Quick Answer: When you consistently take in less energy than training and basic function require, the body downregulates reproductive hormones, and low oestrogen accelerates bone loss regardless of how much loading your exercise provides. This is the mechanism behind stress fractures in otherwise fit, active women. The earliest reliable warning sign is a missing or irregular period, and the fix is eating more rather than training differently.

The assumption underneath most bone advice is that more exercise means stronger bones. It is broadly true and it has an important exception, and the exception catches exactly the people who are doing everything else right.

What is low energy availability, and how is it different from dieting?

Energy availability is the energy left over for your body’s basic functions after exercise has taken its share. The rough calculation is dietary energy intake minus exercise energy expenditure, expressed relative to fat-free mass.

This is a different question from whether you are in a calorie deficit for weight loss, and confusing the two is where the trouble starts.

Consider someone eating 2,000 calories daily who burns 600 in training. Their body has 1,400 calories to run everything else - the heart, the immune system, thyroid function, hormone production, bone remodelling, digestion, temperature regulation, thinking. If their fat-free mass is around 45kg, that works out to roughly 31 calories per kg of fat-free mass. The threshold commonly used in sports science research sits around 30 kcal per kg of fat-free mass, below which measurable physiological disruption starts appearing in studies. That person is right at the edge, and they may well feel fine.

Why this catches people out: low energy availability does not require an eating disorder, extreme dieting, or being underweight. It happens routinely to people who increased their training without increasing their eating - a marathon block, a new class schedule, a summer of higher activity. The intake stayed the same and the demand went up, which is arithmetically identical to eating less.

It also happens to people with a perfectly normal relationship with food who simply have a low appetite, a busy schedule, or a habit of training before breakfast and eating late.

The condition has a formal name in sports medicine: Relative Energy Deficiency in Sport, or RED-S, which replaced the older concept of the female athlete triad. The broader term reflects that the effects extend well beyond bone and menstrual function - immune function, metabolic rate, mood, iron status, cardiovascular markers, and training adaptation are all affected. Bone happens to be where the damage is least reversible, which is why it gets the attention.

The most important thing to understand: the body is not making an error. Down-regulating reproduction and non-essential maintenance when energy is scarce is a sensible adaptation. Bone remodelling is metabolically expensive and not immediately necessary for survival, so it is among the first things reduced.

How does underfueling actually damage bone?

Two pathways, running simultaneously, and the hormonal one does most of the damage.

Pathway 1: The hormonal route.

When energy availability drops, the hypothalamus reduces its pulsed release of gonadotropin-releasing hormone. That reduces luteinising hormone and follicle-stimulating hormone, which reduces ovarian oestrogen production. Oestrogen is one of the primary brakes on bone resorption - it restrains the osteoclasts that break bone down.

With less oestrogen, resorption proceeds faster than formation. The net result is bone loss, and the mechanism is essentially the same as what happens at menopause - which is why a young athlete with amenorrhoea can present with bone density more typical of a woman decades older.

This is why exercise does not compensate. Loading stimulates bone formation, and formation requires energy and a hormonal environment that permits building. Under energy restriction, the signal arrives and the machinery to answer it is not running. Loading a bone that cannot rebuild produces microdamage without the repair, which is the direct path to a stress fracture.

Pathway 2: The nutritional route.

Lower total intake usually means lower calcium and vitamin D, less protein for the collagen matrix that makes up bone’s structural framework, and often reduced fat intake, which impairs absorption of the fat-soluble vitamins including D and K.

Additional contributing factors documented in this population:

• Elevated cortisol from the combination of training stress and energy restriction, which further suppresses bone formation
• Reduced IGF-1, a key anabolic signal for bone
• Altered thyroid hormone, particularly reduced T3, which lowers overall metabolic activity
• Reduced leptin, which appears to act as one of the signals reporting energy status to the hypothalamus

How fast does this happen? Faster than most people assume. Studies have shown measurable hormonal changes within days of severely reduced energy availability, and meaningful bone density changes over months rather than years. The bone lost during a prolonged period of amenorrhoea may not be fully recoverable, even after periods return - which is what makes this different from most training mistakes.

What are the early warning signs?

The useful ones appear well before a fracture, and most of them are easy to dismiss individually.

The single most important sign: menstrual changes.

