What Is RED-S? Doctors Explain the Condition—and Why Women Are at Higher Risk

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. “I had been under-fueling for almost three years,” she wrote. “My body was fighting back any way it knew how.”

But, you don’t need to be an elite-level tennis player to experience the condition. 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.

What Is RED-S?

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.

Anyone, from the elite athlete to someone training for their first half-marathon to the person increasing their lifting sessions in the gym while trying to lose weight, can be at risk of developing RED-S. Remember, the “r” in RED-S stands for relative; so it’s all about not meeting your energy needs, whatever they are.

When underfueling, in the short term, you may experience fatigue, impaired concentration, more frequent illnesses, poor recovery, and irregular periods or none at all, Dr. Matzkin says. When it comes to your sport, training and performance may suffer too. Low energy availability can make it harder to sustain a workout, decrease strength and endurance, or lead to muscle weakness or pain, says Amy West, MD, a sports medicine physician at Northwell Orthopedic Institute.

You’ll also be much more susceptible to injuries, particularly when it comes to your bones. This has impacts on the long term: “Adolescence and young adulthood are critical years for building peak bone mass, so prolonged low energy availability during this period may have consequences later in life,” Dr. Matzkin says. Down the line, that could mean osteoporosis. Meanwhile, missing your period for long periods of time can have impacts on future fertility, Dr. West adds.

Early stages of RED-S can happen pretty quickly, says Amy Stephens, RD, a sports dietitian who specializes in eating disorders. After just one day of underfueling, you’ll feel symptoms of “hanger”; after just a week, your body may stop building muscle and endurance. Then, she says, it could take six to nine months to fully recover.

Why Is RED-S More Common in Women?

RED-S is much more associated with women and girls compared to boys and men. Until 2014, RED-S was referred to as the “female athlete triad,” which highlighted the relationship between undereating, menstrual dysfunction, and decreased bone mineral density. While RED-S can happen to anyone, “female athletes are more vulnerable, especially in sports that emphasize leanness, endurance, aesthetics, or weight categories,” Dr. Matzkin says. (Think: Running, gymnastics, and diving.)

But, that’s not the only reason women may deal with RED-S more than men. There’s also the pressures that women face disproportionately to be thin, says Dr. West, and the fact that women are more likely to have eating disorders.

Signs of RED-S to Watch Out For

RED-S doesn’t happen all at once. Earlier, more subtle signs like fatigue and trouble sleeping pop up when a person has what docs call low energy availability, which occurs when they’re not eating enough to support their activity levels. As someone is continuously in a state of low energy availability, RED-S can occur.

Low energy availability and RED-S can affect almost every system in the body, but these are the areas you may most often see symptoms, according to the doctors.

Earlier signs include:

• Persistent fatigue
• Taking longer to recover after a workout
• Declining performance
• Getting sick more often
• Poor sleep
• Increased hunger
• Mood swings and/or increased anxiety and depression
• Hair loss
• Weight loss

More serious red flags include:

• Irregular or absent periods
• Pain when putting weight on a limb
• Frequent injuries, especially recurrent stress reactions or stress fractures

How to Make Sure You're Eating Enough for Your Activity Level

If RED-S is suddenly on your radar now, don’t worry. You don’t need to completely overhaul your life or your training to avoid it. In fact, there are a few intuitive steps you can take to give yourself a good shot at fueling fully—no calorie tracking necessary. Here’s what you can do.

Check in with your body.

It’s so simple, but occasionally taking stock of how you’re feeling in and out of training can go a long way. The symptoms above are warning signs of not eating enough, but you should also know what a fully-fueled athlete feels like too. A well-fueled athlete has consistent energy all day, mostly only feels hungry around mealtimes, and doesn’t deal with brain fog, Stephens says. Their mood is stable, they recover well between workouts, and they sleep well too. It’s a good idea to check in and reflect on how you’ve been feeling lately—strong and energized and able to “kick” at the end of a workout, or sluggish and hungry, struggling to get in those last few reps.

Plus, if you wanted to dig even deeper, you can also get some more information from an annual blood workup with your doctor. Since RED-S affects so many systems in the body, Stephens says, you might see issues pop up across a range of tests, including your thyroid, white blood cell count, ferritin and iron, vitamin D, or complete blood count. If any of those are out of range, it could be due to underfueling, but they would need to be discussed with your doctor in the context of the rest of your health history and any other testing.

Hit these general guidelines in your meals.

You don’t need to download a macro-tracking app or suddenly start counting calories to avoid RED-S. (For the record: Stephens doesn’t have her athletes do that.) Instead, focus simply on getting three meals per day, with your plate being about one-third protein, one-third carbs, and one-third vegetables each time. Most people don’t eat breakfast, Stephens says, and that’s often one of the quickest culprits when it comes to not having enough food in a day.

Increase energy intake with training demand.

This is another simple reminder, but if you’re someone with a training plan on the horizon, keep in mind that what you’re eating now is not what you’ll be eating in the future, Stephens says. As plans progress and get more and more intense (you’re running longer distances or lifting heavier in the gym or simply adding another day of Solidcore per week), you’ll need to increase the amount of food you eat.

One helpful rule of thumb is that for every hour you spend working out, you should add on an extra 500 to 600 calories to your day, whether that’s with the help of snacks or larger serving sizes at meals, Stephens says. Then, anything in the 2-to-3-hour range will require in-session fueling, like gels on a long run.

When to Go to the Doctor

If you’re noticing any of the above symptoms or suspect that you’re not fueling right, it’s a good idea to see a professional. Earlier signs like fatigue and poor sleep are easy to write off, but checking in and making sure they’re not a result of underfueling can keep low energy availability from developing into a full-on case of RED-S. In addition to setting an appointment with a doc, you can also try increasing your food intake by incorporating more snacks throughout your day or having larger portion sizes at meals. If you’re not already having a carb-rich snack before a workout, try adding that for some extra energy, paired with something high in protein and carbs afterward to support recovery.

Just keep in mind: It’s never too early to bring this up with a doctor or dietitian, whether you want help determining the appropriate calorie and macro goals for your workout demands or for them to evaluate symptoms you’re not so sure about. “Earlier evaluation is better,” Dr. Matzkin says. “We don't want to wait for a stress fracture—this means bone health has already been affected.” Ideally, you’ll talk with a doctor before reaching this point, but two symptoms that should send you to their office ASAP are frequent injuries, especially if you’re having a tough time recovering from them, and period irregularities, Dr. West says.

From there, treating RED-S is all about getting your body back to the right balance of energy intake to energy output and managing any injuries that may have arisen. You’ll likely need to work with a doctor and/or dietitian to help you get enough calories and nutrients and reduce your training demands while you get caught up. In addition to a sports medicine doc and/or dietitian, you may also work with a bone endocrine specialist (if your bone health has been affected) and/or a sports psychologist, Dr. Matzkin says.

Whether you are at risk for RED-S or not, consider this your reminder that fueling is critical for any active person. Being thoughtful about getting enough calories, carbs, and fat for energy, and protein for recovery is your best line of defense against the condition. While everyone’s needs will be different, a rule of thumb, per Dr. West: “Athletes often need more food than they think.”