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Training on too little fuel: what the body cuts back
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In one pass A familiar pattern: a desk job by day, the gym or a run after work, and dinner cut right back to lose fat.
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Chapter 1
The energy left for the body
A familiar pattern: a desk job by day, the gym or a run after work, and dinner cut right back to lose fat. Part of the energy a person eats is spent on training first; only what is left goes to the everyday running costs of keeping the heart beating, holding body temperature, repairing tissue and supporting reproduction. That remainder is called energy availability: intake minus the energy spent on exercise, scaled to fat-free mass (the muscle, bone and organs that remain once fat is set aside).
It is not the same thing as the more familiar energy balance. Energy balance compares everything that comes in with everything that goes out; energy availability asks how much the body still has to spend once training has been paid for. Training more, eating less, or both will shrink that remainder, and too small a remainder disturbs the body's hormonal environment.
The 2023 consensus of the International Olympic Committee (IOC) calls the chain of health and performance problems that low energy availability sets off Relative Energy Deficiency in Sport (REDs).
It is not the same thing as the more familiar energy balance. Energy balance compares everything that comes in with everything that goes out; energy availability asks how much the body still has to spend once training has been paid for. Training more, eating less, or both will shrink that remainder, and too small a remainder disturbs the body's hormonal environment.
The 2023 consensus of the International Olympic Committee (IOC) calls the chain of health and performance problems that low energy availability sets off Relative Energy Deficiency in Sport (REDs).
Mechanism · Steady weight can still mean underfueling
So an unchanged weight, and a body that looks no thinner, do not mean there is enough energy left once training is paid for. Body weight tracks the overall account; energy availability tracks what is left over. The two have to be read separately.Say you are building up to a half marathon: weekly mileage keeps climbing while you still eat the portions you used to cut fat. Your weight holds steady, yet the problem may be hiding in this second ledger.
Evidence · Why the data on men are thin
Energy availability was first studied in women and has only recently been extended to men, where experimental evidence is still thin. In women it is tied to the menstrual cycle, which is not covered here; this story is mainly about men.Trials that actually change participants' energy intake and then track what happens are few, and usually last no more than 5 days; most other data are one-off surveys and observations. So most of what follows about men is still preliminary.
Chapter 2
The hypothalamus turns down the testosterone line
When energy stays short for a long time, the hypothalamus at the base of the brain sets priorities and puts off spending that is not urgent right now, and reproduction is one of those items. The hypothalamus slows its release of gonadotropin-releasing hormone (GnRH), the pituitary gland follows by releasing less (LH), the testes receive fewer instructions, and testosterone falls. Low testosterone of this kind, caused by low energy and starting in the brain rather than the testes, is called functional hypothalamic hypogonadism. Together with low energy and low (or bone stress injury), it makes up the Male Athlete Triad.
Still, this line holds up better against short deficits in men than in women. Current data suggest that men need a more severe energy shortfall than women before reproduction and bone are affected, but exactly how large that shortfall must be is still unclear. The pattern is most common in adolescent and young adult male athletes in endurance sports or sports with weight classes. Some of the trials, though, recruited ordinary men who simply exercise regularly. By the way energy availability is calculated, someone who trains steadily after work while also cutting back on food can push that remainder down just the same.
Still, this line holds up better against short deficits in men than in women. Current data suggest that men need a more severe energy shortfall than women before reproduction and bone are affected, but exactly how large that shortfall must be is still unclear. The pattern is most common in adolescent and young adult male athletes in endurance sports or sports with weight classes. Some of the trials, though, recruited ordinary men who simply exercise regularly. By the way energy availability is calculated, someone who trains steadily after work while also cutting back on food can push that remainder down just the same.