• Periods stopping entirely (amenorrhoea) - the clearest signal, and never something to accept as normal because you train hard
• Cycles becoming longer or irregular - often the first change, and frequently ignored
• Lighter periods than previously
• Periods that never started in a young athlete, or that stopped after coming off hormonal contraception

An important complication: hormonal contraception produces a withdrawal bleed that looks like a period regardless of what your own hormones are doing. If you are on the pill, you have lost your most useful indicator, and the other signs matter more. This is worth raising with a doctor rather than working out alone.

Other signs, roughly in order of how commonly they appear:

• Repeated illness - more colds than usual, or ones that linger
• Persistent fatigue that rest does not resolve
• Feeling cold constantly, particularly hands and feet
• Poor recovery between sessions - soreness lasting longer, sessions that used to feel routine now feeling hard
• Training plateau or decline despite consistent or increased work. This is the one that often finally gets attention, and it is usually well into the process.
• Sleep disturbance, particularly waking in the early hours
• Mood changes - low mood, irritability, anxiety, or a narrowed preoccupation with food and training
• Digestive problems - constipation and bloating are common
• Hair thinning, brittle nails, dry skin
• Reduced libido
• Bone pain or a stress reaction - and by this point the process is well advanced

The pattern that should prompt action: any two or three of these together, particularly alongside a menstrual change, in someone who has recently increased training or reduced intake. Individually each has other explanations. Together they form a recognisable picture.

Who is most at risk, and why it is often invisible

Endurance athletes carry the highest documented risk - distance running, triathlon, cycling, and swimming, because of the sheer volume of energy expenditure. Runners are the most affected of all, because running combines high energy cost with high bone impact, which means the loading damage accumulates in exactly the population whose repair capacity is compromised.

Aesthetic and weight-category sports follow closely - dance, gymnastics, figure skating, and any sport with weight classes or physique judging, where the cultural pressure toward low body weight is explicit.

But the largest group is not competitive athletes at all. It is recreationally active women who:

• Increased training volume without increasing intake
• Train fasted in the morning and eat their first meal at midday
• Follow a restrictive eating pattern - intermittent fasting, low carbohydrate, or a general habit of eating lightly - alongside meaningful training
• Are in a deliberate weight loss phase while training hard
• Are simply busy, with a low appetite and irregular meals

Why it is often missed by everyone involved:

• Body weight is frequently normal. Low energy availability does not require being underweight, and BMI is a poor screening tool for it. Many affected women look exactly as you would expect a fit person to look.
• Performance often improves initially. Losing weight while training frequently produces a short-term performance boost, which reinforces the behaviour just as the damage begins.
• The symptoms are individually unremarkable. Tiredness, feeling cold, and getting colds are hardly alarming on their own.
• Absent periods get normalised. It is still common to hear that periods stopping is a sign of fitness. It is not. It is a sign that reproductive function has been switched off for want of energy.
• Bone loss is silent until a fracture.

The demographic detail that matters most: peak bone mass is built largely by the late twenties. Energy deficiency during adolescence and the twenties means not just losing bone but failing to build the peak that has to last a lifetime - and that window does not reopen.

What Nobody Tells You About Recovering From This

Eating more is the treatment, and it is harder than it sounds. The instruction is simple. Carrying it out, for someone whose habits and often identity are built around training and controlled eating, frequently is not. This is why recovery works better with support - a sports dietitian, a doctor, and where relevant a psychologist - than alone.

The target is an increase, not a number you calculate. Research protocols often use an additional 300 to 600 calories daily as a starting point, with adjustments based on response. Trying to calculate exact energy availability at home is unreliable, because exercise expenditure estimates are poor and fat-free mass measurement is imprecise.

Training usually needs reducing too, at least initially. Not stopped, in most cases - reduced. The equation has two sides and moving only one takes longer. Any diagnosed bone stress injury changes this entirely and requires specific medical guidance about loading.

Periods returning is the marker to watch for, and it typically takes several months - often three to twelve after energy availability is restored. It is the clearest available sign that the hormonal system has come back online.

Weight gain is likely and is part of the treatment. Often 2 to 5kg, and it includes restoration of body fat that was below the level the endocrine system needs. Many people find this the hardest part, and it is worth naming in advance rather than encountering it as an unwelcome surprise.

Bone density recovery is slow and often incomplete. Studies following recovered athletes show improvement over one to three years, and frequently not a full return to expected values - particularly where amenorrhoea lasted years or occurred during adolescence. This is the strongest argument for acting on the early signs rather than waiting for a scan to confirm something.

