due to the decline of normal aging or if it is due to a disease (hypogonadism).