I’ve written before about low energy availability — what it is, who’s at risk, and how to start climbing out of it. That post deals with the state itself: not enough fuel left over, after training, to run the body properly.
This one is about what happens when that state doesn’t get fixed.
Because low energy availability isn’t really the diagnosis. It’s the input. Leave it in place long enough and it stops being a nutrition problem and becomes a whole-body syndrome with a name of its own: RED-S — Relative Energy Deficiency in Sport. That’s the version that costs you bone, hormones, immune function, and eventually the performance you were under-fuelling to protect in the first place.
LEA Is the Exposure. RED-S Is the Outcome.
The distinction matters, and it’s the piece most athletes miss.
Low energy availability describes a mismatch — energy in versus energy burned in training, with whatever’s left over having to cover everything else your body does. Every athlete dips into it sometimes. A brutal training camp, a stomach bug during a big week, a day where the ride went longer than the food you packed. Short, occasional dips are survivable and largely self-correcting.
The 2023 IOC consensus statement frames this as a spectrum, running from adaptable low energy availability at one end to problematic low energy availability at the other. What moves you along that spectrum is dose and duration — how deep the deficit runs, and how long you sit in it. Cross far enough for long enough and your body stops treating it as a temporary shortfall and starts making structural decisions: shutting down systems it considers optional so it can keep funding the ones it doesn’t.
That downshift, across multiple body systems at once, is RED-S.
One note on terminology, because you’ll see both versions in the wild: the original acronym was RED-S, and the 2023 update dropped the hyphen, making it REDs. Same syndrome. The IOC first recognised it as a distinct condition in 2014, updated the guidance in 2018, and updated it again in 2023 — the science has moved fast, and most of what’s below reflects that most recent picture.
What Your Body Shuts Down, and Roughly in What Order
Nothing about this happens all at once, which is exactly why it’s so easy to miss. The early changes are quiet and the loud ones arrive years later.
Weeks: hormonal and metabolic. Reproductive hormones are among the first things to go, because reproduction is the most metabolically expensive thing your body isn’t currently required to do. In women that shows up as cycles getting lighter, longer, irregular, or stopping altogether. In men it’s a drop in testosterone, often accompanied by loss of libido, low mood, and flatness in training. Resting metabolic rate starts to fall too. Your body quietly spends less, which is a big part of why the scale can stop moving even as the deficit deepens.
Months: bone, immunity, gut, mood. Bone turnover shifts toward breaking down more than it builds. You start catching everything going around, and holding onto it longer. Digestion slows, and so does wound healing. That saddle sore or road rash that used to clear in days now takes weeks. Sleep gets worse. So does mood, irritability, and your ability to hold focus, and the relationship there runs both directions: under-fuelling degrades mental health, and degraded mental health makes fuelling harder.
Years: the ones you can’t easily undo. This is where RED-S stops being a bad season and becomes a medical history. Stress fractures. Bone mineral density that lands in osteopenic or osteoporotic territory decades before it should. Chronically suppressed metabolic rate. For an adult, under-fuelling means losing bone you already have, and some of that comes back. For a teenager, it means never building it and you’re capped for life at a lower peak.
The cruel part is that the timeline of the damage runs opposite to the timeline of the warning signs. By the time something breaks, the underlying deficit has usually been running for a long time.
Why Endurance Cyclists Are Unusually Exposed
Every endurance sport carries RED-S risk. Cycling stacks the odds in a particular way.
The energy cost is enormous and easy to underestimate. A five-hour ride burns more than most people can comfortably eat back, especially when appetite is blunted for hours afterwards, which it reliably is. Skip a proper meal after a long ride and you can rack up a serious deficit without ever feeling like you restricted anything.
It’s non-weight-bearing. This is the one that catches people out. High-impact, weight-bearing sports like gymnastics and soccer build bone; cycling, where your legs never take your body weight against the ground, doesn’t provide that stimulus at all. The consequence shows up in the data. Road cyclists consistently measure lower bone mineral density than both physically active non-athletes and athletes in weight-bearing sports. So cycling gives you none of the bone-building stimulus that would partially offset an energy deficit, and then adds crash risk on top. Two forces pushing the same direction.
The research on this isn’t subtle. In a study of fifty competitive male road cyclists, researchers found low energy availability in 28% of them and low lumbar spine bone density in 44%. Energy availability turned out to be the single strongest predictor of that bone density score. Separate work in elite cyclists across career stages found that low bone density may not fully recover even after a professional career ends.
And it’s a weight-sensitive culture. Watts per kilogram makes weight a performance variable in a way it simply isn’t in most sports, which gives under-eating a rationalisation that sounds like strategy. That’s a genuinely dangerous combination, and it’s worth naming plainly.
Two more things worth flagging: this is not a women’s issue. The 2023 consensus put real emphasis on how the syndrome presents in male and Para athletes, and one recent study of endurance athletes found low lumbar bone density was actually more prevalent in the men than the women. That same consensus also called out how much RED-S overlaps with overtraining syndrome. This means a lot of athletes who think they’re overtrained are actually under-fuelled, and are treating it by resting more instead of eating more.
