About age 30 to 35, most men (and some women) notice they are gaining weight around the middle. Their pants become tight and at some point no longer fit. The words "pot belly," "beer belly," or "spare tire" are sometimes used to describe the medical condition called "abdominal obesity." This sort of fat accumulation greatly increases the risk of cardiovascular and other diseases.

Low testosterone = abdominal fat gain

As it turns out, there is a scientific explanation for the tendency toward abdominal obesity among middle- aged men. As men age, their levels of free testosterone decline, and levels of estrogen and insulin increase. This is partly because aging men convert much of their testosterone into estradiol, a form of estrogen. Of the remaining testosterone, much is bound to sex hormone-binding globulin, a protein in the blood, and is not biologically active. Studies have shown that men with low free testosterone have higher rates of coronary artery disease, mental depression, and dementia (Tan et al 2004).

The idea behind testosterone replacement therapy is to restore the level of free testosterone to that of a healthy 25-year-old to counteract the effects of increased estrogen. Studies have shown that fat cells, particularly abdominal fat cells, convert testosterone to estradiol (Schneider et al 1979; Kley et al 1980; Killinger et al 1987; Khaw et al 1992). The more belly fat a man accumulates, the greater the conversion of his testosterone into estradiol. As long as free testosterone is low and the ratio of estrogen to insulin is high, most aging men will store fat around their belly (Abate 2002).

Clinical studies have shown that testosterone replacement therapy can provide a variety of benefits.

In one study of 86 men aged 50 to 70, waist-to-hip ratio and blood pressure markedly decreased after 60 days of testosterone therapy (Li et al 2002).

Another testosterone-replacement study in middle-aged obese men showed improved waist-to-hip ratio along with a decrease in plasma insulin and an increase in glucose disposal, suggesting improved insulin sensitivity (Marin et al 1992).

In another trial, abdominally obese middle-aged men showed improved glucose control, decreased abdominal body fat, and improved sexual function after testosterone therapy (Boyanov et al 2003).

Given that these studies looked only at testosterone levels, one can only speculate about what the results might have looked like if excess estrogen and insulin had also been suppressed.