Evidence · Eight weeks of US Army Ranger training
The most direct observations come from the US Army Ranger course: two groups of healthy, lean young men (49 and 48 men) went through repeated cycles of food restriction and refeeding over 8 weeks, with average daily energy deficits of about 1200 and 1000 kcal. By the end, group 1, which had the larger deficit, had testosterone close to castrate levels, (T3, the active form of thyroid hormone) below normal, and insulin-like growth factor 1 () cut in half. Luteinizing hormone stayed suppressed throughout, which points to the hypothalamus and pituitary as the starting point rather than a fault in the testes themselves.After refeeding, T3, testosterone and IGF-1 rose again quickly, even while the heavy workload, sleep loss and heat and cold continued. In group 2, which ate more, the drop in thyroid function and the rise in cholesterol were milder. The limits need stating: the trainees were also carrying heavy workloads, sleep loss and heat and cold, so the energy deficit was not isolated, and the two groups were not randomized.
Evidence · Four days of low energy availability
Another small trial put 6 physically active men through two 4-day periods of low energy availability in randomized crossover order: one by eating less only, and one by eating less and exercising more. In both, leptin (a signal sent by fat tissue that reflects energy stores) fell by more than half and insulin by more than a third, and it made little difference where the deficit came from.Testosterone, , and ghrelin, however, showed no significant change. The authors read this as a sign that these hormones may hold up better against short deficits in men than in women; but with only 6 people over only 4 days, finding no change may simply reflect too small a sample.
Mechanism · Two kinds of low testosterone
Men who train intensely over long periods can have low resting testosterone and changes along the hypothalamic-pituitary-gonadal axis; in most published studies, though, the drop still sits within the clinical normal range, often at its low end.A 2020 review by Hackney splits exercise-related low testosterone into two kinds: an acute kind, linked to low energy availability or overtraining, which usually resolves once it is dealt with; and a kind that persists for years, which he calls the Exercise Hypogonadal Male Condition. This classification is the author's own proposed framework, and the mechanisms behind it are not yet worked out; before blaming training, other causes such as head injury or anabolic steroid use need to be ruled out. So a single low testosterone result does not, on its own, tell you the cause.
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Chapter 3
Where else it saves: thyroid, bone, power
Beyond the reproductive line, the body saves in other places too. One is the thyroid: in young men who went short on food for weeks, the active thyroid hormone fell below normal; T3 is one of the main signals that set the pace of metabolism across the body. Another is bone: bone is taken down and rebuilt every day, and when energy runs short, the building side slows while the breaking-down side shows signs of speeding up. A third is muscle output: in a trial that cut energy step by step in male endurance athletes, explosive power such as jumping dropped first, and endurance followed later.
These often go wrong together. Among elite distance runners, men with low testosterone (and women whose periods had stopped) had more bone injuries, and those rated high risk by the Triad or REDs tools had lower T3. That suggests they may share one cause, the same energy shortfall; but the data come from a one-off survey and cannot prove cause and effect.
These often go wrong together. Among elite distance runners, men with low testosterone (and women whose periods had stopped) had more bone injuries, and those rated high risk by the Triad or REDs tools had lower T3. That suggests they may share one cause, the same energy shortfall; but the data come from a one-off survey and cannot prove cause and effect.
Evidence · Power drops before hormones move
This trial took 12 male endurance athletes through three steps of falling energy availability, 10 to 14 days each, with the deficit created by adding training. Explosive power fell first: vertical jump height dropped from 0.32 m to 0.28 m, while endurance output fell only at later steps.The hormones responded more slowly and less consistently. Compared step by step, only at the most severe step fell significantly, while testosterone only trended down; with a different statistical method that looked at all three steps together, the drop in testosterone did reach significance. The authors' summary: compared with performance, hormones are more resilient to changes in energy, changed little over about two weeks, and varied greatly between individuals.
Limits: the sample was small; the three steps were progressive; some athletes could not complete the full 14 days of the third step; and body composition was measured by bioelectrical impedance rather than a more precise scan.
