Energy availability — the master variable every runner must understand
Calories in minus calories out is the wrong lens. The right one is EA — what is left over for your body after the run — and it governs everything from bones to hormones to race performance.
Every meaningful decision in runner nutrition — how much to eat, whether to run fasted, whether race weight is helping or hurting — routes through a single variable: energy availability (EA). EA is defined as dietary energy intake minus exercise energy expenditure, divided by fat-free mass, and it measures how much fuel is actually available for all non-exercise physiology: bone remodelling, hormone production, immune function, reproduction, thermoregulation. If the number is too low, these systems are quietly starved.
Anne Loucks' laboratory work (Journal of Clinical Endocrinology & Metabolism, 2003) established that EA below 30 kcal/kg fat-free mass per day suppresses luteinising hormone pulsatility, basal metabolic rate, and bone formation markers within five days in healthy women. The International Olympic Committee's RED-S consensus statements (Mountjoy et al., BJSM, 2014, 2018, 2023) expanded this into the Relative Energy Deficiency in Sport framework — a syndrome that affects both sexes and touches menstrual function, bone density, cardiovascular health, immunity, metabolic rate, psychological health, endurance, coordination, concentration, and injury risk.
The operational target every distance runner should know: 45 kcal per kg of fat-free mass per day as the optimal zone. A 60 kg runner with ~20% body fat has ~48 kg fat-free mass, needing ≥2,160 kcal after training expenditure is subtracted. If that runner burns 800 kcal on a long run, total intake that day needs to clear ~2,960 kcal just to maintain the 45 floor — not a weight-loss calculation, a don't-break-things calculation.
The most common way runners fall below the EA threshold is not dramatic under-eating. It is a gradual mismatch: training volume climbs during a marathon build, intake stays static because "I'm being disciplined," and EA silently drops 35 → 30 → 25. The symptoms creep in over weeks: blunted workouts, stubborn niggles that don't resolve, disturbed sleep, missed periods in female runners, colds that won't shake.
Every other topic in this guide — race-day carb intake, protein distribution, plant-based fuelling, masters nutrition — sits on top of an EA foundation. If EA is broken, the downstream optimisations compound nothing.