Relative energy deficiency in sport (RED-S), the syndrome of physiological consequences from inadequate caloric intake relative to training expenditure, affects a substantial minority of competitive endurance athletes, particularly women. Hair loss is one of the more visible manifestations alongside menstrual disruption, bone density loss, and immune dysfunction. The connection between training intensity and hair status often goes unrecognised by both athletes and their physicians.

The mechanism involves multiple overlapping pathways. Inadequate energy availability suppresses HPG axis function, reducing estrogen production in women and potentially affecting hair follicle estrogen signalling. Inflammatory cytokines elevate with chronic energy deficiency. Iron status often suffers due to combined inadequate intake and increased loss through sweat and exercise. The cumulative effect is a classic telogen effluvium presentation in athletes who frequently dismiss it as 'normal' for their training volume.

Practical management requires addressing the underlying energy deficiency. Sports dietitian consultation is often essential, many endurance athletes substantially underestimate their actual energy needs. Treating the iron component independently of resolving the energy issue rarely produces sustained hair recovery. For athletes unwilling to substantially increase caloric intake, the hair loss is unlikely to resolve. The conversation about training-related hair loss benefits from framing it as a sign that overall physiological function is compromised, not just a cosmetic concern in isolation.