What Is Low Energy Availability and Its Effects?

Low energy availability is a state in which the body does not have enough fuel left over after exercise to support its normal biological functions. It happens when someone’s food intake, minus the calories burned during training, leaves too little energy per kilogram of lean body mass for everyday processes like hormone production, bone maintenance, and immune defense. The threshold most commonly cited as problematic is below about 30 kilocalories per kilogram of fat-free mass per day. While the concept was first studied in female endurance athletes, it affects men, recreational exercisers, adolescents, and military personnel alike, and its consequences reach far beyond weight loss or fatigue.

How Energy Availability Is Calculated

Energy availability is not the same as total calorie intake or a simple calorie deficit. It is calculated by taking how much energy you eat, subtracting the energy you burn during exercise, and dividing what remains by your fat-free mass (essentially everything in your body that is not stored fat).¹ The result tells you how much energy is left over per kilogram of lean tissue for all the other work your body needs to do: running your brain, keeping your heart beating, repairing tissue, producing hormones, and maintaining your immune system. A healthy, adequately fueled person typically has an energy availability of 45 kilocalories per kilogram of fat-free mass per day or higher. When that number drops below roughly 30, the body starts making trade-offs, diverting energy toward the most immediately survival-critical processes and shutting down or slowing others.

This distinction matters because two people can eat the exact same number of calories yet have very different energy availability depending on how much they exercise and how much lean mass they carry. A runner eating 2,500 calories a day who burns 1,200 through training has much less energy available for basic physiology than a sedentary person eating 2,000 calories. That is why low energy availability is so common in athletic populations: the exercise side of the equation drains fuel that the body’s maintenance systems need.

The Body’s Triage System

When energy runs short, your body does not simply slow down evenly across the board. Instead, it prioritizes. Processes that keep you alive right now, like fueling the brain and maintaining core temperature, get protected. Processes that matter for long-term health but are not urgent, like reproduction, bone remodeling, and full immune surveillance, get downgraded. Evolutionary biologists describe this as an energy trade-off: because every calorie spent on one function is a calorie unavailable for another, the body evolved to protect immediate survival even at the cost of future health.²

This explains why the effects of low energy availability are so wide-ranging. It is not a single disease with a single symptom. It is an energy crisis that ripples through nearly every system in the body, and the signs can be subtle for months before they become obvious.

Hormonal Disruption and the Reproductive System

The reproductive system is one of the first casualties when energy availability drops, precisely because reproduction is not essential for immediate survival. In women, the brain’s signaling hormone for the menstrual cycle, luteinizing hormone (LH), is released in pulses, and the frequency of those pulses is directly tied to energy availability. Research in exercising women showed that low energy availability reduced LH pulse frequency by about 10% during waking hours, and this disruption was caused by the energy shortfall itself, not by the physical stress of exercise.³ That finding was an important turning point in the field: it showed that the problem is not training too hard, but eating too little relative to training.

A follow-up study confirmed that energy availability, averaged across a menstrual cycle, predicted LH pulse frequency, with each unit decrease in availability corresponding to a measurable decrease in pulse rate.⁴ When these pulses become too infrequent or irregular, the downstream consequences include menstrual irregularities and, in more severe cases, a complete loss of periods known as functional hypothalamic amenorrhea. The commonly cited threshold for preventing this is energy availability of at least 30 kilocalories per kilogram of fat-free mass per day, though disruptions can begin even slightly above that number.⁵

Men are not exempt. Low energy availability in male endurance athletes is linked to decreased testosterone, reduced bone density, and lower resting metabolic rate.⁶ The disruption follows a similar pattern through the hypothalamic-pituitary-gonadal axis, leading to a condition sometimes called functional hypogonadotropic hypogonadism. Signs include drops in testosterone and LH, changes in sperm production, and self-reported decreases in libido.⁷ Because testosterone also supports bone density and muscle maintenance in men, its suppression under low energy availability creates a cascade of downstream problems.

Metabolic Slowdown and Thyroid Function

One of the body’s earliest responses to an energy shortfall is to reduce its resting metabolic rate, essentially turning down the thermostat to conserve fuel. A key player in this process is the thyroid hormone triiodothyronine (T3), the active form of thyroid hormone that regulates how quickly your cells use energy. Research has established that energy availability, not exercise stress, determines thyroid hormone levels. When energy availability was maintained at adequate levels, exercise at various intensities did not affect thyroid hormones. But when energy availability was cut to very low levels for just four days, T3 dropped regardless of whether the subjects were exercising or not.⁸

Subsequent work identified a threshold of roughly 25 kilocalories per kilogram of fat-free mass per day in women, below which thyroid disruption becomes evident.⁸ Short-term severe low energy availability also triggers decreases in insulin-like growth factor 1 (IGF-1), a hormone important for tissue growth and repair, alongside the suppression of the reproductive axis and reduced protein turnover in connective tissues.⁹ The practical upshot: someone with low energy availability may feel cold, fatigued, and sluggish even when their training load has not changed. Their body is burning fewer calories at rest, which ironically makes the energy deficit harder to notice because weight loss may slow down or stall.

