The narrative around athlete leanness and performance has a structural problem: the sports where low body weight is most advantageous are often the same sports where low energy intake causes the most physiological damage. RED-S is not about eating disorders — it is about energy accounting, and it affects male athletes nearly as commonly as female ones.
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Energy Availability: The Key Variable
RED-S is defined by low energy availability (LEA) — the energy remaining for physiological function after the cost of exercise training is subtracted from total energy intake:
Energy Availability (EA) = Energy Intake (kcal) − Exercise Energy Expenditure (kcal) / Fat-Free Mass (kg)
Target EA for optimal function: ≥45 kcal/kg FFM/day. The threshold below which significant hormonal and metabolic disruption occurs: <30 kcal/kg FFM/day.
A 60 kg female athlete with 20% body fat (48 kg FFM) who consumes 2,000 kcal and trains for 90 minutes at 500 kcal expenditure has an EA of: (2,000 − 500) / 48 = 31.25 kcal/kg FFM — just above the clinical threshold, but within the range where subclinical effects accumulate.
This is not a rare edge case. Studies across multiple endurance sports suggest 20–50% of female and 20–30% of male athletes regularly operate below the 45 kcal/kg FFM optimal threshold — most of them without awareness that their energy intake is insufficient relative to their training load.
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The Mountjoy Consensus: Nine Affected Systems
The 2014 Mountjoy et al. consensus statement (updated 2018) — published by the IOC — identified nine physiological systems impaired by chronic LEA:
1. Endocrine: suppressed LH/FSH pulsatility → reduced oestrogen (females) and testosterone (males); elevated cortisol; suppressed IGF-1 2. Bone: reduced osteoblast activity, elevated bone resorption markers, reduced BMD — the pathway to stress fracture 3. Metabolic: reduced resting metabolic rate (down-regulation of thermogenesis and NEAT), T3 suppression 4. Haematological: reduced red blood cell production (iron-deficiency anaemia risk amplified) 5. Growth and development: impaired linear growth in adolescents; delayed puberty 6. Gastrointestinal: slowed gastric motility, bloating, constipation 7. Immunological: increased URTI frequency, impaired wound healing 8. Psychological: mood disturbance, reduced executive function, impaired concentration 9. Cardiovascular: bradycardia, reduced cardiac output, orthostatic hypotension
The performance consequences — frequently the last thing practitioners assess — include: reduced strength, reduced endurance, impaired coordination, reduced training adaptability, and increased injury risk.
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RED-S in Male Athletes: The Under-Recognised Presentation
RED-S was previously framed as the "Female Athlete Triad" (low energy availability, menstrual dysfunction, low BMD). The Mountjoy consensus explicitly expanded this to both sexes — a recognition that the hormonal and metabolic consequences of LEA are not sex-specific.
In male athletes, the presentation differs:
- Testosterone suppression (not oestrogen loss) is the primary hormonal consequence
- No equivalent of amenorrhoea as a visible clinical marker — LEA in males is harder to detect without blood testing
- Libido decline and mood changes are often the earliest subjective symptoms
- Stress fracture in bone occurs in males at lower deficits than in females, because males lack the baseline BMD buffer from higher peak bone mass at entry
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The Protein Interaction
Low energy availability compounds protein utilisation problems: in a hypocaloric state, dietary protein is diverted from anabolic use (muscle protein synthesis) toward gluconeogenesis (energy production). This means an athlete in LEA requires *higher* absolute protein intake — not lower — to maintain muscle mass, because the protein efficiency ratio is reduced.
The practical implication: athletes who restrict calories to achieve body composition targets while simultaneously increasing training volume are creating a double bind — reduced energy availability suppresses MPS via hormonal pathways, and the protein that is consumed is partially burned as fuel rather than used for tissue repair.
For athletes calculating protein requirements across different energy availability states — including during controlled cutting phases where EA must stay above the 30 kcal/kg FFM threshold — the protein intake calculator at winsport.uk/tools/nutrition/protein-calculator provides weight-adjusted protein targets by goal. During a deficit phase, targeting the upper end of the recommended range (2.2–2.4g/kg) helps compensate for the reduced protein efficiency that accompanies caloric restriction.
Do you currently calculate energy availability — not just caloric deficit — when your athletes report unexplained fatigue, illness frequency, or stalled adaptation?