Energy Availability and RED-S — When Footballers Don’t Eat Enough

Introduction

The image of an athlete who doesn’t eat enough seems paradoxical — sport demands high energy output, and most athletes appear well-fuelled. Yet inadequate energy availability is one of the most common, most damaging, and most under-recognised problems in football. Relative Energy Deficiency in Sport (RED-S) — the clinical syndrome caused by chronic underfuelling relative to training demands — impairs performance, disrupts hormones, damages bone, and compromises immune function, often without the athlete or their support staff recognising the cause.

The Science

Energy availability (EA) is defined as:

EA = Energy Intake − Exercise Energy Expenditure, per kg of fat-free mass (FFM)

It represents the energy left over for all physiological functions after training is accounted for. The threshold for low energy availability is <30 kcal/kg FFM/day. Below this threshold, the body enacts metabolic suppression — down-regulating energy-costly processes including hormonal function, bone remodelling, reproductive function, protein synthesis, and immune response.

RED-S (formerly called the Female Athlete Triad in its limited original formulation) describes the systemic consequences of chronic low energy availability:

Endocrine disruption:

• In females: suppressed LH/FSH → oestrogen deficiency → menstrual irregularity or amenorrhoea
• In males: suppressed testosterone → reduced anabolic signalling, impaired recovery, reduced libido
• Both sexes: elevated cortisol, suppressed IGF-1, disrupted growth hormone pulsatility

Bone health impairment: Low energy availability reduces bone formation (osteoblast activity) and increases bone resorption (osteoclast activity), reducing bone mineral density over months. In female athletes, oestrogen deficiency compounds the effect. The clinical result: stress fractures — particularly tibial and metatarsal stress fractures — are significantly more common in athletes with low EA. A player who has had two metatarsal stress fractures in two years almost certainly has an undiagnosed energy availability problem.

Cardiovascular effects: Chronic low EA reduces cardiac left ventricular mass, impairs endothelial function, and reduces blood flow capacity — all reducing aerobic performance capacity.

Immune impairment: Low EA reduces secretory IgA, natural killer cell activity, and systemic immune competence — increasing infection frequency.

Psychological features: Low EA is associated with increased food preoccupation, elevated anxiety, irritability, and impaired cognitive function. These psychological features can be both cause and consequence of the energy deficit.

What Research Says

Mountjoy et al. (2014) published the IOC consensus statement that established RED-S as the current clinical framework in British Journal of Sports Medicine, formally extending the concept beyond female athletes and beyond the reproductive axis to encompass all health consequences. Their framework identified 9 body systems affected by low energy availability and provided the diagnostic criteria that guide clinical assessment.

Slater et al. (2016) surveyed 109 female recreational exercisers in New Zealand in International Journal of Sport Nutrition and Exercise Metabolism using the LEAF-Q questionnaire, classifying 45% as at risk of low energy availability — showing that low energy availability risk is not confined to elite athletes.

Heikura et al. (2018) studied national- and world-class female and male distance athletes, finding that amenorrhoeic women and men with low testosterone had lower sex hormones and triiodothyronine and roughly 4.5-fold more bone injuries than other athletes — showing that low energy availability has clinically relevant consequences in males as well as females.

Did You Know? In a survey of English Football League professional players, approximately 18% of responding players reported eating patterns consistent with disordered eating — skipping meals, deliberate restriction, or binge-purge behaviours — motivated primarily by concerns about body weight and body composition targets set by coaches. The interaction between performance culture, weight expectations, and player welfare is a poorly managed risk area at many professional clubs. Nutritional surveillance and confidential dietary assessment should be part of routine player health monitoring.

Applied to Football

Recognising and correcting RED-S in football:

1. Monitor for red flags: stress fractures, amenorrhoea, and weight loss combined. Any player who sustains two stress fractures, a female player with irregular or absent periods, or a male player showing unexplained testosterone decline should trigger nutritional assessment.
2. Calculate energy availability when in doubt. 7-day dietary recall + exercise energy expenditure (estimated from GPS data) allows approximate EA calculation. Values below 30 kcal/kg FFM/day require intervention.
3. Review weight and body fat targets. Coaches or clubs who set arbitrary body fat targets without nutritional support are a primary cause of RED-S in athletes. Body composition targets must be accompanied by sufficient energy intake to support both performance and physiological health.
4. Increase carbohydrate and total energy on heavy training days. The most common pattern in football RED-S is inadequate fuelling on high-load days. Players who eat “healthy” small meals consistently across training weeks often fail to match intake to demand on double-session days.
5. Involve a registered sports dietitian. RED-S assessment and treatment requires professional nutritional expertise. It is not manageable through general healthy eating advice.

Key Takeaways

• Energy availability = (Energy Intake − Exercise Energy Expenditure) / FFM; below 30 kcal/kg FFM/day is low
• RED-S affects both male and female athletes; consequences include hormonal disruption, bone loss, and immune suppression
• Stress fractures recurring in the same player are a cardinal RED-S red flag requiring urgent nutritional assessment
• Male RED-S manifests as testosterone suppression, elevated cortisol, and impaired recovery — often undiagnosed
• Body composition targets without nutritional support are a primary driver of RED-S in football academies and professional clubs

References

• Mountjoy, M., Sundgot-Borgen, J., Burke, L., Carter, S., Constantini, N., Lebrun, C., … & Ljungqvist, A. (2014). The IOC consensus statement: beyond the female athlete triad — Relative Energy Deficiency in Sport (RED-S). British Journal of Sports Medicine, 48(7), 491–497.
• Slater, J., McLay-Cooke, R., Brown, R., & Black, K. (2016). Female recreational exercisers at risk for low energy availability. International Journal of Sport Nutrition and Exercise Metabolism, 26(5), 421–427.
• Heikura, I. A., Uusitalo, A. L. T., Stellingwerff, T., Bergland, D., Mero, A. A., & Burke, L. M. (2018). Low energy availability is difficult to assess but outcomes have large impact on bone injury rates in elite distance athletes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 403–411.

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