Bone Health Under Threat

Bone might seem static, but it is constantly being broken down and rebuilt. When energy availability is low, the balance tips toward breakdown. Short-term low energy availability increases markers of bone resorption (the breaking-down part) and decreases markers of bone formation, and cross-sectional research on people with chronically low energy availability shows lower bone mass, weaker bone structure, and a higher risk of stress fractures.¹⁰

A controlled study comparing low energy availability to a low-carbohydrate, high-fat diet found that both conditions impaired bone formation markers, though the low-carb diet had a larger effect. Low energy availability reduced a key bone formation marker (P1NP) by about 14% and increased bone resorption markers around exercise.¹¹ This is worth noting because it suggests that what you eat, not just how much, matters for bone. Low carbohydrate availability may compound the damage caused by insufficient total energy, a point researchers are beginning to explore more carefully.¹²

The bone effects are especially concerning because they can accumulate silently. A stress fracture may be the first obvious sign of a problem that has been building for months or years. And unlike some hormonal disruptions, lost bone density is difficult and slow to regain.

Muscle, Glycogen, and Athletic Performance

Athletes often tolerate low energy availability in pursuit of a lighter, leaner body, believing it will improve performance. The evidence suggests the opposite. A study of trained women found that just ten days of low energy availability impaired the rate of muscle protein synthesis, both the contractile (force-producing) and the structural (support and signaling) components of muscle.¹³ In other words, the muscles were rebuilding themselves more slowly after training, which over time would undermine strength, power, and the body’s ability to adapt to training.

Beyond protein synthesis, muscle glycogen, the stored carbohydrate that fuels high-intensity effort, is also compromised. A systematic review of evidence from functional sports concluded that low energy availability and reduced muscle glycogen consistently impair both performance and recovery, and emphasized the need to think about carbohydrate availability as a distinct concern from total energy.¹⁴ The performance decline is not always dramatic at first, which is part of what makes it insidious. An athlete may attribute a slightly slower recovery or a plateau in training gains to overtraining or mental fatigue rather than recognizing inadequate fueling as the root cause.

Gut Problems and Immune Suppression

People with chronically low energy availability frequently report gastrointestinal symptoms: bloating, feeling uncomfortably full after small meals, constipation, and abdominal discomfort. These are not psychosomatic. When the body is conserving energy, it slows gut motility, delays gastric emptying, and alters the hormones that regulate digestion.¹⁵ The cruel irony is that these symptoms can make it even harder to eat enough, creating a feedback loop: the less you eat, the worse eating feels, the less you want to eat.

The immune system is another area hit by chronic underfueling. Athletes with low energy availability tend to get sick more frequently, picking up upper respiratory infections and other illnesses that interrupt training. The suppressed levels of hormones like IGF-1 and thyroid hormones, combined with reduced protein turnover, leave the immune system with fewer resources to mount effective defenses.

Cardiovascular Signals

The cardiovascular effects of low energy availability are an area where the evidence is still catching up to the concern. A systematic review of physically active women found mixed results: a couple of studies pointed toward unfavorable blood lipid profiles associated with low energy availability, while others found no significant lipid differences. One study identified twenty harmful associations between lipid markers and clinical signs of hormonal imbalance and altered body composition, both characteristic of low energy availability. On the vascular side, however, most studies showed no significant changes in markers like arterial stiffness or heart-rate variability, though one study did flag abnormalities in flow-mediated dilation.¹⁶ The honest read of this evidence is that low energy availability probably moves cardiovascular risk markers in an unhealthy direction, but the research is young and not yet conclusive enough to quantify the long-term heart risk.

Psychological Terrain

The relationship between low energy availability and mental health is tangled. A cross-sectional study of female athletes found that low energy availability on its own was not significantly associated with depression, either current symptoms or lifetime prevalence. However, there was a significant association between depression and disordered eating or eating disorders.¹⁷ This distinction is important. Low energy availability can arise from deliberate restriction driven by an eating disorder, from ignorance about how much fuel training demands, from logistical barriers to eating enough, or from appetite suppression caused by training itself. When the cause is disordered eating, depression and anxiety are often part of the picture. When the cause is accidental underfueling, the psychological toll may be less severe but still present as irritability, poor concentration, and reduced motivation.

Recognizing which pathway is at play matters for treatment. Someone whose low energy availability stems from not knowing they need to eat more after practice requires education and meal planning. Someone whose low energy availability is driven by a fear of weight gain requires psychological support alongside nutritional guidance.

