What happens when athletes chronically under-eat?
Eat too little for your training load long enough and your body starts closing accounts: bone density, sex hormones, immunity and performance all degrade. It is called RED-S, it is the predictable failure mode of aggressive dieting, and it happens to men too.
The number that matters isn’t calories eaten, it’s energy availability — what’s left to run your body on after training takes its cut, scaled to fat-free mass. In controlled feeding studies, the pulsing of luteinizing hormone that drives the menstrual cycle held steady at roughly 30 kcal per kg of fat-free mass per day and came apart below it. Five days was enough to see the disruption.
Sustain that for months and the damage compounds. Sex hormones fall, cycles get irregular or stop, and bone stops being maintained — athletes lose density in exactly the years they should be banking it, and stress fractures follow. A 2025 meta-analysis pooling 59 studies and about 6,000 athletes found roughly 45% were running low energy availability, with worse endurance and explosive power, blunted response to training, more illness-related missed sessions, and more bone stress injuries.
The original framing was the female athlete triad: low energy availability, menstrual dysfunction, low bone density. The IOC widened it to Relative Energy Deficiency in Sport because neither the symptom list nor the sex restriction held up. Among 813 male athletes, low-availability markers tracked with cardiovascular and psychological problems, poorer training response, and reduced endurance — and that group’s average BMI was higher than the well-fueled group’s. Lean is not the tell.
Where it gets shakier
Energy availability is genuinely hard to measure. Food diaries underreport badly, exercise expenditure is estimated, and body composition carries its own error. Most of the syndrome evidence is cross-sectional and self-reported, and the symptoms are nonspecific — sleep debt, training load, and ordinary life stress produce a similar picture. A 2024 critique argued that RED-S gets diagnosed circularly, since the symptoms used to identify it have other plausible causes. That argument is about the model’s breadth, not about whether underfueling hurts.
The takeaway
This is where aggressive cutting advice ends up if you run it long enough. Watch for periods that stall or stop, dropped libido, recurring minor illness, nagging bone pain, sleep that quit working, and training that stopped improving despite more work. Those are signals to eat more, not to push harder.