The Trap: It Works, Right Up Until It Doesn’t
Here’s why smart, disciplined athletes walk into this.
In the early phase, under-fuelling often looks like it’s working. Weight comes down, power-to-weight goes up, climbing feels better, and you get external validation for it. Nothing in the first few months tells you anything is wrong. The feedback loop actively rewards you.
Then the returns invert. Power stalls at a lower number than it should. Recovery between sessions stretches from a day to three. Intervals you used to hit become negotiable. You add training load to fix it, which deepens the deficit, which degrades performance further. That’s the spiral, and by the time it’s obvious, the physiological cost has been accruing for a long while.
Discipline is what gets most people into this. It’s not a failure of willpower, it’s usually the opposite.
What This Actually Looks Like Day to Day
You don’t need a lab to spot the pattern. What you’re looking for is a cluster, not a single item:
- Menstrual cycles that changed, got lighter, became irregular, or stopped
- Loss of libido, low mood, or a persistent flatness that doesn’t lift on rest days
- Feeling cold constantly, especially hands and feet
- Getting sick more often, and staying sick longer
- Cuts, saddle sores, and road rash healing slowly
- Poor sleep despite heavy training
- Nagging bone or shin pain, or a stress fracture with no obvious mechanism
- Power that’s plateaued or dropped despite consistent, well-structured training
- Recovery that takes noticeably longer than it used to
- Food-related anxiety, or riding to “earn” or “offset” meals
Any one of these has other explanations. Several at once, in a hard-training athlete, is a pattern worth taking seriously.
A Word on the Threshold Number
You’ll find a specific energy availability value in the literature where problems reliably start showing up. I’m deliberately not building this article around it because in practice, athletes turn that number into a target to hover just above rather than a floor to stay well clear of. It’s a research threshold, derived from group averages, and your individual tolerance depends on training load, history, sex, age, genetics and stress. It was never meant to be a daily budget.
The more useful question isn’t am I above the line. It’s are my cycles normal, is my sleep intact, am I healing, is my training producing what it should, and do I feel warm and clear-headed and interested in things. Your body is running a far better assessment than any calculator.
Recovery Takes Longer Than the Damage Suggests
The honest version: getting out of RED-S is measured in months and years, not weeks, and different systems recover on different clocks.
Energy levels and mood tend to respond first, sometimes within weeks of eating properly. Menstrual function usually takes months. Bone is the slow one, remodelling is a years-long process, and depending on how long the deficit ran and when in life it happened, some of it may not fully come back.
The core of recovery is unglamorous: eat more, train less for a while, and take the resulting temporary drop in fitness as the price of a career instead of a season. Adding load-bearing and resistance work matters especially for cyclists, since it addresses the bone stimulus that riding never provided.
And this is genuinely the point to involve professionals: a sports physician, a registered dietitian who works with endurance athletes, and, where disordered eating is part of the picture, a mental health clinician. RED-S involves bone density, endocrine function and often the athlete’s relationship with food. That’s not a self-managed project, and it’s not something an article can substitute for. If you recognise yourself in the list above, the next step is an appointment, not a new plan.
FAQ
What’s the difference between LEA and RED-S?
Low energy availability is the cause — not enough energy left over after training to support normal physiological function. RED-S is the resulting syndrome when that deficit runs deep enough for long enough, affecting bone, hormones, immunity, metabolism, mental health and performance together.
Can men get RED-S?
Yes. It was originally framed around female athletes as an extension of the Female Athlete Triad, but the current consensus covers male and Para athletes explicitly. In men it typically shows up as low testosterone, loss of libido, low mood, and — particularly in cyclists — low bone mineral density.
How long does it take to develop?
There’s no fixed timeline. Hormonal and metabolic changes can appear within weeks of a sustained deficit; bone and immune consequences typically take months to years. Depth of deficit and duration both matter.
Is RED-S reversible?
Largely, but not entirely, and not quickly. Energy, mood and menstrual function generally recover with sustained refuelling and reduced training load. Bone mineral density is the slowest to respond, and losses sustained over years — or during adolescence — may not fully reverse.
Do I have RED-S or am I just overtrained?
They overlap heavily, and the 2023 consensus specifically flags the confusion. The practical difference: if rest alone hasn’t fixed it, or if it’s accompanied by menstrual changes, feeling cold, poor healing or frequent illness, energy availability is the more likely culprit. Either way, it’s worth assessing properly rather than guessing.
Keep Reading
If you’re rethinking your own fuelling after this, start with the low energy availability post for the foundations, then have a look at the gran fondo training plan for what adequate fuelling actually looks like across a long day on the bike. The full Nutrition and Training archives are a good starting resource.
The short version, if you take nothing else: the athletes who last aren’t the ones who ate the least. They’re the ones who stayed fuelled long enough to keep showing up.
This article is educational and isn’t a substitute for individual medical advice. If any of this sounds like you, please talk to a sports physician or registered dietitian.