Under-Fueling Is the Runner's Silent Tax: RED-S, Explained for Women

The most common serious mistake in women's distance running is not overtraining. It is under-fueling — and because it works quietly, it often goes unnoticed until a stress fracture or a missing period forces the issue.

The condition has a name: Relative Energy Deficiency in Sport (RED-S), driven by low energy availability — not eating enough to cover both your training and the basic business of keeping your body running. Here is why it matters, and why women are hit first and hardest.

What low energy availability actually is

Energy availability is what is left over for your body after exercise burns its share. When that number stays too low — whether from intentional restriction, disordered eating, or simply not eating enough to match hard training — the body quietly starts shutting down "non-essential" systems to protect itself. Bones, hormones, metabolism, and eventually performance all pay the tax.

It is far more common than you would think

This is not a rare, elite-only problem. Across studies, low energy availability affects roughly 40–82% of female endurance runners, with the youngest athletes (18–24) at greatest risk. In elite endurance athletes specifically, Melin and colleagues found 63% had low energy availability, 60% had menstrual dysfunction, and 45% had impaired bone health — despite normal body weight. You cannot spot it from the outside.

What it costs

The consequences are connected, and they are not small:

• Bone. Between a third and three-quarters of affected athletes have low bone density or bone stress injuries. Athletes who have lost their period suffer around 4.5 times the bone-injury rate, per Heikura and colleagues. This is the thread connecting RED-S to our bone density piece.
• Your period. A lost or irregular period is not a badge of fitness or a harmless side effect of training — it is one of the clearest warning lights that you are under-fueled.
• Metabolism. Low energy availability suppresses resting metabolic rate and thyroid hormone — the body turning the thermostat down.
• Performance. The cruel irony: under-fueling to get lighter and faster backfires. In studies, only athletes with normal menstrual function improved over time, while under-fueled athletes lost training days to injury and, in severe cases, ran meaningfully slower.

Why women first

Women are more vulnerable through several sex-specific mechanisms: the menstrual cycle itself is energy-demanding, low energy availability can switch off reproductive function (functional hypothalamic amenorrhea), estrogen loss directly weakens bone, and eating-disorder prevalence is higher. It is not a willpower story — it is physiology.

The warning signs, and how it's screened

Watch for missing or irregular periods, recurrent stress fractures, always feeling cold, stalled progress, poor recovery, and frequent illness. Clinicians use a validated questionnaire, the LEAF-Q, developed by Melin and colleagues, as a first-line screen (about 78% sensitivity, 90% specificity) — but reproductive function is often the more honest marker than any food diary.

What to do about it

The fix is the opposite of dieting: fuel the training you actually do.

• Eat enough. Research points to a target around 45 kcal per kg of fat-free mass per day to protect health.
• Fuel around your runs, not just at meals — especially before and after hard or long efforts.
• Treat a missing period as a signal, not an achievement, and get it checked.
• Get real help. Education alone often is not enough to change behavior — a sports dietitian and physician are the right team, especially if disordered eating is in the picture.

The honest caveats

Measuring energy availability precisely is genuinely hard, which is why clinicians lean on reproductive and hormonal markers. This is not something to self-diagnose or tough out — if several warning signs sound familiar, that is a conversation with a doctor and a dietitian, not a blog. But the core message is simple and safe for everyone: under-fueling is not discipline. It is a tax on the running you love.

Common questions

Is losing my period normal for a runner?
No. It is one of the most reliable signs of under-fueling and is linked to a much higher bone-injury risk. Get it evaluated.

Can I have RED-S at a normal weight?
Yes. In elite runners, low energy availability and its consequences were common despite normal body weight — it is about energy balance, not appearance.

Will eating more make me slower?
The evidence points the other way: under-fueling costs training days and, in severe cases, race performance. Fueling adequately protects both health and speed.

How much should I be eating?
Studies use roughly 45 kcal/kg fat-free mass/day as a health target, but individual needs vary — a sports dietitian can personalize it.

Fueling underpins every stage. Treadly's Seasons guides cover running through a woman's whole life — from your menstrual cycle to perimenopause and menopause.

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Treadly's coaching is built to fuel the work, not ration it — and points you to the science behind why that matters. If you have warning signs, please see a clinician.