Glossary

Low Energy Availability

Updated April 9, 2026

Low energy availability happens when dietary intake does not cover both exercise cost and the body’s remaining physiological work. It matters because the first signal is often worse training, worse recovery, or worse iron levels, which is why Low Energy Availability in Female Endurance Athletes is one of the site’s more useful performance reads. A person can look disciplined on paper and still underfuel hard enough to damage adaptation.

What the term actually means

Energy availability is usually expressed as dietary intake minus exercise energy expenditure, divided by kilograms of fat-free mass. The classic screening threshold for women is below 30 kcal per kg fat-free mass per day, while about 45 kcal per kg fat-free mass per day is often treated as a better physiological target.¹ Those numbers are screening tools with clear limits. Day-to-day variation, logging error, and mixed training weeks all complicate the picture.

The point is simpler than the formula. After training takes its share, the body still has to run hormone production, protein synthesis, immune function, thermoregulation, sleep recovery, and bone turnover. When that budget stays too low, the body starts cutting from places that athletes notice only after performance falls.

The International Olympic Committee treats low energy availability as the main exposure that drives the broader Relative Energy Deficiency in Sport picture.² That framing matters because it keeps the focus on the cause. The issue is inadequate fuel relative to demand.

What the research says

This problem can show up fast. Fahrenholtz and colleagues ran a 14-day randomized crossover study in female endurance athletes and found that low energy availability impaired performance in two exercise tests and raised cortisol by 22%, with some performance cost still present after three days of refueling.³ That is short enough to matter during a heavy block, a training camp, or a poorly planned cut.

The muscle side matters too. Areta and colleagues found that 10 days at 25 kcal per kg fat-free mass per day reduced both myofibrillar and sarcoplasmic muscle protein synthesis compared with 50 kcal per kg fat-free mass per day in trained women.⁴ In plain language, the body became less willing to build and repair tissue even though protein intake inside the trial was controlled.

Male athletes are not exempt. Hooper and colleagues summarized evidence showing that male endurance athletes with low energy availability had lower testosterone and poorer training support than inactive controls.⁵ That helps explain why testosterone, cortisol, and training output often move together in poorly fueled endurance or hybrid athletes.

How it shows up in real training

Low energy availability rarely starts as a dramatic crash. It usually starts as drift. Long runs feel flat. Threshold sessions get worse at the same effort. Resting irritability rises. Glycogen feels perpetually low. Recovery between sessions stretches out even when recovery time on a wearable looks normal.

What to do about it

The fix is almost never a supplement first. It is more food in the places where the deficit actually formed. That often means more carbohydrate around training, fewer fasted sessions, better post-workout nutrition, and a smaller or shorter fat-loss phase.

Start with training days. Add fuel before and after the work that is currently going badly. If the athlete is an endurance athlete, total carbohydrate is usually the first miss. If the athlete is dieting hard, total energy and protein together may both be low. If appetite is poor, liquid calories or lower-volume meals can do more than another lecture about food quality.

The common mistake is waiting for visible weight loss or obvious illness before acting. Low energy availability can damage training quality long before the mirror changes. Keep glycogen, iron levels, cortisol, and recovery time in the same frame, because the pattern matters more than any single symptom.