When it comes to exercise, there’s one thing that’s just as important as the training itself: fueling. Having the right nutrients—and enough calories—to sustain your activity and recover from it are critical for staying healthy, feeling your best, and keeping your body functioning in tip-top shape. After all, fueling your body isn't just about performing well today; it's about being able to keep doing what you love for years to come.
Eating can sometimes feel like an afterthought, especially when you’re busy with training, but consistently underfueling can actually result in a condition called relative energy deficiency in sport, or RED-S for short. And women have a much higher risk of developing RED-S compared to men. In a 2026 study in the Journal of Science and Medicine in Sport, researchers found that 70 percent of high-level female athletes were at risk of developing RED-S.
Top-ranked tennis player Emma Navarro recently revealed that she is one of those athletes dealing with the condition. Earlier this month, she posted on Instagram, detailing her experience, which included extreme mood swings, brain fog, a weakened immune system, constant headaches, and bouts of dizziness. Exercises she could do with ease in the past suddenly felt impossible.
She was also hungry, all the time. Here’s what you need to know about RED-S, including how to spot—and stop—it early, according to experts. Meet the experts: Elizabeth Matzkin, MD, is an orthopedic surgeon and chief of women’s sports medicine at Brigham and Women’s Hospital.
Amy West, MD, is a sports medicine physician at Northwell Orthopedic Institute. Amy Stephens, RD, is a sports dietitian who specializes in eating disorders. There’s a simple explanation behind what causes RED-S: It occurs when someone is not eating enough to meet the demands of their training—or even their body’s basic physiologic needs, says Elizabeth Matzkin, MD, an orthopedic surgeon and chief of women’s sports medicine at Brigham and Women’s Hospital.
Your body needs energy not only to sustain you, say, on a run or in a weightlifting session, but it also needs energy to support bone health, hormone production, metabolism, immune function, and cardiovascular health. When you’re consistently not consuming enough food to support your activity levels, over time, your body won’t have the energy it needs to support those critical functions, leading to a wide range of symptoms that define RED-S—which is more than medical jargon for undereating. It reflects what happens when your body is chronically underfueled and can lead to long-term health issues.
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