Evidence · Five days shifted bone turnover
A small pilot trial had young, recreationally trained men spend 5 days in each of three conditions, two of which were low energy availability, created by eating less plus daily cycling. Compared with the control, P1NP, a blood marker of bone formation, fell clearly (by about 15% and 25%, against about 4% in the control); CTX, a marker of bone breakdown, rose by about 7% in the low-protein condition while it fell in the control. Eating a higher amount of protein did not prevent these changes.Keep the limits in mind: only 7 men completed all three conditions, each lasted only 5 days, and cycling is not weight-bearing; what was measured were blood signals of bone turnover, not , let alone fractures. What it shows is a direction: when energy runs short, the balance between breaking down and building bone starts tipping toward breakdown within days.
Evidence · Low testosterone and bone injury
Among national- and world-class distance runners, men with low testosterone and women whose periods had stopped had about 4.5 times the rate of bone injury seen in the others (a large gap in women, a moderate one in men). Those rated high risk by the Triad or REDs assessment tools had lower , and among men these high-risk groups also had more past fractures. The authors argue that looking at reproductive function is more objective than estimating energy availability from food and training diaries. This was a cross-sectional study with only 10 men in the low-testosterone group, so it cannot prove cause and effect.Another study followed 156 male college distance runners, of whom 42 (27%) had a total of 61 bone stress injuries. The researchers combined energy availability, body mass index (), and past bone stress injury into one cumulative risk score: each point higher went with a roughly 27% to 37% higher risk of a later bone stress injury, and for the past-injury item alone, each point higher went with about 57% higher risk. BMI and bone density on their own showed no significant link, and the data come from only two universities.
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Chapter 4
Underfueling, short sleep or overload: why the signals overlap
For someone who works and trains, all three can land in the same stretch of weeks: a deadline eats into sleep, a busy day means a skipped meal, and the weekend is spent making up missed sessions. Constant tiredness, training that stalls, a lower sex drive: signals like these can have more than one cause. The symptoms recorded in research on underfueling and on overtraining overlap heavily, and both can start changing hormones at the level of the hypothalamus and pituitary; while short sleep on its own can also push down testosterone in young men.
So symptoms alone make it hard to tell which one is at work. The energy side offers one clue: in the Ranger course above, hormones climbed back quickly once eating was restored. Review authors also caution that low energy intake should be ruled out before overtraining is diagnosed. In practice, then, whether you are eating enough is usually the first question to ask; sleep is a separate possible cause.
So symptoms alone make it hard to tell which one is at work. The energy side offers one clue: in the Ranger course above, hormones climbed back quickly once eating was restored. Review authors also caution that low energy intake should be ruled out before overtraining is diagnosed. In practice, then, whether you are eating enough is usually the first question to ask; sleep is a separate possible cause.
Evidence · Only low sex drive told groups apart
A questionnaire validation study in male athletes (LEAM-Q) included 310 athletes from 10 countries. The researchers used clinical markers to place some of them in a Training on too little fuel: what the body cuts back group, then checked whether self-reported symptoms could tell them apart from the controls: dizziness, illness and fatigue could not; only low sex drive separated the two groups.Two points matter when reading this study. First, the clinical markers used to define the Training on too little fuel: what the body cuts back group included low testosterone, so part of that group's lower testosterone comes from the definition itself. Second, the questionnaire is only a flag that further clinical assessment is needed, and cannot be used for diagnosis. Low sex drive and low testosterone have also long been treated as markers of the Exercise Hypogonadal Male Condition, and how that relates to low energy has not been sorted out.
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Evidence · Hardest trainers reported lower libido
An online cross-sectional survey asked 1077 healthy men about their endurance training habits and their libido. Those who had trained at the highest intensity and for the longest were more likely to have low libido scores; by , the low- and mid-intensity groups had 6.9 and 2.8 times the odds of normal or high libido, respectively, compared with the highest-intensity group.This is only an association and cannot show that training lowers libido: the study measured neither hormones nor energy intake, relied on one-off self-reports, and had wide .
