RED-S — Relative Energy Deficiency in Sport — is not an eating disorder. It’s an energy math problem. When you consistently burn more calories through training than you consume through food, your body starts shutting down non-essential functions to conserve energy. “Non-essential” from a survival perspective includes things that are very essential from a running perspective: bone density, hormone production, immune function, and metabolic rate.

The result is a runner who trains consistently but gets slower, gets injured more often, and can’t figure out why.

What RED-S Looks Like

In women: Irregular or absent menstrual periods (amenorrhea) are the most visible sign. Many female runners treat this as normal — “I run a lot, so it makes sense.” It doesn’t. Amenorrhea in active women is a sign of energy deficiency, not fitness. It’s associated with reduced bone density, increased stress fracture risk, and impaired recovery.[1]

In men: Reduced testosterone, decreased libido, fatigue, and declining performance. These symptoms are less visible and less commonly associated with under-fueling, so they’re often attributed to “overtraining” — which is technically correct, because under-fueling creates the same downstream effects as overtraining.

In both: Recurring stress fractures, slow recovery from illness, persistent fatigue, mood disturbances, declining performance despite consistent training, poor sleep quality, and gastrointestinal issues.

Why Runners Are Vulnerable

Accidental under-fueling. Most RED-S in recreational runners isn’t intentional dieting — it’s simply not eating enough to match training demands. Running 40 miles per week burns approximately 4,000 additional calories beyond basal metabolism. If your eating habits don’t scale with your training, the deficit accumulates.

Intentional restriction for performance. “Race weight” culture encourages runners to lose weight for faster times. For some runners, this creates a caloric deficit that exceeds what the body can sustain alongside training. The initial performance improvement from weight loss can mask the gradual degradation happening underneath.

The lighter-faster trap. Losing weight does improve running economy (fewer kilograms to move per stride). But below a certain threshold, the performance gains from weight loss are overtaken by the performance losses from RED-S — reduced power output, impaired recovery, decreased bone density. The curve goes up, then sharply down.

The Training Connection

RED-S isn’t just a nutrition problem — it’s a training load problem. The same runner eating the same diet can be fine at 30 miles per week and in energy deficit at 50 miles per week. The variable isn’t what they’re eating; it’s how much they’re burning.

This is why RED-S often appears when training volume increases — for a new training cycle, a marathon buildup, or any period of intensified preparation. The runner didn’t change their diet, but their energy expenditure changed.

Warning Signs

  • Recurrent injuries. Especially stress fractures or tendon issues that keep coming back despite doing everything “right” in training.
  • Declining performance. Training consistently but getting slower. RPE is rising for the same paces.
  • Fatigue that rest doesn’t fix. A recovery week doesn’t help. A full week off doesn’t help. The fatigue is metabolic, not muscular.
  • Mood and cognitive changes. Irritability, difficulty concentrating, loss of motivation. Your brain runs on glucose too.
  • For women: absent or irregular periods. This is not a normal response to running. It requires medical attention.

What to Do

Eat more. This is the simplest prescription and often the hardest to implement. Adding 300-500 calories per day (particularly around training sessions) is often sufficient to correct mild energy deficiency. Prioritize carbohydrates and protein around training.

Reduce training volume temporarily. If energy intake can’t increase enough (appetite suppression is a symptom of RED-S), reducing training volume narrows the deficit from the other side.

See a healthcare provider. If you suspect RED-S — particularly if menstrual irregularities or recurring stress fractures are present — medical evaluation is important. Bone density scans, hormone panels, and metabolic testing can quantify the extent of the deficit and guide recovery.

Don’t treat it as a willpower problem. RED-S is a physiological condition, not a discipline failure. Your body is doing exactly what it’s designed to do when energy is insufficient: conserving resources by downregulating expendable functions. The fix is providing more energy, not pushing harder.

Healthcare provider note: If anything in this article conflicts with guidance from your healthcare provider, follow your provider’s advice — they know your situation, we don’t.