Why Measuring Energy Availability Is So Difficult

One frustrating reality about low energy availability is how hard it is to measure accurately in real life. The calculation requires three inputs: energy intake, exercise energy expenditure, and fat-free mass. Each one is prone to significant error. People routinely underestimate how much they eat (or overestimate, depending on their goals). Exercise energy expenditure is difficult to measure outside a laboratory setting. And fat-free mass estimates vary depending on the technique used, with different machines sometimes giving meaningfully different results for the same person.¹⁸

Because of these measurement challenges, clinicians increasingly rely on screening questionnaires rather than trying to calculate a precise number. The most widely used is the Low Energy Availability in Females Questionnaire (LEAF-Q), which asks about injuries, gastrointestinal function, and reproductive symptoms as indirect indicators of energy status. The LEAF-Q is now recommended in the International Olympic Committee’s clinical assessment tool for Relative Energy Deficiency in Sport (REDs).¹⁹ Male-specific versions (LEAM-Q) and broader tools like the REDs Clinical Assessment Tool Version 2 are also being used, particularly in pre-participation screening for young competitive athletes.²⁰

Who Is at Risk Beyond Elite Athletes

Low energy availability is often framed as an elite sport problem, but it shows up in some unexpected populations. Military personnel are a prime example. Combat training, long marches, and field exercises create enormous energy expenditures, and soldiers often cannot eat enough to keep up, especially in the field. The combination of high physical demand and restricted access to food produces classic energy deficits.²¹ A study of male Army ROTC cadets found that 62% had clinically low energy availability (below 30 kilocalories per kilogram of fat-free mass per day) and not a single participant reached the optimal threshold of 45 or above.²² These were not people trying to lose weight; they simply were not eating enough to match the demands of their training.

Adolescent athletes are another vulnerable group. A study of competitive adolescent climbers found that average energy availability was only 27.5 kilocalories per kilogram of fat-free mass per day, with 63% of participants falling below the 30-kilocalorie threshold. Among the female climbers with energy availability below that threshold, more than a third showed signs of menstrual dysfunction.²³ For teenagers still growing, the stakes are even higher because low energy availability during adolescence can compromise the accumulation of peak bone mass, potentially setting up a lifetime of skeletal fragility.

Carbohydrate Availability as a Separate Problem

A growing area of interest in sports nutrition is whether low carbohydrate availability has its own set of health and performance consequences, separate from and on top of low total energy availability. The two often go hand in hand, since athletes cutting calories tend to reduce carbohydrates most aggressively. But there is evidence that inadequate carbohydrate intake may independently harm bone markers, hormone function, and exercise performance even when total calorie intake is closer to adequate.¹² The bone study mentioned earlier illustrates this: a low-carb, high-fat diet impaired bone formation markers even more than a calorically restricted diet of similar energy availability.¹¹ This means athletes and coaches who focus exclusively on total calorie counts may be missing a piece of the puzzle. Someone eating enough total energy but almost entirely from fat and protein could still be compromising their health.

The Time-Lag Problem in Recovery

One of the most frustrating aspects of low energy availability is that the damage does not always appear when the energy shortfall happens, and the recovery does not always begin the moment you start eating more. Hormonal changes can set off physiological effects on different timelines, so a bone loss triggered by months of suppressed estrogen or testosterone may not show up on a scan until much later, and it may take years of adequate fueling to reverse, if it fully reverses at all.²⁴

This variable temporal relationship between hormonal disruption and its downstream effects is one reason early detection matters so much. By the time symptoms become clinically obvious, such as a stress fracture or complete loss of menstrual periods, the underlying hormonal and metabolic disruption has usually been accumulating for a long time. The return of menstrual periods after improving energy intake is often treated as a sign of recovery, but researchers caution that hormonal normalization and full restoration of bone, metabolic, and reproductive health may operate on very different clocks. Someone who gets their period back within a few months of eating more may still have bone density that takes years to rebuild. Patience and continued monitoring, not just symptom resolution, are part of the recovery process.

Accidental Underfueling and How to Spot It

Perhaps the biggest misconception about low energy availability is that it always involves deliberate restriction. Many athletes, soldiers, and active people end up underfueled simply because they do not realize how much energy their activity demands. Appetite is a poor guide during intense training: hard exercise can suppress hunger for hours, and people tend to underestimate the caloric cost of endurance activity. Add in early-morning training sessions, travel, and busy schedules that make eating inconvenient, and accidental low energy availability becomes easy to stumble into.

Recognizing the warning signs can help. In women, the most reliable early indicator is a change in menstrual patterns: cycles becoming irregular, lighter, or disappearing altogether. In men, the signs are more ambiguous but can include persistent fatigue out of proportion to training load, decreased interest in sex, frequent illness, and unexplained drops in performance. Both sexes may notice unusual food cravings, difficulty sleeping, and a sense of being cold all the time, which reflects the metabolic slowdown driven by falling thyroid hormone levels. Recurring injuries, particularly stress reactions or fractures in weight-bearing bones, should raise a red flag in any active person, regardless of sex.

The fix is conceptually simple, eat more, but practically complicated. Increasing energy intake without guidance can feel counterintuitive for someone who has internalized the idea that lighter means faster. Working with a sports dietitian who can help estimate energy needs and design a realistic eating plan around training is generally the most effective approach. For adolescents, involving parents and coaches creates an environment where adequate fueling is treated as part of the training plan rather than something left to the athlete to figure out alone.