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Mechanism · Where overtraining meets underfueling
Training overload comes in three grades: functional overreaching, non-functional overreaching, and the long-term maladaptation of overtraining syndrome.A 2021 narrative review looked back at 21 eligible studies of training overload or overtraining, and in 18 of them found signs of too little energy or carbohydrate: some showed intake falling during the overload period, others a difference between two groups; none of these studies was designed to test this question. The authors argue that many negative outcomes blamed on training overload may in fact be underfueling.
On the sleep side: one week of only 5 hours a night lowered daytime testosterone in healthy young men by about 10% to 15%. That study did not look at energy intake; what it shows is only that short sleep is an independent possible cause in its own right.
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Chapter 5
Fast weight loss is not a win: reading it and when to see a doctor
In the weeks before a wedding, a photo shoot or a health check, some people want one last push. A common belief goes: the more you train and the less you eat, the faster the weight comes off, and weight coming off means success. The problem is that when weight comes off too fast, it is harder to build muscle at the same time. In one randomized trial, two groups of athletes lost about the same amount of weight, but the group that lost it slowly also gained fat-free mass, while the group that lost it fast did not.
The scale also cannot show the cost paid in hormones, bone and strength. No single lab test or questionnaire can confirm Relative Energy Deficiency in Sport; the IOC assessment tool first screens, then sorts risk by signs and symptoms, and leaves the diagnosis to a doctor. Bone stress injury is itself one corner of the Male Athlete Triad, and the low energy in the Triad can come with disordered eating. So a bone stress injury (especially at the femoral neck, sacrum or pelvis), a lasting drop in sex drive or morning erections, or growing anxiety about food and weight are all worth having a doctor assess.
The scale also cannot show the cost paid in hormones, bone and strength. No single lab test or questionnaire can confirm Relative Energy Deficiency in Sport; the IOC assessment tool first screens, then sorts risk by signs and symptoms, and leaves the diagnosis to a doctor. Bone stress injury is itself one corner of the Male Athlete Triad, and the low energy in the Triad can come with disordered eating. So a bone stress injury (especially at the femoral neck, sacrum or pelvis), a lasting drop in sex drive or morning erections, or growing anxiety about food and weight are all worth having a doctor assess.
Evidence · Slow versus fast weight loss
This trial randomly split 24 elite athletes into two groups that lost 0.7% and 1.4% of body weight per week, with both groups doing strength training 4 times a week. In the end both groups lost about 5.5% of body weight; fat-free mass rose by about 2% in the slow group and barely changed in the fast group. The authors concluded that athletes who want to gain muscle and strength while losing weight are better served by a slower pace.Limits: the sample was small, the participants were elite athletes, the two groups' interventions lasted different lengths of time (about 8.5 weeks for the slow group, about 5.3 for the fast group), and the result has not been tested in the general population.
Evidence · One natural bodybuilder's contest prep
A single-person case report followed one drug-free male bodybuilder (aged 26 to 27) for six months before and six months after a competition. During contest preparation his body fat fell from 14.8% to 4.5%, testosterone fell from 9.22 to 2.27 ng/mL, and his mood-disturbance score rose sharply. After the competition, testosterone returned to 9.91 ng/mL, body fat to 14.6%, and the mood score came back down; but the strength lost during preparation had still not fully returned after 6 months of recovery.This is one person, useful only as an illustration of what this path can cost; it cannot be generalized to anyone else.
