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Cutting calories can lower testosterone
For a man above about 20 percent body fat, eating below maintenance and losing fat, ideally with resistance training to preserve or build muscle, improves testosterone, free testosterone, and other hormone markers of vitality and fertility. For a man at 15 percent or less, cutting below maintenance consistently, six or seven days a week, for two or three weeks or more, is very likely to reduce testosterone and free testosterone, which could potentially hurt fertility.
Testosterone has a reference range of roughly 300 up to 900 nanograms per deciliter, or 1,200 in some countries, but that range is only one indication of hormone health. Free testosterone and the testosterone to estrogen ratio matter too, and men ideally don't have testosterone really high and estrogen really low, because estrogen is very important in men.
Why does low body fat lower testosterone?
When energy intake stays below energy use, the hypothalamus reduces its pulses of GnRH, the signal that starts the testosterone cascade. Leptin, a hormone released by fat tissue, falls with body fat and is one of the signals that tells the brain about energy stores. Rossow and colleagues followed natural bodybuilders preparing for a contest in a 2013 study in the International Journal of Sport Nutrition and Exercise Metabolism. Testosterone fell sharply during preparation and recovered afterward.
Why does extra body fat lower testosterone?
Fat tissue contains the enzyme aromatase, which converts testosterone into estradiol, a form of estrogen. More fat tissue means more conversion. Higher estradiol then signals the pituitary to release less LH, the hormone that drives the testes to make testosterone. Insulin resistance and inflammation, both more common with excess body fat, also lower testosterone.
Why is estrogen important in men?
Men make estrogen mainly by converting testosterone with aromatase. In 1994, Eric Smith and colleagues reported in the New England Journal of Medicine a man with a mutation in his estrogen receptor gene. He was very tall, his growth plates had never closed, and his bones were thin. The case showed that estrogen drives bone growth and bone density in men.