Under-Fuelled: Energy Availability, RED-S, and Why “Eat Less, Train More” Backfires

The grades in this post: Strong means consistent results from more than one controlled trial. Limited means a single small trial, or one without a proper control group. Mixed means studies disagree, or the effect depends on who you are. Mechanistic means it makes biological sense but hasn’t been tested as an outcome in people. None means I found no evidence either way. Consensus means expert panels agree, but it rests on judgement more than trials.

The short version

• Energy availability is what’s left over: the energy you eat, minus what training burns, per kilogram of lean mass. When it’s too low, the body conserves. You can be weight-stable and still be short.
• It’s common, and not only in women. The IOC says REDs affects female and male athletes. It’s been found in elite footballers and in middle-aged club and recreational athletes in England. You can’t see it by looking: in one UK study, the women flagged as at risk were on average heavier, not lighter.
• A few days of low energy availability changed hormones in controlled trials. In exercising women, T3 fell, reverse T3 rose, the brain’s signals to the ovaries were disrupted, and bone formation dropped. Men’s hormones held up better in short trials, but low testosterone and low bone density turn up in under-fuelled male athletes over longer periods.
• It’s the energy, not the exercise. In the trials that separated the two, low energy, not exercise stress, drove the hormone changes.
• Carbohydrate matters for longer sessions, and possibly for bone. Total daily protein matters more than precise timing.
• Screening tools find risk, not a diagnosis. The IOC’s tool ends with a doctor. No single test settles it, including the ones I run.
• See a doctor if your periods stop or become irregular, you’ve had a stress fracture, or you’re a man with a persistently low sex drive while training hard, or if food and weight have started to run your life.

What energy availability is

Energy availability is the energy eaten minus the energy spent in exercise, divided by fat-free (lean) mass (Loucks 1994; Espinar 2026). It’s what’s left to run your hormones, bones, immune system and repair once training is paid for.

It isn’t energy balance, which shows up over time on the scales. In one controlled trial, lowering energy availability disrupted women’s reproductive hormone signals with no significant difference in body weight between conditions (Loucks & Thuma 2003, as summarised in Espinar 2026). The scales are a poor guide.

The threshold is softer than it looks. Low energy availability is usually defined as below 30 kcal per kg of fat-free mass per day (about 126 kJ/kg FFM/day) (Espinar 2026). That figure comes from short lab trials in young, habitually sedentary women, where reproductive hormone pulses were unaffected at 30 kcal/kg lean mass/day but disrupted below it (Loucks & Thuma 2003). A 2026 review argues it shouldn’t be used as a diagnostic cut-off: responses vary between people, and intake, exercise expenditure and lean mass are all measured with considerable error in real life. Food records, for example, typically under-record intake by 5–21% (Espinar 2026, citing earlier work).

Grade: the definition is consensus opinion. A threshold effect on hormones is strong trial evidence in short lab studies of women. Using it as a fixed cut-off for an individual is mixed at best.

Not all low energy availability is harmful. The IOC’s 2023 model describes a continuum from “adaptable” to “problematic“, depending on how deep and how long the deficit is, and on individual factors such as medical history, diet and training (Burke 2023; Mountjoy 2023). A short, planned deficit isn’t months of chronic under-fuelling. Grade: consensus opinion.

Who it affects

The IOC’s 2023 consensus describes REDs as affecting female and male athletes, and notes that more than 170 original studies since 2018 have added to the field, including more data on men (Mountjoy 2023). Its prevention review says REDs is common across sports and performance levels (Torstveit 2023).

• Elite footballers. A 2026 meta-analysis found average intake deficits of up to 741 kcal (about 3.1 MJ) a day in men and 544 kcal (about 2.3 MJ) in women, persisting after adjusting for under-reporting (Mendes 2026). That’s energy balance, not availability, but it points the same way.
• Club and recreational athletes, including middle-aged ones. In 55 endurance athletes in East Anglia (average age about 45; 46% recreational), 77% of the women screened “at risk” on the LEAF-Q questionnaire, and 31% of the men had low lumbar spine bone density, defined as a Z-score of −1 or below. That’s a looser cut-off than a clinical diagnosis. Forty per cent of participants restricted calories, yet 93% felt they fuelled well (Gowers 2025).
• Physically active women. Of 1,154 Spanish women aged 15–45 from 78 recreational and competitive sports, 40% were classed as at risk by questionnaire (Torres Dos Ramos 2025).
• Men. In a clinic sample of men aged 15–30, those with markers of low energy availability had about three times the odds of reporting psychological illness, and more often reported cardiovascular symptoms, a poorer training response and poorer endurance (Holtzman 2025). Reviews of male endurance athletes and cyclists report energy deficits, lower spine bone density and lower testosterone (Schofield 2021; Cupka 2023).

Two cautions. Most of these studies are cross-sectional and use questionnaires, which flag risk, not confirmed REDs. And the East Anglia authors note that peri-menopausal symptoms may have inflated the older women’s scores (Gowers 2025).

