Age-associated testosterone decline is closely associated with deep abdominal fat (visceral fat), a component of the metabolic syndrome. Fat tissue is an extremely active hormonal modulator, particularly for testosterone and estrogen. An enzyme in fat tissue known as aromatase converts testosterone into estradiol, the major estrogen in humans. Excess aromatase activity decreases testosterone and increases estrogen levels, which can result in a number of deleterious body changes for men. Low serum testosterone concentrations are closely correlated with high body mass index (BMI), along with elevated ratios of body fat to lean mass.