Relative Energy Deficiency in Sport (RED‑S) is a clinical syndrome resulting from insufficient energy availability relative to the energy expended in exercise.

It extends the concept of the Female Athlete Triad to include males and broader physiological consequences.

Energy Availability (EA)

Energy availability is the central measurable concept in RED‑S. It’s defined as the amount of dietary energy remaining for basic physiological functions after the energy cost of exercise has been subtracted. Formally:

where:

• $EI$ = energy intake (kcal/day)
• $EEE$ = exercise energy expenditure (kcal/day)
• $FFM$ = fat‑free mass (kg)

Thresholds commonly used in the literature are approximate: $EA < 30\ \text{kcal/kg FFM/day}$ is considered low and associated with impaired physiological function, whereas ~$45\ \text{kcal/kg FFM/day}$ is often cited as an estimate for adequate availability for most athletes.

Example calculation: an athlete with $EI=2000$ kcal, $EEE=800$ kcal, and $FFM=45$ kg has

which would indicate low energy availability.