In recent years, the popularity of testosterone replacement therapy (TRT) has greatly increased. The consensus believed by many is that testosterone levels begin to decline in men about age 40, dropping 0.2 to 2 percent every year after that. This decline in testosterone, however, varies with individuals. Not all men show drastic drops in their testosterone levels. What often determines the rate of decline of testosterone is general health and negative health behavior. If your health is bad because of various types of illness, you have a far greater chance of also having low testosterone levels. Indeed, some scientists think that the low testosterone levels shown by many men are more of a reflection of their general health status than any particular perturbation in their hormone system. Bad habits that could also induce low testosterone levels in men include not getting enough sleep. If you get less than 7 hours of sleep a night, you have a high likelihood of having low testosterone levels, especially past age 40. Having too much body fat also influences the rate of testosterone production, favoring the conversion of testosterone into estrogen because of increased activity of the aromatase enzyme that resides in body fat and acts to convert testosterone into estrogen. Nutrition also plays a role. If your diet doesn't contain at least 20% fat, you will likely show lower testosterone levels even if you are young. Excessive stress promotes the release of other hormones that interfere with testosterone release, such as cortisol.
Still, for many men testosterone levels inexorably decline for no clear reasons. One theory about this is that the Leydig cells of the testes, where most of the testosterone in a man's body is synthesized, are bombarded for years by oxidants that circulate in the blood. This results in the gradual destruction of the Leydig cells that makes them far less responsive to the effects of luteinizing hormone (LH), a pituitary hormone that travels from the brain to the testes, where it dictates the activity of an enzymatic cascade that results in testosterone production. This relationship between oxidation and testosterone production hasn't been explored much but will be in an upcoming article in Applied Metabolics. Suffice to say now that animal studies show that if you temper the effects of oxidants through supplying antioxidants to the animals, the usual decline in testosterone starting at middle age doesn't happen. Whether this would work for humans has never been explored.
According to medical texts, about 30 to 40 percent of men show low testosterone levels or "Low T" by the time they reach age 40. In my experience, this figure is far higher as I have met or spoken with few men over age 40 who have not shown signs characteristic of declining testosterone levels. These can range from a failure to make any muscular or strength gains in the gym; increased rate of training injuries; depression; lack of . . .