Not Eating Enough for Your Workouts? Here's What to Know About RED-S

A top tennis player and a world-class runner both learned the hard way that underfueling can backfire. Experts explain the warning signs, who’s at risk, and the simple habits that can help.

You may not be aware of it, but your body needs enough fuel to support both your workouts and its everyday functions. If you’re consistently not eating enough, it could lead to a condition called relative energy deficiency in sport, or RED-S.

Tennis star Emma Navarro and long-distance runner Jake Smith have both opened up about dealing with the condition.

Navarro recently detailed her experience on Instagram, sharing her symptoms such as mood swings, brain fog, frequent headaches, and dizziness. “I had been under-fueling for almost three years,” she wrote. “My body was fighting back any way it knew how.” She has also decided to step away from competition until 2027.

While the condition is more widely recognized in women, Smith was also diagnosed with it after suffering two stress fractures in four months. The runner said he believed for years that being “lighter is faster,” so he sought to lose weight, but that eventually proved detrimental.

So, What is RED-S?

According to Dr. Elizabeth Matzkin, chief of women’s sports medicine at Brigham and Women’s Hospital, RED-S happens when someone isn’t eating enough to meet the demands of their training, or even their body’s basic needs. Generally, the body needs energy not just for workouts, but also to support the bones, hormones, metabolism, immune system, and heart.

This is not to say the condition only affects athletes. Anyone can be at risk, from a half-marathon trainer to someone just looking to lose weight at the gym.

Who’s at Risk?

Women are usually more vulnerable to the condition, especially in sports that emphasize leanness, such as running and gymnastics.

But as in the case of Smith, men are just as vulnerable, but the symptoms are easily missed. For women, changes in their period can be a signal, while for men, it can be lower testosterone, reduced libido, and fatigue.

Signs to Watch For

If you’re an active person who works out regularly, some early signs to watch out for are persistent fatigue, slower recovery after workouts, declining performance, getting sick more often, poor sleep, increased hunger, mood changes, and hair loss.

More serious red flags include irregular or missing periods, pain when putting weight on a limb, and frequent injuries, especially stress fractures.

How to Fuel Well

In order to avoid RED-S, start by monitoring your body and how it feels. According to sports dietitian Amy Stephens, you’re well-fueled if you can sustain your energy throughout the day, mostly feel hungry only around mealtimes, sleep well, and recover well between workouts.

When to See a Doctor

Don’t wait for a stress fracture before seeing a doctor, Matzkin says, noting, “Earlier evaluation is better.” If you’re observing some symptoms, see a doctor or a dietitian.

The good news is that RED-S can be reversed with the right support. Smith has since recovered and is racing better than ever. “I’m heavier, I’m faster, I’m happier,” he said.

As sports medicine physician Dr. Amy West puts it, “Athletes often need more food than they think.” If you think you’re underfueling, take the first step by consulting a health professional