It doesn't look like what you'd expect
She's fit. She trains regularly. She watches what she eats. And she hasn't had a regular period in eight months.
RED-S — Relative Energy Deficiency in Sport — is the condition that develops when the body's energy intake is chronically insufficient to cover both basic metabolic needs and training demands. It was defined by the International Olympic Committee in 2014, replacing the older "Female Athlete Triad" framing because that term was too narrow. This isn't just elite athletes. It's recreational runners who increased their mileage. It's gym-goers who added two more sessions without increasing food intake. It's anyone whose calorie-out now substantially exceeds what's going in.
What actually happens in the body
When the body senses an energy deficit, it doesn't just let performance decline. It prioritizes survival. Hormonal systems involved in reproduction are shut down first — they're metabolically expensive and non-essential for immediate survival. LH and FSH levels drop. Estrogen falls. Ovulation stops, and periods become irregular or absent. This is not the body being dramatic. This is a coordinated, protective response.
The skeletal consequences follow from the estrogen loss: bone mineral density decreases, stress fracture risk increases. The cardiovascular system adapts by lowering resting heart rate and blood pressure in ways that can look like fitness but are actually adaptation to energy scarcity. Immune function drops. Thyroid hormones shift toward conservation. And psychologically, irritability, depression, and cognitive impairment often develop — sometimes before the physical signs become obvious.
A 2024 comprehensive review in Endocrine Reviews documented the full endocrine manifestations of RED-S, confirming multi-system suppression beginning with the hypothalamic-pituitary-gonadal axis and extending to thyroid function, growth hormone, and cortisol dysregulation. The IOC 2014 consensus statement that established the RED-S framework explicitly replaced the Female Athlete Triad because REDs more accurately captures the non-reproductive health consequences and applies to both sexes. For women, the menstrual disruption component remains the most visible clinical marker — and one of the most consistently ignored by coaches and non-specialist clinicians.
One particularly important nuance: you can have RED-S without an eating disorder. Some women develop it through well-intentioned training increases without matching food intake. Others fall into it through gradual restriction that never reaches clinical thresholds for an eating disorder but still chronically under-fuels exercise demands. The condition doesn't care why the gap exists — only that it does.
Warning signs that often get missed
Irregular or absent periods in an active woman are the most common presentation — and the most commonly dismissed. "It's just your training" is not an appropriate clinical response. Amenorrhea in an active woman warrants evaluation for RED-S, not reassurance.
Other signals: persistent fatigue that doesn't improve with rest, recurrent stress fractures or slow healing, frequent illness suggesting immune suppression, unusual cold sensitivity, declining performance despite consistent training, and mood changes including irritability or depression. Hair loss is also a commonly missed sign — caloric restriction affects the hair growth cycle in a similar way to thyroid dysfunction, and the two can be confused.
What to tell your doctor
- If you're training regularly and your period has become irregular or stopped, tell your doctor directly — and ask specifically about RED-S as a possible cause, not just thyroid or other hormonal causes.
- Request a DEXA scan if you've had more than one stress fracture or have been amenorrheic for over six months — bone density loss can be significant and long-lasting if not addressed early.
- Ask for a referral to a sports dietitian who is familiar with RED-S — recovery requires increasing energy availability, which is nuanced and different from general nutrition advice.
- Be honest about your training load and calorie intake. The clinical picture of RED-S is often missed when athletes underreport training and overreport eating — something that happens from embarrassment, not deception.
If you're concerned about RED-S, ask your primary care doctor for a referral to a sports medicine physician or a specialist in athlete health. Recovery typically requires reducing training volume, increasing caloric intake under dietitian guidance, and close monitoring of bone density and hormonal markers. This is not a condition to manage alone.
Sources
- Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687-697.
- Dipla K, et al. Relative Energy Deficiency in Sport (REDs): Endocrine Manifestations, Pathophysiology and Treatments. Endocr Rev. 2024;45(5):676-708. doi:10.1093/edrv/bnad034.
- Statuta SM. Relative Energy Deficiency in Sport (RED-S) in the Active Female. Am Fam Physician. 2022;106(1):12-13.
- Logue D, et al. Low energy availability in athletes: A review of prevalence, dietary patterns, physiological health, and sports performance. Sports Med. 2018;48(1):73-96.
- De Souza MJ, et al. Misunderstanding the female athlete triad. Br J Sports Med. 2014;48(7):563-4.