Fuel your training or the gains stall
High confidence
Fuel your training and the body adapts to it; under-fuel it (low energy availability / RED-S) and the training response falters, endurance performance drops, and health and injury risk rise. In a controlled trial, adequately-fuelled runners improved 10k time about 5-6% during intensified training while an under-fuelled group did not, despite similar aerobic gains.
Why it works
Low energy availability suppresses endocrine function (thyroid, sex/steroid hormones), raises stress hormones, and redirects energy from adaptation toward survival; inadequate carbohydrate degrades exercise economy and the capacity to train and race at intensity.
What it means in practice
Pair training plans with nutrition matched to the load; flag chronic under-fueling as a performance and health risk, not a shortcut. Qualitative/mechanistic; avoid quoting one clean percentage as if universal.
The evidence
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The IOC panel ties low energy availability to impairment across multiple systems, bone health among them, alongside menstrual function, metabolic rate, immunity, protein synthesis, and cardiovascular health. It names low energy availability as the upstream driver that links under-fueling to weaker bone and higher stress-injury risk in athletes. This is expert consensus rather than primary data.
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Chronic low energy availability impairs sports performance and the adaptive training response through endocrine effects (suppressed thyroid and sex hormones, elevated stress hormones) and reduced muscle force, as the body redirects energy toward survival functions. This is the RED-S-and-performance angle, not just bone health.
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High-carbohydrate and periodized-carbohydrate groups improved 10km race time (about 6.6% and 5.3%), while the low-carbohydrate/high-fat group showed no performance improvement despite a rise in peak aerobic capacity, because low carbohydrate availability impaired exercise economy at race pace. The finding was independently reproduced in 2020.
n=21
Why we call confidence high
The IOC RED-S consensus (Mountjoy 2018) and a 2020 review (Logue) establish that low energy availability impairs performance and adaptation; Burke 2017 (independently reproduced 2020) is a controlled trial showing adequate/periodized carbohydrate availability delivered a real endurance-performance gain while low-carb negated it.
Where it applies
Endurance athletes and recreational-to-elite distance runners; the RED-S / low-energy-availability evidence spans both sexes.
Does not apply to: deliberate short-term 'train-low' carbohydrate-periodization protocols, which are a targeted advanced tactic, not chronic under-fueling.
Last reviewed Jul 15, 2026. See how we score.