Low Energy Availability: How Under-Fueling Alters Your Endocrine System

What energy availability means (and how it’s calculated)

Energy availability (EA) is not the same as energy balance. It is the energy left over for all non-training physiological functions after the cost of exercise is subtracted. The formula is straightforward: EA = (energy intake − exercise energy expenditure) / fat-free mass, expressed in kcal per kilogram of fat-free mass (FFM) per day.

Energy balance can look fine while EA is dangerously low. An athlete who eats enough to maintain bodyweight may still be under-fueling: if training burns a large share of intake, the residual energy available to the heart, brain, bones, immune system, and reproductive axis can fall below what those systems need. This is why bodyweight is a poor screening tool for low energy availability (LEA), and why weight-stable athletes can still be at risk.

A healthy reference point for EA is roughly ≥ 45 kcal/kg FFM/day, which supports normal physiological function in most athletes. The clinically relevant problem begins when EA is driven down, either by deliberate restriction, appetite suppression from heavy training, or simply failing to scale intake to a rising training load. Calculating EA requires an honest food log, a reasonable estimate of exercise energy expenditure, and a body-composition measure for FFM. None of these are perfectly precise, but the directional signal is important.

The ~30 kcal/kg FFM threshold and what happens below it

One of the most-cited papers in this field comes from Loucks & Thuma (2003): luteinizing hormone (LH) pulsatility is disrupted when energy availability falls below approximately 30 kcal/kg FFM/day. This was not an observational association. In a tightly controlled design, exercising women had their EA manipulated directly, and LH pulse frequency dropped while pulse amplitude rose once EA crossed below that threshold.

LH is the upstream signal that drives the entire reproductive cascade. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses; the pituitary answers with LH and FSH; the gonads respond with estrogen, progesterone, or testosterone. Disrupt the pulse pattern at the start, and everything else is subject to change. In theory, the body interprets low energy availability as a famine signal and concludes this is a poor time to reproduce.

The roughly 30 kcal/kg figure is a useful landmark, but there is variability. Individual sensitivity varies, and the consensus literature treats it as an approximate inflection point.

RED-S beyond the menstrual cycle (bone, metabolism, immunity)

Bone is a central casualty of RED-S. Low estrogen or testosterone, combined with energy scarcity and elevated cortisol, pushes bone remodeling toward resorption. The result is reduced bone mineral density and therefore a higher stress-fracture risk.

RED-S impairs resting metabolic rate, gastrointestinal function, glycogen storage, protein synthesis, immunity, cardiovascular health, and psychological wellbeing (Mountjoy et al., 2023). Performance itself suffers: reduced endurance capacity, impaired training response, lower power, and depressed mood. Under-fueling is often pursued in search of leanness or a lighter racing weight even though it directly undermines the adaptations the athlete is often training for. You cannot out-train a calorie deficit that has switched off the machinery of adaptation.

Male athletes and RED-S

RED-S was characterized largely in women. However, the underlying endocrine biology in males is shared: chronic LEA suppresses the hypothalamic-pituitary-gonadal axis similarly to women.

In male athletes, LEA can lower testosterone, reduce libido, impair bone health, depress metabolic rate, and blunt training adaptation. Diagnosis often rests on subtler markers: declining performance despite consistent training, low testosterone on bloodwork, recurrent illness, stress fractures, and persistent fatigue. These are easy to dismiss individually and only cohere when LEA is considered as the common cause.

In endurance and weight-sensitive sports, leanness is prized and under-fueling is often worn as a badge of dedication. Male athletes may push EA low without ever suspecting an endocrine consequence, precisely because the syndrome is framed as a “female” issue.

How LEA masquerades as “discipline” and as overtraining

LEA is dangerous partly because it hides behind two flattering disguises. The first is discipline. In a sports culture that rewards restraint and leanness, eating less can appear as commitment rather than risk.

The second disguise is overtraining. The symptom overlap is substantial: fatigue, stalled or declining performance, poor recovery, frequent illness, low mood, and disrupted sleep all appear in both LEA and non-functional overreaching. An athlete and coach who see these signs naturally reach for the standard fix, more rest, and less volume. If the true driver is under-fueling, rest alone will not resolve it, and the deficit can continue to impair bone and endocrine function while everyone waits for recovery that may not happen.

The differentiator is fueling history. This is the practical reason to track readiness and recovery trends over time: a persistent downward drift that does not respond to reduced load is a signal to interrogate energy availability, not just training stress. The fix for LEA is calories and, often, professional support, not another deload week.

Practical: spotting and reversing low energy availability

Spotting LEA starts with the right questions, not a scale. Watch for the cluster: menstrual changes in women, feeling cold, recurrent sickness, stress reactions or fractures, plateauing performance, persistent fatigue, and recovery markers that stay depressed despite rest.

Reversal is conceptually simple and [can be] behaviorally hard: increase energy availability. That means eating more, training relatively less, or both, until EA is consistently restored toward healthy levels. The reproductive axis can begin recovering within weeks of restored EA, but full recovery of bone density and metabolic function can take many months and is not always complete. Earlier intervention often means a faster, more complete return.

RED-S is a clinical diagnosis. Suspected cases warrant assessment by a sports physician and a sports dietitian, ideally with bloodwork and bone-density measurement, because self-managed under-fueling is precisely how the problem started. The goal is simple: catch the deficit while it is still adaptable, before it becomes problematic.