Earlier this year, pro triathlete Lionel Sanders withdrew from Ironman Lake Placid, explaining he is dealing with injuries likely stemming from RED-S, relative energy deficiency in sport. This announcement came as a shock to the triathlete community for a few reasons: RED-S is a relatively under-discussed topic previously known as the "female athlete triad," as main symptoms for diagnosis include loss of periods, or experiencing highly irregular periods and loss of bone density.
Before Sanders opened up about his struggles with RED-S, age-group triathlete Nick Bennett had already shared his own story to help bring more awareness to the issue.
"Lionel Sanders has been huge in this," he says. "It was kind of amazing to see someone that popular come out and admit they're struggling with it."
Bennett, 19, bought himself a ticket for his first Ironman for his 18th birthday last year, which he completed in November 2024 in Florida. But what he didn't know was his head-first dive into the sport would soon have him sidelined with an injury and a diagnosis of RED-S, which is more prevalent in triathletes than many realize. And recent research shows there is still a lack of information about how RED-S can impact young male athletes, making early screening and detection important.
RED-S is a syndrome where athletes experience impaired physiological or psychological functioning caused by severe and/or prolonged low energy availability where there's a mis-match between dietary intake and the amount of energy you're expending, explains Allison Yamamoto, sports nutritionist, CEO and founder of Koa Athletic Health, who herself spent years struggling with RED-S.
"When I first heard what it was, nobody had talked about it, and now I feel like it's one of those things that's extremely common," Bennett says, especially within the endurance community since Sanders spoke up. "There's a lot of people willing to step out and say, 'Yes, I'm suffering with it,' especially male athletes."
Essentially, RED-S is characterized by poor health and declining athletic performance caused by underfueling - when your dietary intake doesn't support the amount of energy your body is expending during training - according to Boston Children's Hospital. It's important to note that RED-S can affect athletes of any level and gender.
As endurance athletes, RED-S can be especially prevalent in triathletes due to the amount of energy output training and competition requires. "When we're not meeting our energy needs, our physiological processes will downregulate," Yamamoto says, which means other non-essential functions in the body slow to conserve energy, resulting in changes such as bone loss (which can lead to stress fractures), reduced immune function, and reduced testosterone in males.
And, prevalence estimates suggest RED-S in endurance athletes is relatively high, but the amount of awareness among both athletes and practitioners, although growing, doesn't match how often it's actually occurring, says Yamamoto. "Oftentimes, athletes aren't appropriately screening for it because they don't know it's something they should be doing or they don't think they're somebody who could be at risk of it," she says.
This underfueling and physiological downregulation caused Bennett injuries which led to a RED-S diagnosis. After a successful year of racing in 2024, Bennett had a full schedule of races for 2025. But that all changed in February of this year, when he sustained an injury to both quads during a half marathon that forced him to drop out of the race.
Ahead of the race, he says that he felt great during his peak week and he was the most fit he's ever felt. During that week, he had a routine blood test done. When the results came back, a day before he left for his race, his doctor told him his testosterone levels were 59. Both he and his doctor didn't think that could be possible based on how he was feeling, so they did a re-draw.
He traveled to the race, and after a messy start, hit his stride. But soon, things took a turn. "My left leg seized up, and I couldn't move it. Then, I turned a corner and my right leg did the same thing," he says. Since he couldn't move, he sat down for a while, then walked back to the start, unable to finish the race. After talking to some people he knew with a medical background, they suspected he might have a tear or strain in his quads. But then his second lab results came back the next day - his testosterone levels were 61 and his creatine kinase (ck) levels were elevated, between 1,200 and 1,500, signaling muscular breakdown. He knew something was wrong.
"Everything was out of whack, so at that point [my family and I] were like, 'OK, we need to find a medical team that knows what they're doing,'" Bennett says. He started working with a team at Boston Children's Hospital, which accepts patients into their early 20s, and while the department he works with is called the Female Athlete Program, one of the specialties is RED-S.
"I saw them and they immediately knew what it was," he says. Additionally, doctors suspected rhabdo and that he tore his quads, both of which were later confirmed. He also did a metabolic test, which showed he was burning 500 more calories than doctors originally suspected when he was at rest, which partially explained why he was at such an energy deficit.
