Testosterone decline isn't due to aging (in which case we'd be screwed), argues Massachusetts General Hospital endocrinologist Frances Hayes; it's due to what happens to us as we age – so we've got a big say in the matter. Weight gain is at the top of that list. We're not talking about obesity, which is no one's idea of an aphrodisiac, but the undramatic creep that turns a lithe 160-pound collegian into a 190-pounder 20 years later. You might look fine (in clothes anyway), but the extra weight does a number on your hormones.
In 2012, Hayes coauthored a study that looked at 900 men for one year, with one group reforming their diets and exercising a modest 2.5 hours a week to lose weight, another group taking blood sugar–lowering meds, and a control group doing nothing. The latter two groups saw no changes, but the dieter-exercisers dropped 15 percent of body weight and, correspondingly, their testosterone levels rose 15 percent. The number of men in this group whose T count, according to the standard medical thinking, put them in the red zone was cut almost in half.
An even bigger whammy: When European researchers tracked 2,700 men for four years to see how weight and testosterone levels lined up, "we found that having your BMI increase by, say, 5 percent was equivalent to a decade of aging," says Hayes. Closer to home, she adds, "I've definitely had patients who had success normalizing their testosterone by losing weight without having to go on hormone-replacement therapy."
Extra fat secretes the hormone aromatase, Hayes explains, and it converts some of the androgens in your body into estrogen. That then signals the brain to slow production of more androgens. The fatter we get, the more womanly we become. No surprise to any guy who's pulled off his shirt at the beach and revealed middle-aged man boob.