For most of the time the medical community has been paying attention to relative energy deficiency in sport, the condition has been understood as a women’s problem. The original framework was the female athlete triad, identified in the 1990s — the cluster of disordered eating, menstrual dysfunction and low bone mineral density that emerged in endurance athletes, gymnasts, dancers and distance runners. The triad became RED-S, and then REDs, as the diagnostic framework expanded to capture a broader range of consequences. But the underlying assumption — that this is something that happens to women — never really shifted.
Dr Stacy Sims, the New Zealand-based exercise physiologist whose work forms the basis of our wider interview series, thinks that assumption has caused real harm to male athletes for two decades.
“Yes, men also suffer from low energy availability,” she told Unfiltered. “But the symptomology is different than women. So far, when we see all the symptomology of REDs, it’s based on female data — which is different. So we’re seeing cardiovascular perturbations, gut perturbations, loss of menstrual cycle, mood disorders. Those happen in men. But they appear in a different fashion.”
The clinical problem is straightforward and consequential. The diagnostic criteria most clinicians use to identify REDs were developed from female case data. Apply those criteria to a male athlete and a substantial proportion of cases will be missed — not because the athletes don’t have the condition, but because the markers being checked are the wrong ones.
Where the diagnostic framework came from
The female athlete triad was a genuinely important clinical observation when it emerged. The pattern — low energy availability driving menstrual dysfunction driving bone density loss — turned out to be common in female athletes across a wide range of sports, and the framework gave clinicians something to look for. It was a step forward.
But the framework was always going to struggle when applied to male athletes, for the simple reason that several of its core diagnostic markers don’t exist in male physiology. There’s no menstrual cycle to lose. The cardiovascular changes that show up in undernourished female athletes look different in men. The mood and behavioural shifts present differently. Apply the framework anyway and you end up looking for symptoms that aren’t there, while missing the ones that are.
When the medical literature expanded the female athlete triad into RED-S — and subsequently REDs — to acknowledge that male athletes were also affected, the diagnostic criteria didn’t get rebuilt from the ground up. They got extended. The female-default symptoms remained the canonical markers; men’s distinct presentation got bolted on as a footnote rather than treated as its own clinical picture. The result is that clinicians who screen male athletes for REDs are often working with a framework that wasn’t designed to catch them.
Why men’s symptoms look completely different
Two of the most striking sex differences in REDs presentation are in mood and cardiovascular markers, and both illustrate why the female-default framework misses male cases.
On mood, the standard female symptom cluster includes anxiety, depression and the kind of low-energy mood disturbance that’s relatively easy to recognise as something is wrong. Male athletes with low energy availability, Sims says, often present completely differently. “The mood disorders are different — they become more aggressive when they become depressed.” Aggression doesn’t read as a symptom of an eating-and-energy disorder; it reads as a personality issue, a coaching problem, a discipline issue. Male athletes whose underlying problem is REDs frequently get treated as having anything else, because the symptom doesn’t look like the textbook description.
On cardiovascular markers, the divergence is even cleaner. In undernourished female athletes, low-density lipoproteins (LDL cholesterol) rise. That’s a well-documented signal in the female REDs literature. In undernourished male athletes, LDL doesn’t necessarily move in the same way — what tends to rise is triglycerides. A clinician running standard bloods on a male athlete and using a female-default REDs screen would look at LDL, find nothing alarming, and miss the triglyceride pattern that’s actually flagging the condition.
The same logic applies across other markers. Gut perturbations exist in both sexes but present differently. The cardiovascular profile diverges in ways that aren’t subtle. The hormonal disruption — the male equivalent of menstrual cycle dysfunction — manifests as suppressed testosterone, but the threshold at which testosterone starts dropping in undernourished men sits at a calorie intake low enough that many male athletes never reach it, even when their training-to-fuelling ratio is genuinely problematic.
Which leads to the underlying physiological reason men get protected, in a way, from the symptoms women develop fastest.
Why women crash at higher calorie intakes than men
The threshold question is one of the most striking findings in modern exercise physiology, and it underpins the entire REDs sex-differences picture.
Sims describes a critical threshold at which low calorie intake starts disrupting the endocrine system. The threshold is set by the kisspeptin neurons in the hypothalamus — a system that’s responsible for regulating puberty, menstruation and broader endocrine function. Women have two areas of receptive kisspeptin neurons; men have one. The reason is that female physiology requires a more sensitive monitoring system because it’s regulating a menstrual cycle that men don’t have to support.