Older adults: I found no direct evidence in exercisers over 60. A study of runners aged 65–80 has published its protocol but not results (Oreská 2026). In East Anglia, low bone density in men went with older age and longer training history, but the design can’t say whether energy was the cause (Gowers 2025). Grade: none, so far.

Grade for “men and recreational exercisers too“: mixed to consensus opinion. The pattern is consistent across observational studies; prevalence depends heavily on how it’s measured.

What it does, graded

The honest caveat. Much of the foundational evidence comes from small, short lab trials, mostly in young women. Some researchers argue REDs is better treated as a theoretical construct than a validated syndrome, because fatigue, irregular periods, low bone density and frequent illness can also come from training load, stress or poor sleep. The IOC group replies that long-term trials of starving athletes would be unethical, and that decades of observational and clinical data show the same cluster of problems (Espinar 2026). Both points are fair. Under-fuelling clearly changes hormones and bone markers; the boundaries of “REDs” as a diagnosis are still being argued.

The signs to notice

None is specific on its own. Several together, in someone training hard, are worth taking seriously.

• Periods that stop or become irregular. In women, menstrual change is one of the clearest signals. Hormonal contraception can hide it (Espinar 2026).
• Low sex drive in men. In the male questionnaire’s validation, low sex drive was the one symptom that separated under-fuelled men from controls (Lundy 2022), and in elite men it went with higher odds of a later bone stress injury (Heikura 2025; very wide confidence interval). But in a clinic sample, men with markers of low energy availability were actually less likely to report gonadal problems (odds ratio 0.49) (Holtzman 2025). Its absence doesn’t rule anything out.
• Stress fractures or bone stress injuries (Heikura 2025; Papageorgiou 2018).
• Frequent illness, dizziness, fatigue, gut symptoms (Lundy 2022; Melin 2014).
• Training going backwards: poorer response, poorer endurance, more irritability (Holtzman 2025).
• Low mood, anxiety, or food and weight taking over your thinking (Pensgaard 2023).

Grade: consensus opinion, supported by questionnaire validation studies.

Why “eat less, train more” backfires

It’s the pattern I see most often in people who want to lose weight: cut food, add sessions. It pushes on both sides of the sum at once.

• The body doesn’t care which side the deficit came from. In exercising men, leptin and insulin fell by the same amount whether the deficit came from diet or exercise (Koehler 2016). In women, low energy, not exercise stress, disrupted reproductive hormones (Loucks 1998). Limited two small trials, one compared with earlier data.
• Faster loss cost muscle. In a small randomised trial in elite athletes doing strength training, the slower-loss group gained lean mass while the faster group’s didn’t change, despite similar total weight loss (Garthe 2011). Limited one small trial.
• People don’t notice. Forty per cent of the East Anglia athletes restricted calories while 93% believed they fuelled well (Gowers 2025).
• Body composition gets more attention than it deserves. The IOC’s review found higher body fat went with poorer endurance performance and gaining muscle helped across sports, but found no body composition cut-off that marks a performance advantage; its influence “should not be overstated” (Mathisen 2023). Consensus opinion
• Recovery is slower than the damage. One day of very heavy eating didn’t restore disrupted LH pulses or T3 in women (Loucks & Verdun 1998). Limited one small trial, no control group.

If weight loss is genuinely the goal, a slower, smaller deficit protects training better than a crash.

Carbohydrate around training

The IOC’s 2023 update highlights “the growing role of low carbohydrate availability” in REDs, not just low total energy (Mountjoy 2023).

• During longer sessions, carbohydrate helps. Of 61 randomised, placebo-controlled studies, 82% found a performance benefit, and the benefit grew with exercise duration (Stellingwerff & Cox 2014). Strong trial evidence for endurance exercise.
• For about an hour, the mouth may be enough. At that duration, the authors explain the benefit by carbohydrate in the mouth stimulating the brain, so type and amount seemed irrelevant. For sessions over about 2 hours, where glycogen runs down, they attribute it to high delivery rates, helped by mixed glucose and fructose, tailored to individual tolerance (Stellingwerff & Cox 2014). Strong trial evidence for performance; the mechanisms are the authors’ interpretation.
• Low carbohydrate may cost bone, even with enough energy. In 30 world-class race walkers, 3.5 weeks of a ketogenic diet, matched for energy with a high-carbohydrate diet, raised a bone-breakdown marker and lowered bone-formation markers; formation hadn’t recovered after carbohydrate was reintroduced (Heikura 2020). Limited one small trial, bone markers only.
• Hard, high-volume training uses the most glycogen. The ISSN says muscle glycogen is maximised by a high-carbohydrate diet and depleted most by high-volume exercise (Kerksick 2017). The joint position statement of the American College of Sports Medicine and North American dietetic bodies gives guidelines on the type, amount and timing of food around training, and recommends a registered dietitian for a personal plan (Thomas 2016). Consensus opinion

Protein: the daily total matters more than the clock

• Total beats timing. A meta-regression of randomised trials found that once total protein was accounted for, timing protein around training made no significant difference to strength or muscle growth; total protein was the strongest predictor of muscle gain (Schoenfeld 2013). A published critique called the result inconclusive (Beale 2016). Mixed to strong: timing not shown to matter; total intake consistently does.
• More protein helps, up to a point. Across 49 trials and 1,863 people doing resistance training, extra protein added modestly to strength and lean-mass gains. For lean mass (not strength), the effect was larger in trained people and smaller with age, and plateaued above a certain daily total. Training itself was a far bigger driver, and timing, dose and source played “a minor, if any, role“. The trials only included people eating enough energy (Morton 2018). Strong trial evidence for a modest effect.
• Spread it through the day, says the ISSN, which calls total daily protein “a primary area of emphasis” (Kerksick 2017). Consensus opinion

Protein doesn’t fix low energy availability: if total energy is short, some protein is likely burnt for fuel. (Mechanistic and untested in the sources I read.)

