Does high-volume endurance training with inadequate energy availability suppress the HPG axis and lower testosterone in men?

Research indicates that high-volume endurance training, when combined with inadequate energy availability, leads to a suppression of the hypothalamic–pituitary–gonadal (HPG) axis and a subsequent decline in testosterone levels in men. This condition, often termed functional hypogonadotropic hypogonadism or the "exercise-hypogonadal male condition" (EHMC), is a core component of Relative Energy Deficiency in Sport (RED-S).

Clinical and effectiveness evidence

Studies of male endurance athletes, including long-distance runners and cyclists, demonstrate a high prevalence of low energy availability (LEA), defined as an intake of less than 30 kcal/kg of lean body mass per day.

• In some cohorts, 47% to 77% of male endurance athletes meet the criteria for LEA.
• Chronic high training volumes in this energy-depleted state are consistently associated with significant reductions in basal free and total testosterone.
• Research shows that these athletes often maintain testosterone levels at the lower end of the clinical range (or below), which can manifest as reduced libido, impaired recovery, and decreased bone mineral density.

Mechanistic explanations

The suppression of testosterone is driven by a neuroendocrine adaptation to conserve energy.

• GnRH Pulsatility: The hypothalamus senses a deficit between energy intake and the energy cost of exercise. This triggers a reduction in the pulsatile release of gonadotropin-releasing hormone (GnRH).
• Pituitary Response: Reduced GnRH leads to diminished secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary.
• Testicular Output: Because LH is the primary signal for Leydig cells in the testes to produce testosterone, this "top-down" suppression results in lower circulating testosterone levels without the compensatory rise in LH typically seen in primary testicular failure.
• Metabolic Shift: This HPG suppression often coincides with other survival-oriented adaptations, such as reduced triiodothyronine (T3) and elevated cortisol.

Bottom line

High-volume endurance training without sufficient caloric intake suppresses the HPG axis, leading to lower testosterone levels and impaired reproductive health. Ensuring energy availability—specifically matching caloric intake to exercise expenditure—is critical for maintaining hormonal balance and metabolic health in male athletes.