The consequence is that women crash at calorie intakes more than twice as high as the threshold for men. “When women start to drop their calories below 35 calories per kilogram of fat-free mass, the leptin neurons are downregulated,” Sims explains. “We start to see a downregulation of thyroid. Within even four days of this, we see a reduction in luteinising hormone pulse, which then feeds forward into menstrual cycle dysfunction. For men, that threshold when we start to see a little bit of endocrine dysfunction — like a little bit of lower testosterone — is 15 calories per kilogram of fat-free mass.”
The practical implication is sharp. A male athlete eating at a level that would crash a female athlete’s endocrine system within days will, often, look fine on paper. Hormones in range. Bloods unremarkable. Performance holding up. He’s not below his threshold. He’s well above it. The same restriction applied to a female athlete produces hormonal disruption almost immediately, which is why the female framework caught the condition first — the signal was loud, fast and unmistakable.
But this protection isn’t the same as immunity. Male athletes do develop REDs, often at calorie intakes that look generous to anyone applying a female-default mental model. The problem is that the symptoms emerge later in the trajectory, and they emerge as the alternative cluster Sims describes — increased aggression, triglyceride rise, gut symptoms, suppressed testosterone — rather than as the clearly recognisable female pattern.
The evolutionary logic is worth pausing on. “From a biological perspective, in times of low calorie, men would lean up and get fitter and stronger to go fight the beast and try to bring calories home,” Sims says. “But women would get fatter and their menstrual cycle would stop because they needed to store calories and they couldn’t reproduce. Because why would you reproduce when there aren’t enough calories around to feed who was already there?”
That asymmetric calibration — male physiology built to function efficiently under scarcity, female physiology built to protect reproductive capacity by shutting down at the first sign of it — is why the same condition produces dramatically different clinical pictures. It’s not that male and female athletes have different conditions. It’s that they have the same condition, expressing itself through systems that were always designed to behave differently under stress.
The clinical problem
The implication for sports medicine is that the diagnostic criteria need to be substantially rebuilt — or at least substantially supplemented — to catch male cases reliably. A clinician screening a male endurance athlete for low energy availability shouldn’t be looking for the absence of a menstrual cycle, or LDL elevation, or the depressive mood pattern that flags the condition in women. They should be looking for triglyceride elevation, behavioural changes that may include increased aggression rather than depression, suppressed testosterone, gut symptoms, and a careful read of the training-to-fuelling ratio across recent months.
This isn’t theoretical. The male athletes most at risk — endurance runners, cyclists, climbers, weight-class combat sports athletes — are operating in environments where the cultural pressure to lean out is strong and where the existing screening frameworks weren’t designed to catch them. The conversation about RED-S and REDs has been changing slowly to reflect male presentation. The IOC consensus statement on REDs has updated multiple times since 2014. But the lag between the academic literature and what gets screened in real-world sports medicine practice is substantial.
The other dimension is that the REDs framework intersects with the broader question of how women should fuel their training — and with how perimenopause changes the picture for older female athletes, where the underlying hormonal environment is shifting independently of training load.
What this means in practice
For athletes, the practical implication is that the standard self-screening questions for REDs may not be the right ones if you’re male. Loss of menstrual cycle is the canonical female warning sign and there’s no male equivalent that’s anything like as clear. The closest male equivalents — declining performance despite stable training, persistently low libido, mood changes that family and partners are picking up on, gut symptoms, recurring soft-tissue injuries — are all individually unremarkable and only mean something when they cluster.
For coaches, the practical implication is to take aggression and behavioural shifts in male athletes seriously as a possible health signal rather than treating them as character issues to be managed. The athlete who’s becoming snappier, more confrontational, more easily frustrated under training load may be telling you something about his energy availability, not his discipline.
For clinicians, the practical implication is the harder one. The diagnostic frameworks taught in sports medicine training are still female-default in their core markers, and rebuilding the screening protocols around male-specific presentation is the work of a generation. In the meantime, the clinicians working with male athletes who are willing to read the underlying physiology — and willing to take male triglyceride elevation, mood and aggression changes, and suppressed testosterone seriously as potential REDs markers — are the ones most likely to catch the condition before it produces lasting damage.
The condition is the same. The presentation is different. The medical infrastructure to handle that difference is, slowly, being built.
Roar: How to Match Your Food and Fitness to Your Unique Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life and Next Level: Your Guide to Kicking Ass, Feeling Great, and Crushing Goals Through Menopause and Beyond by Dr Stacy Sims (Rodale) are out now. Visit drstacysims.com.
