Relative Energy Deficiency in Sport (REDs) is startlingly common and often unintentional. Last year I trained for my first ultramarathon and underestimated how much I’d need to change my nutrition to survive 31 trail miles, not just my training. I learned the hard way that intense activity — even for weekend athletes — requires real fuel, in a culture that thinks the point of exercise is to burn calories.
Relative Energy Deficiency in Sport (REDs) can affect athletes of any level, age, or gender, and most people at risk have never heard of it. It was first identified as the “female athlete triad” (low energy availability, amenorrhea, low bone density) — a recent diagnosis in medical history partly because women were excluded from organized sports until Title IX in 1972, and sports science still skews male. We now know underfueling affects everyone.
One study of non-elite Boston Marathon runners found 42.5% of women and 17.6% of men showed signs of low energy availability — and those athletes were slower and more injury-prone. Sometimes underfueling is intentional (“lighter is better” dogma in endurance sports); often it’s just a mistake, especially as training volume increases without a corresponding increase in food. Popular diets like keto and intermittent fasting make it easy to tip into deficit without noticing.
Left unaddressed, REDs disrupts hormones, bone density, mental health, and cardiovascular health — and it’s much harder to reverse than to prevent. Watch for stress fractures, missing periods, profound fatigue, underperformance, or unexplained frequent illness. When I finally worked with a coach who taught me to fuel properly, I immediately ran faster, longer, and felt better — it felt like magic. If you coach, parent, or teach young athletes, how you talk about food and bodies is where this starts.