There’s no protein post on this site yet. Creatine is covered in The Biochemistry of Structure, and the supplements with good evidence are in part 1.

Screening and testing: what they can and can’t tell you

I run functional tests and blood chemistry for a living, so I have an interest here. I’ve graded testing to the same standard.

The IOC tool (REDs CAT2). The IOC says there is “no singular validated diagnostic test” for REDs. CAT2 runs in three steps: screening, a traffic-light severity rating from weighted signs and symptoms, then a physician-led diagnosis and treatment plan (Stellingwerff 2023). In 213 elite athletes, 55% were green, 36% yellow, 5% orange and 4% red; the higher categories had more missed periods and bone stress injuries, and lower T3, testosterone and bone density (Heikura 2025). That study was partly run by the tool’s developers. Grade: consensus opinion, with early supporting data from its authors. Built for elite athletes with a medical team.

Questionnaires. The LEAF-Q (women) correctly classified 78% of at-risk and 90% of not-at-risk endurance athletes in its original validation (Melin 2014). It’s less reliable in team, aesthetic and power sports; gut conditions or polycystic ovary syndrome can give false positives, and hormonal contraception false negatives (Espinar 2026). In the LEAM-Q (men), only low sex drive reliably distinguished cases (Lundy 2022), and there’s no universal cut-off (Espinar 2026). Grade: useful for flagging risk; not diagnostic.

Blood tests. CAT2 uses T3, testosterone and cholesterol, plus bone density by DXA scan (Heikura 2025). A 2026 review argues sex hormones, LH, FSH and T3 should be first-line, but says there’s no agreed minimal panel and hormones vary a lot between people (Espinar 2026). In elite athletes, men with testosterone in the lowest quarter of the reference range were more likely to be in the higher-risk categories, and men with T3 in the lowest quarter more likely to have had a bone stress injury; the authors say more research is needed to confirm this (Heikura 2026). Grade: mixed. Useful alongside history and symptoms; no single result confirms or excludes REDs, and a “normal” result doesn’t rule it out.

The tests I run. The REDs studies I read used blood tests, DXA and resting metabolic rate. None used the urine hormone, stool or organic acid tests in my programme, and I’ve found no evidence validating them for diagnosing low energy availability. Grade: none. I’d use testing to check things that look like under-fuelling or travel with it, and as a baseline to track over time. That isn’t a REDs diagnosis, and anyone with red flags should see a doctor first.

Working it out at home. Unreliable in practice. Intake is under-recorded, wearables misjudge intermittent and resistance exercise, and lean mass depends on the method (Espinar 2026). In one re-analysis, counting all daily activity, not just training, moved an athlete from adequate to low (Espinar 2026, citing Taguchi & Manore 2022). Grade: mechanistic and untested as a self-screening tool.

When to see a doctor

The IOC tool ends with a physician-led diagnosis (Stellingwerff 2023). See your GP, or a sports medicine doctor, if:

• your periods have stopped or become irregular;
• you’ve had a stress fracture or bone stress injury;
• you’re a man with a persistently low sex drive while training hard;
• you have dizziness, frequent illness or performance falling away despite rest;
• food, weight or exercise feel out of control. Eating disorders are part of the REDs picture and need specialist care (Pensgaard 2023). Beat, the UK’s eating disorder charity, offers support and information on its website.

For men with training-related low testosterone, a review recommends nutrition and changes to training; testosterone drugs aren’t available to athletes in sports governed by the World Anti-Doping Agency (Hooper 2018). The cornerstone of treatment is finding and correcting the energy shortfall (Torstveit 2023). Grade: consensus opinion.

How much? What the research used

On the sources: the ISSN position stands list authors with supplement-industry affiliations (Kerksick 2017 includes one). I’ve used them for figures, and the IOC and independent trials for whether things work, as in part 1.

Where I land

Under-fuelling is common, often invisible on the scales, and not confined to young female athletes. The short-term hormone changes are well shown in controlled trials; the long-term picture rests more on observational studies, and the diagnostic boundaries are still argued.

The practical message doesn’t depend on settling that. If you train hard, eat enough to cover it, keep carbohydrate around longer sessions, get total protein right before worrying about timing, and lose weight slowly if at all. If the warning signs are there, see a doctor first; testing comes after, for tracking, not for diagnosing REDs.

Next in the series: making weight, and what the evidence says about cutting for a weigh-in.

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