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Testosterone in the body is secreted in a small amount by the adrenal cortex but primarily by the testicular Leydig cells. This testosterone circulates in the body in a couple of forms - bound or free. That is, your body contains both types. The Mayo Clinic defines the differences, pointing out that the majority of circulating testosterone in the body is bound to sex hormone-binding globulin (SHBG), and for men is also termed testosterone-binding globulin. As it circulates in the body, most testosterone join tightly to escort molecules and are defined as "bound". A smaller amount of testosterone is albumin bound and an even smaller portion exists as free hormone, which moves about in the body unescorted. Percentage wise, that works out to in this manner for men - roughly 44% is bound to Sex Hormone Binding Globulin (SHBG), with 50% bound to albumin and 2-5% "'free" in circulation.
Originally it was thought that the unbound, or free testosterone (which only makes up approximately 4 percent of the testosterone in the body) of was the biologically active form of testosterone. Mayo notes that "testosterone is weakly bound to serum albumin and dissociates freely in the capillary bed, thereby becoming readily available for tissue uptake. All non-SHBG-bound testosterone is therefore considered bioavailable."
Life Labs Medical Laboratory Services notes: "studies have shown consistent reductions in bioavailable testosterone of up to 50% in elderly males. In some patients, pathologically low levels have been seen in association with declining libido and poor sexual performance. Diminished levels may also be a contributing factor to symptoms of andropause, the male equivalent of menopause. Recent studies have also associated decreased levels with depression in the elderly as well as osteoporosis."
Clearly you want that bioavailable testosterone circulating in your body. The decreases noted in these studies cited by Life Labs, affect not only things such as libido, mental health and bone structure but also the muscle system. The translation is this - you want higher bioavailable testosterone in the body as lower levels causes all kinds of mischief to the male body (and mind). The good news is the recent discovery that all non-SHBG-bound testosterone is bioavailable.
Factors such as excess stress (some stress is good on occasion) and excessive alcohol intake can suppress bioavailability of testosterone in your body.
Staying lean and mean is also a plus factor for keeping the testosterone available and working hard. When you let too much body fat accumulate these higher body fat levels leads to more estrogenic activity in the body, the opposite of testosterone activity.
For more information go to Natural Testosterone Supplement for Men