Athletes classified with low energy availability sat lower on body mass, BMI and bone density and higher on cortisol, while leptin did not separate the groups

What energy availability is

Energy availability is the energy left for the body's basic functions once the cost of training has been taken out of what was eaten, usually expressed per kilogram of fat free mass. It is the quantity that matters because the body appears to defend or downgrade functions such as reproduction, bone turnover and thyroid signalling based on what is left over rather than on total intake.

Drawn from background physiology, not from this paper.

Why they ran it

The authors note that when low energy availability falls below threshold levels it has been associated with declines in physiological functions such as bone health and endocrine function, and in turn with diminished physical capability in athletes. They set out to pool the existing comparisons of body composition and hormonal status between athletes classified as having low energy availability and those who were not, in order to identify which physiological characteristics travel with energy imbalance.

Drawn from the paper's introduction.

Eleven cross sectional studies met the inclusion criteria, together covering 1006 athletes, of whom 509 were classified as having low energy availability. The literature search covered January 2007 to July 2023 across seven databases.

Athletes in the low energy availability group tended to sit lower on body weight, body mass index and total body bone mineral density Z score. Several physiological markers were lower in that group, cortisol was higher, and leptin did not differ significantly between the groups.

The authors conclude that athletes with low energy availability may experience impairments in physiological function including alterations in gonadal hormones and bone mineral density markers, and frame the pooled picture as an association with adverse health outcomes rather than a demonstrated sequence.

The numbers

Studies pooled	11 cross sectional studies
Search window	January 2007 to July 2023
Athletes included	1006 total, 471 female and 535 male
Classified with low energy availability	509 of 1006
Leptin	no statistically significant difference between groups

Why this might happen

Open question The authors do not claim to know why.

The authors frame the pooled differences as alterations in gonadal hormones and bone mineral density markers accompanying energy imbalance, but the design they pooled cannot say whether low energy availability produced those differences, whether the body composition that defines the classification produced them, or whether both follow from something else.

The leptin result sits awkwardly inside any single account. Leptin tracks fat mass closely and would be expected to fall alongside the lower body mass the authors report, and it did not separate the groups here.

Drawn from the abstract and stated conclusions.

What this does not show

This does not show that low energy availability causes lower bone density or higher cortisol. Every one of the 11 pooled studies is cross sectional, meaning the marker and the classification were measured at the same moment in the same athletes. That design cannot order cause and effect, and the arrow could run either way or be driven by something shared by both.
This does not show that leptin is unaffected by low energy availability. A pooled result that fails to reach statistical significance is not evidence that two groups are the same. It remains compatible with a real difference the pooled comparison was not precise enough to detect, and no equivalence testing is reported.
This does not show that half of all athletes have low energy availability. The 509 out of 1006 figure comes from studies designed to compare a classified group against a comparison group. It reflects how the primary studies recruited, not how common the state is in any athletic population.
This does not give a threshold, a cut off or a marker anyone can screen with. The pooled comparison starts from classifications the original studies applied, and those definitions are not interchangeable across studies. Nothing here validates any single marker as a diagnostic test.

Where this leaves us

The physiological picture that travels with a low energy availability classification is now pooled rather than anecdotal, and it is a body composition and stress hormone picture rather than a leptin one.

Athletes in 11 cross sectional studies published between 2007 and 2023, roughly half female and half male.

Prospective work that measures athletes before and during a period of restricted energy availability, rather than comparing groups already classified, is what would turn this from a portrait into a sequence.

Caveats worth holding

The abstract reports the sex split as 49.4 percent female and 50.6 percent male, but 471 and 535 of 1006 are 46.8 percent and 53.2 percent. The 50.6 percent figure is the one that correctly describes 509 of 1006 classified with low energy availability, so the sex percentages appear to have been transcribed from that line. The raw counts are used here and the percentages are not.
The literature search closes in July 2023, so a paper published in August 2026 pools nothing from the intervening three years.
Only 11 studies were pooled, and the primary studies applied different definitions of low energy availability.
Several results are described in the abstract as directional tendencies without effect sizes or intervals attached, so they are not reproduced here as figures.
No open full text was reachable, so this rests on the abstract.