Performance typically improves after recovery, which surprises people who feared that eating more would make them slower. Better recovery, fewer illnesses, more training days completed, and restored hormonal function generally add up to more progress than the deficit ever produced.

Bone density scans have limits in young women. A DEXA scan gives useful information, and in premenopausal women results are reported as Z-scores comparing you to age-matched peers rather than the T-scores used for osteoporosis diagnosis in older adults. Interpretation needs a clinician who understands the context.

Practical steps to protect bone while training hard

Fuel around training rather than only after it.

• Eat something before morning training, even a small amount. Consistently training fasted and eating late extends the daily period of low energy availability, and there is evidence that the timing matters, not just the daily total.
• Eat within an hour or two afterwards, including both carbohydrate and protein
• Do not let the gap between finishing a session and eating stretch across the afternoon

Get the specific nutrients bone requires:

• Calcium: roughly 1,000mg daily for most adult women, higher in some groups. Dairy, fortified plant milks, tinned fish with bones, leafy greens, tofu set with calcium.
• Vitamin D: commonly supplemented at 800 to 2,000 IU daily, particularly in autumn and winter at northern latitudes where skin synthesis effectively stops. Worth testing rather than guessing.
• Protein: 1.2 to 2.0g per kg body weight for active people. Bone is roughly half protein by volume, and the collagen matrix needs building material.
• Adequate carbohydrate. Low-carbohydrate diets in endurance athletes have been associated with markers of increased bone resorption in several studies, independent of total energy.
• Enough dietary fat for fat-soluble vitamin absorption and hormone production

Keep the loading appropriate rather than maximal:

• Resistance training two to three times weekly with meaningful loads - this is the highest-value bone stimulus
• Some impact work, but not as a substitute for adequate fuelling
• Recovery days that are genuinely recovery days

Monitor the things that tell you early:

• Track your cycle. It is free, it takes seconds, and it is the single best indicator available.
• Note how often you are getting ill and how recovery feels week to week
• Watch for creeping training volume without a matching change in eating - the most common way this starts

When to see a doctor: three or more missed periods, any suspected stress fracture, bone pain that worsens with activity, or two or more of the warning signs together. Ask specifically about energy availability - it is well recognised in sports medicine and less consistently on the radar in general practice.

FAQ: Underfueling and Bone Health, Answered

Can this happen if my weight is normal or if I am not thin?

Yes. Low energy availability is about the gap between intake and expenditure, not about body size. People at every body weight can be underfuelled, and normal weight is one of the main reasons this is missed.

I am on the pill and my periods are regular. Am I protected?

The bleed on hormonal contraception is a withdrawal bleed and does not tell you whether your own hormonal axis is functioning. Combined oral contraceptives have not been shown to reliably protect or restore bone density in this context. Raise it with a doctor rather than treating regular bleeding as reassurance.

How long until bone density improves once I eat more?

Hormonal markers can shift within weeks and periods often return within three to twelve months. Measurable bone density change takes a year or more, and recovery is frequently partial rather than complete.

Is it just a female issue?

No. Men experience relative energy deficiency too, with reduced testosterone and similar effects on bone. It is less studied and less recognised, and the absence of an obvious signal like a menstrual cycle makes it harder to detect early.

Should I get a bone density scan?

Worth discussing with a doctor if you have had three or more consecutive missed periods, a stress fracture, or a long history of restricted eating alongside training. A baseline measurement makes future comparison meaningful. This article is general information, not medical advice - please speak to a healthcare professional about your own situation.

TL;DR:

• Training hard on insufficient energy suppresses oestrogen, and low oestrogen accelerates bone loss regardless of how much loading you do
• It happens at normal body weights and without disordered eating - most often when training increased and intake did not
• Missing or irregular periods are the earliest reliable signal; on hormonal contraception you lose that indicator
• The fix is eating more, usually alongside reduced training, with weight gain as part of the treatment
• Bone density recovery is slow and often incomplete, which is why acting on early signs matters more than confirming with a scan

The uncomfortable part of this topic is that it affects the most committed people rather than the least. If your training has grown over the last year and your eating has not, or if your cycle has changed and you have quietly filed it under being fit, that is worth a conversation with a doctor this month rather than after a fracture. This is general information, not medical advice.