Research · Injury & Recovery

Most Young Athletes Who Are Underfuelling Are Not Dieting

By Muhammad Dzulhisham · MSc Sports Coaching, NSCA-CSCS

When a coach hears that a young athlete is underfuelling, a specific picture arrives: a teenager, usually a girl, deliberately eating less to be lighter. That picture drives the response — watch for dieting, watch for weight loss, refer if you see it.

It is the wrong picture most of the time, and a study of 464 female adolescent athletes aged 13 to 18 shows how wrong.¹

The finding that should change the screening question

Athletes were classified as having clinical indicators of low energy availability if they met criteria for primary or secondary amenorrhoea, or for clinical underweight status. Thirty athletes — 6.5% — met that bar. Then the researchers checked whether those thirty looked like the picture above.¹

Twenty-four of thirty were not restricting on purpose. The authors call this the inadvertent pathway to low energy availability, and their conclusion is that it needs nutrition education rather than eating-disorder screening.¹ A child who has added two training sessions a week and not changed what they eat has arrived at the same physiological place as a child who is dieting, by a route no behavioural questionnaire will catch.

The bone numbers in that same cohort are the reason this is not merely a performance issue. A lumbar spine Z-score averaging −1.30 in adolescence is a problem being created during the window when most lifetime bone mass is laid down.

What the syndrome actually covers

The 2023 International Olympic Committee consensus statement on Relative Energy Deficiency in Sport is the current reference point, and it is worth noting how much the field has moved: the panel reports more than 170 original research publications advancing REDs science since the previous 2018 statement.²

Three things in that statement matter for anyone coaching young athletes. First, REDs is explicitly a syndrome affecting female and male athletes exposed to low energy availability — inadequate energy intake relative to exercise energy expenditure. Second, the 2023 update added emerging evidence on the role of low carbohydrate availability specifically, further evidence of the interplay between mental health and REDs, and more data on the impact of low energy availability in males. Third, the panel produced updated Health and Performance Conceptual Models plus a new Physiological Model, designed to represent the difference between problematic and adaptable exposure to low energy availability, moderated by individual factors.²

That last distinction deserves emphasis because it is frequently lost. Not every period of low energy availability is pathological. The consensus frames outcomes as depending on severity, duration and individual moderating factors — which is why a clinical assessment tool with risk stratification exists, and why a single questionnaire score is not a diagnosis.

On the male side, a review of low energy availability in male endurance athletes reports associations with reduced testosterone, reduced bone density and lowered resting metabolic rate.³ The condition is not sex-specific, and a screening habit built solely around menstrual function will miss every affected boy in the squad.

Why the prevalence numbers disagree so violently

Anyone reading around this topic will hit wildly different figures, and the reason is methodological rather than mysterious.

The second figure comes from a randomised controlled trial in 83 female endurance athletes aged 15 to 18 training more than ten hours a week, where baseline low energy availability by questionnaire was 63.8%.⁴ These are not contradictory findings. One counts athletes who have already crossed into clinical consequences; the other counts athletes screening positive for risk. Quoting either as the prevalence of REDs is where the confusion starts, and it is worth being precise about which one is being cited.

The one intervention with trial evidence

That same trial is the most directly useful piece of evidence for a coach, because it tested something a club could actually do. Eighty-three athletes were randomised; 45 received six nutrition education lectures from a registered dietitian, and 38 received none. Everyone was reassessed at six months.⁴

In the intervention group, energy availability rose, sports nutrition knowledge scores rose, and LEAF-Q scores fell, all significantly. Energy intake, weight, fat-free mass and resting metabolic rate all increased significantly. Eating attitude scores did not differ between groups.⁴

Read those two sentences together and they corroborate the first study from the opposite direction. Six lectures changed how much these athletes ate and what their bodies did with it, without changing their eating attitudes — because attitude was not the problem. Knowledge was.

What this changes in practice

Screen for the training change, not just the eating behaviour. The most common route into low energy availability is a volume increase with no corresponding increase in intake. Preseason, squad promotion and the start of an exam-year training block are the moments to ask.

Ask boys too. Reduced testosterone, bone density and resting metabolic rate are documented in male endurance athletes. A protocol organised around menstrual function is a protocol with a blind spot.

Treat leanness sports as a higher-prevalence setting. Clinical indicators appeared at 10.9% in leanness sports against 2.1% elsewhere — a five-fold difference within the same study.

Education is a legitimate intervention, not a soft option. Six sessions produced measurable physiological change in a randomised trial. That is a stronger evidence base than most things clubs spend money on, and it starts with the basics of what and when young athletes eat rather than with supplements.

Be careful which prevalence figure you quote. A questionnaire-positive rate near 64% and a clinical-indicator rate of 6.5% describe different things. Conflating them either panics parents or, worse, makes the whole topic easy to dismiss.

Where this evidence stops

The 464-athlete study is cross-sectional, so it cannot establish that low energy availability caused the lower bone density rather than both sharing a common cause, and the bone measurements came from a subset of 209 rather than the full sample. Its classification of low energy availability rested on amenorrhoea or underweight status — specific, but insensitive, and it will have missed affected athletes who were menstruating and of normal weight.

The education trial is small at 83 participants, ran for six months, was conducted at a single site, and measured energy availability partly through the same questionnaire it was teaching athletes about — a route by which knowledge gains could inflate the apparent effect. It is a promising result, not a settled protocol.

And the diagnostic boundary matters here more than usual. Amenorrhoea, stress fractures and persistent fatigue in a young athlete need medical assessment; REDs has a clinical assessment tool and a risk-stratification process for good reason. What a coach can reasonably own is the part the evidence points at most clearly — noticing when training went up and food did not.