Low energy availability harms bone and hormones in both male and female endurance athletes, so lighter off-season training is a reason to scale session fuel, not to restrict

IOC consensus statement naming impaired bone health as one of the best-documented outcomes of Relative Energy Deficiency in Sport (REDs) in both women and men, and noting that low carbohydrate availability specifically raises bone-resorption markers in male endurance athletes — extending the bone-stress-injury risk picture beyond training load alone into fuelling and energy availability.

Review defining energy availability as dietary energy left for physiological function after the cost of exercise; LEA can come from more exercise, less intake or both and disrupts endocrine function. Most evidence is cross-sectional; prospective studies manipulating EA are small and typically last 5 days or less; evidence in males is limited.

Bone injuries were about 4.5-fold more prevalent in amenorrhoeic (13) than eumenorrhoeic (22) women and higher in low-testosterone (10) than normal-testosterone (14) men; athletes flagged higher-risk by RED-S and Triad tools had more career fractures and days lost. Diary-estimated energy availability was difficult and error-prone; hormonal and menstrual markers were more objective. Cross-sectional, elite athletes.

28% had low energy availability by questionnaire and clinical interview; 44% had low lumbar spine bone density; the 10 with chronic LEA had lower testosterone; LEA without prior load-bearing sport was associated with the lowest bone density. Cross-sectional, men only, cyclists not triathletes.

Athletes with menstrual dysfunction (15) spent more hours per day in energy deficit than those with regular cycles (10) despite similar 24-hour energy availability; hours in deficit were associated with lower resting metabolic rate ratio, lower oestradiol and higher cortisol. Small, cross-sectional reanalysis.

20 athletes with suppressed resting metabolic rate spent more hours in a large energy deficit across the day than 11 with normal RMR, despite similar 24-hour energy availability; larger deficits were associated with higher cortisol and a lower testosterone:cortisol ratio. Small, cross-sectional.

29 studies, 51 trials of single 30-120 minute exercise bouts: exercise had a trivial effect on subsequent absolute energy intake, so people generally did not compensate in the following hours and a short-term deficit resulted; responses varied. Mostly non-athletes, single sessions.

Sports Medicine 48(5):1031-1048. Proposes the 'glycogen threshold hypothesis': 'train low, compete high' benefits (augmented cell signalling, gene expression, oxidative enzyme adaptation) are especially apparent when training sessions are deliberately commenced within a specific muscle-glycogen range, not simply from restricting carbohydrate generally.

Joint position statement (ACSM with dietetics organisations) giving guidance on type, amount and timing of food, fluids and supplements across training and competition scenarios, and recommending referral to a registered dietitian for a personalised plan.