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Clinical · How the assessment tool grades risk
The 2023 IOC REDs Clinical Assessment Tool, version 2 (CAT2), states plainly that REDs has no single validated diagnostic test. It works in three steps: screening first; then sorting severity into four levels, green, yellow, orange and red, based on signs and symptoms; and finally diagnosis and treatment led by a doctor. Each indicator is weighted by scientific support, clinical severity and practical usability.In men, testosterone below the reference range is listed as a severe primary indicator and counts as two primary indicators; subclinically low testosterone, in the lowest quarter of the reference range, and low are primary indicators. Bone stress injuries count as a primary indicator only above a threshold: in the past two years, one at a high-risk site such as the femoral neck, sacrum or pelvis, or two at other sites, or at least half a year out of training because of them; below that threshold they count as a secondary indicator. Lower sex drive and fewer morning erections are listed only as potential indicators, are not scored, and the evidence behind them is still thin.
The tool also states that every indicator needs other causes besides low energy ruled out first, and that the tool cannot be used on its own to make a diagnosis.
References · 12
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- Mountjoy, M., Ackerman, K. E., Bailey, D. M., et al. (2023). 2023 International Olympic Committee's consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 57(17), 1073-1098. 10.1136/bjsports-2023-106994
- Nattiv, A., De Souza, M. J., Koltun, K. J., Misra, M., Kussman, A., Williams, N. I., Barrack, M. T., Kraus, E., Joy, E., & Fredericson, M. (2021). The Male Athlete Triad—A consensus statement from the Female and Male Athlete Triad Coalition Part 1: Definition and scientific basis. Clinical Journal of Sport Medicine, 31(4), 335-348. 10.1097/JSM.0000000000000946
- Koehler, K., Hoerner, N. R., Gibbs, J. C., Zinner, C., Braun, H., De Souza, M. J., & Schaenzer, W. (2016). Low energy availability in exercising men is associated with reduced leptin and insulin but not with changes in other metabolic hormones. Journal of Sports Sciences, 34(20), 1921-1929. 10.1080/02640414.2016.1142109
- Murphy, C., Bilek, L. D. D., & Koehler, K. (2021). Low energy availability with and without a high-protein diet suppresses bone formation and increases bone resorption in men: A randomized controlled pilot study. Nutrients, 13(3), 802. https://doi.org/10.3390/nu13030802 10.3390/nu13030802
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- Jurov, I., Keay, N., & Rauter, S. (2022). Reducing energy availability in male endurance athletes: A randomized trial with a three-step energy reduction. Journal of the International Society of Sports Nutrition, 19(1), 179–195. https://doi.org/10.1080/15502783.2022.2065111 10.1080/15502783.2022.2065111
- Heikura, I. A., Uusitalo, A. L. T., Stellingwerff, T., Bergland, D., Mero, A. A., & Burke, L. M. (2018). Low energy availability is difficult to assess but outcomes have large impact on bone injury rates in elite distance athletes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 403–411. https://doi.org/10.1123/ijsnem.2017-0313 10.1123/ijsnem.2017-0313
- Stellingwerff, T., Heikura, I. A., Meeusen, R., Bermon, S., Seiler, S., Mountjoy, M. L., & Burke, L. M. (2021). Overtraining syndrome (OTS) and relative energy deficiency in sport (RED-S): Shared pathways, symptoms and complexities. Sports Medicine, 51(11), 2251-2280. 10.1007/s40279-021-01491-0
- Leproult, R., & Van Cauter, E. (2011). Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA, 305(21), 2173-2174. 10.1001/jama.2011.710
- Garthe, I., Raastad, T., Refsnes, P. E., Koivisto, A., & Sundgot-Borgen, J. (2011). Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. International Journal of Sport Nutrition and Exercise Metabolism, 21(2), 97-104. 10.1123/ijsnem.21.2.97
- Stellingwerff, T., Mountjoy, M., McCluskey, W. T., Ackerman, K. E., Verhagen, E., & Heikura, I. A. (2023). Review of the scientific rationale, development and validation of the International Olympic Committee Relative Energy Deficiency in Sport Clinical Assessment Tool: V.2 (IOC REDs CAT2)—by a subgroup of the IOC consensus on REDs. British Journal of Sports Medicine, 57(17), 1109-1121. 10.1136/bjsports-2023-106914
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