In men with body fat below 10% and women with body fat below 20%, levels of hormones that support muscle growth and tissue repair are consistently lower than in individuals with higher body fat.
Mechanism
Synthesis from 1 study
When body fat gets very low, fat cells make less leptin, which tells the brain to slow down sex hormone production — but in athletes who train hard and eat enough, this suppression might not happen, as shown in 10.1016/j.clnu.2021.02.028 where testosterone went up even as fat went down.
Most probable mechanism
When body fat drops below 10% in men or 20% in women, the fat cells make less of a hormone called leptin, which tells the brain that energy stores are low. This causes the brain to reduce signals that tell the testes or ovaries to make sex hormones like testosterone or estrogen, leading to lower hormone levels. This is supported by studies showing leptin levels drop with low body fat and correlate with reduced reproductive hormone output, though direct evidence in very lean athletes is limited.
Adipose tissue mass decreases below critical thresholds (10% in men, 20% in women), leading to reduced leptin secretion from adipocytes — a hormone that signals energy sufficiency to the brain.
Low leptin levels reduce stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) neurons, decreasing pulsatile GnRH release.
Reduced GnRH signaling lowers pituitary secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which in turn decreases gonadal production of testosterone in men and estrogen in women.
The suppression of anabolic hormones occurs despite resistance to energy deficit from resistance training, as shown in 10.1016/j.clnu.2021.02.028 where testosterone increased despite fat loss, suggesting that leptin-mediated suppression may be overridden by other factors like training status or caloric intake patterns.
Evidence from Studies
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