"Looking back now, I'm noticing things over the Ironman training that I'm like, 'I should've picked up on this a long time ago,'" he says. "But it was things that were normalized in training that I see other people going through, and now I'm like, 'You shouldn't be ignoring that.'"
While you can get a RED-S diagnosis from your medical team via labwork, there are also ways you can self-screen which can be helpful when it comes to getting a diagnosis. Stagnation in training, having low libido, having digestive issues, and feeling fatigue or burnout can all be signs you're experiencing RED-S, Yamamoto explained. And while the diagnosis originally was geared toward females, there is now a realization in the medical and sport community this isn't just happening to females. "Because it's management of your training and your nutrition, any athlete can experience RED-S," Yamamoto says.
Additionally, certain psychological factors like hesitation to take a rest day, feeling guilty for taking time for lower-intensity training or rest from training, and adhering to food rules or avoiding certain foods or food groups in general can signal RED-S. "It really is a whole-body impact," she says. Everything from performance to training response to mood and anxiety levels to immune and digestive functioning can be affected by RED-S.
Some things that Bennett now recognizes are signs of RED-S include fatigue and irritability. He says he would train in the mornings and then lie on the couch without energy to do anything else for the rest of the day, or find himself easily frustrated by anything other than his training. He also found himself ignoring injuries and pushing through pain he shouldn't have been, like what he suspects was a broken rib he didn't rest to let heal. He also now realizes that especially during his high-volume weekends, he wasn't meeting nutritional demands his training required, which put him in an energy deficit he couldn't get out of.
"There were multiple small injuries where I'd say, 'Oh, it's just my training volume,' but it was actually a lot more severe issues that shouldn't be happening if you're healthy," he says.
For athletes of any level, the expectation is often that if you're hitting a plateau, you should be pushing harder to get through it, but it's likely stagnation or recurring niggles are early indicators you may be experiencing RED-S. (And for a detailed checklist of what to look for, Jill Colangelo lays it all out in this primer.)
"Really tuning into those symptoms, even if you're an athlete who is capable of 'pushing through' whatever it is you're experiencing, it's better to respond faster than nursing an injury or other health consequences from RED-S," she says.
Previously, Bennett was someone who if the training plan called for 10 miles, he'd push through no matter how he felt. But now he realizes that if something is feeling off, it's better to lower volume or take a rest day than risk injury.
He now works with a team of personal trainers, nutritionists, and psychologists to ensure he is moving forward in the healthiest way possible, and can continue his goals without risking his physical or mental health.
"You need to listen to your body because your training plan doesn't know how you feel," he says.
Following his diagnosis, he was out of exercise for about six weeks, and his testosterone levels bounced back extremely quickly. His team agreed it was time to try exercise again, and then it became about finding the balance between rest and activity. He's also focused on eating more calorie-dense foods to keep up with his energy expenditure.
Severity of RED-S and treatment plans vary widely person to person according to Yamamoto, but nutrition intervention is typically the place treatment starts. To help athletes recover, it's key to invest in restoring that energy availability through nutrition, and also ensure you have an intentional training program that allows for de-loading and diversification. And, more often than not, some sort of psychological support can be very beneficial. Yamamoto suggests finding someone, such as a mentor, coach, or therapist, who can help navigate and work through some of the aspects of the standards and relationships to food and exercise.
For about five months, Bennett was out of exercise, going from 15 to 20 hour training weeks to none, and he had to cancel three races so far this year, which he says also negatively affected his mental health. He's just now getting back to training, with his goal to complete the Ironman 70.3 in North Carolina this October.
Since sharing his story, he's had people reach out to offer support and to say they're glad more people are talking about their own experience with RED-S.
"No one wants to admit their testosterone is 50, but it's something that needs to be talked about, especially with how young the athletes are getting in the sport. ... You have 18-year-olds that want to do Ironmans and it's great," he says. "But it needs to be talked about that it's not the healthiest thing to be doing, and you need to keep an eye on your health because in the end that's the most important thing."