Low Energy Availability and REDs: The Underdiagnosed One

Not eating enough for your training load is a syndrome with a name, a consensus statement and a long list of consequences.

What this is

Low energy availability means not enough energy left over for normal physiological function once training has taken its share. The IOC's 2023 consensus statement describes the resulting syndrome — Relative Energy Deficiency in Sport, REDs — as a spectrum from adaptable to problematic, with consequences across energy metabolism, reproductive function, musculoskeletal health, immunity and cardiovascular health.

It doesn't require an eating disorder, and it doesn't require being underweight. It commonly arrives by accident: training volume goes up, appetite lags, and intake quietly fails to follow.

What the evidence says

  • Low energy availability causes measurable physiological harm

    Gold· human data

    The IOC consensus documents effects on reproductive function, bone, metabolic rate, immunity, glycogen synthesis and cardiovascular and haematological health. This is a well-characterised syndrome, not a training fad.

  • Missing periods in an athlete is a red flag, not a badge

    Gold· human data

    Amenorrhoea in this context signals suppressed reproductive function and predicts bone consequences. The old idea that it's a normal sign of leanness or hard training is wrong and has cost people their bone density.

  • It affects men, and is under-recognised there

    Gold· human data

    The 2023 statement explicitly covers male athletes. Without menstrual disruption as a signal, presentation is often via poor recovery, low libido, frequent illness or stalled performance.

  • Recovery is possible with increased energy availability

    Silver· human data

    Restoring intake and reducing load restores function for many, though bone density lost during the period of deficiency may not fully recover — which is the argument for catching it early.

  • "Leaning out" always improves performance

    Not supported

    Beyond a point it reliably degrades it, via exactly the mechanisms above. Many athletes get slower on the way to a target weight.

What this means in practice

  • Match intake to training load, including on rest days — those are when adaptation happens.
  • Treat any of these as a reason to seek help: lost periods, repeated stress fractures, frequent illness, persistent fatigue, stalled progress despite training more.
  • Fuel around sessions rather than training fasted by default if you're already training a lot.
  • Use our TDEE calculator as a starting estimate for what a heavy training week actually costs — most people under-count it.
  • Get bone health assessed if you've had amenorrhoea or a stress fracture.

When to see a doctor

  • Periods that have stopped or become irregular during a training block.
  • A stress fracture, or bone pain that worsens with impact.
  • Recurrent infections, persistent fatigue, or performance declining as training increases.
  • Anxiety about food, rigid rules, or compensating for eating with exercise — seek specialist support.

The honest summary

Under-fuelling is the most consequential and least glamorous problem in endurance and physique sport. Lost periods are the clearest early signal and the one most often celebrated instead of investigated.

Work it out

See also

Frequently asked questions

Is it normal to lose your period when training hard?
Common, yes. Normal, no. It signals suppressed reproductive function with downstream consequences for bone, and it's one of the clearest signs of low energy availability. It warrants medical assessment.
Can men get REDs?
Yes, and the 2023 IOC consensus statement covers male athletes explicitly. It's recognised later in men because there's no menstrual signal — it usually shows as poor recovery, illness, low libido or stalled performance.
How do I know if I'm under-fuelling?
The signals are functional rather than numerical: recovery, illness frequency, menstrual function, bone injuries, mood, libido and whether performance improves when training increases. A sports dietitian can assess it properly.

Sources

Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.

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