Relative energy deficiency in sport, or RED-S, occurs when someone does not eat enough to meet the demands of their training or their body's basic needs, according to Elizabeth Matzkin, MD, an orthopedic surgeon and chief of women's sports medicine at Brigham and Women's Hospital. The body requires energy to sustain bone health, hormone production, metabolism, immune function, and cardiovascular health beyond just powering workouts.
A 2026 study in the Journal of Science and Medicine in Sport found that 70 percent of high-level female athletes were at risk of developing RED-S. Women face higher risk than men, particularly in sports that emphasize leanness, endurance, aesthetics, or weight categories like running, gymnastics, and diving. Amy West, MD, a sports medicine physician at Northwell Orthopedic Institute, notes that women also face disproportionate cultural pressures to be thin and are more likely to have eating disorders.
Top-ranked tennis player Emma Navarro recently disclosed on Instagram that she dealt with RED-S after underfueling for nearly three years. She experienced extreme mood swings, brain fog, a weakened immune system, constant headaches, and dizziness. Exercises she once performed easily became impossible.
In the short term, underfueling causes fatigue, impaired concentration, more frequent illnesses, poor recovery, and irregular periods or none at all, Dr. Matzkin says. Training performance may suffer as well. Low energy availability can make it harder to sustain a workout, decrease strength and endurance, or lead to muscle weakness or pain, Dr. West says. Bone injuries become more likely, which matters during adolescence and young adulthood when peak bone mass forms. Prolonged low energy availability during this period may lead to osteoporosis later, Dr. Matzkin says. Missing periods for long periods can affect future fertility, Dr. West adds.
Amy Stephens, RD, a sports dietitian who specializes in eating disorders, says symptoms appear quickly. After just one day of underfueling, you'll feel symptoms of "hanger." After one week, your body may stop building muscle and endurance. Recovery could take six to nine months.
To prevent RED-S, Stephens recommends eating three meals per day with each plate being about one-third protein, one-third carbs, and one-third vegetables. Most people skip breakfast, which Stephens identifies as a quick culprit for underfueling. As training plans intensify, increase the amount of food you eat accordingly. A fully-fueled athlete has consistent energy all day, feels hungry mostly around mealtimes, avoids brain fog, maintains stable mood, recovers well between workouts, and sleeps well, Stephens says.