RED-S: What Relative Energy Deficiency in Sport Means for Triathletes

Three-discipline training volume, a culture of tracking calories and “training load”, and a sport that rewards leanness in some disciplines make triathletes a genuine risk group for RED-S — Relative Energy Deficiency in Sport. It’s a condition that’s often missed because it doesn’t look like a single injury; it looks like a run of bad luck.

What RED-S Actually Is

RED-S describes what happens when an athlete’s energy intake doesn’t cover the cost of exercise plus the energy the body needs for everything else — hormone production, bone maintenance, immune function, tissue repair. The technical term is low energy availability: energy intake minus exercise energy expenditure, relative to fat-free mass. When that number stays too low for too long, the body starts deprioritising “non-essential” functions to protect what’s left.

The International Olympic Committee first published a consensus statement on this in 2014 (as an expansion of the older “Female Athlete Triad” model, since the same underlying mechanism affects men too), and updated it in 2018 and again in 2023 as more research came in. Estimates of how common it is among elite endurance athletes vary hugely — from roughly 15% to 80% depending on the study and sport — which itself tells you how often it goes unrecognised by athletes, coaches and even clinicians.

Why Triathletes Are a Particular Risk Group

High weekly training volume across three disciplines makes it genuinely easy to under-eat relative to output without noticing, especially during heavy block-building or race-season training.
Sharp jumps in volume — the classic base-to-build transition, or a sudden increase heading into a long-course build — are a specifically flagged risk period.
Long-course events without matching increases in nutrition compound the problem over months, not days.
A sport culture that often equates leanness with performance can push athletes toward restriction that looks like discipline rather than a warning sign.

Warning Signs

Losing your period, or your period becoming irregular, if you’re a woman not on hormonal contraception that would explain it.
Recurrent injuries, especially stress fractures or bone stress injuries that keep coming back in different locations.
Unexplained fatigue or a performance plateau/decline despite consistent, well-structured training.
Getting ill more often than usual — colds, infections, slow-healing minor injuries.
Poor recovery between sessions that used to feel manageable.
Low libido, mood changes, or difficulty concentrating.

No single sign on its own confirms RED-S — plenty have other explanations — but two or three together, especially recurrent stress fractures plus a missed period or persistent fatigue, are worth taking seriously rather than training through.

What To Do If This Sounds Familiar

Don’t try to self-diagnose or self-treat by simply “eating more” without guidance — see your GP and ask specifically about RED-S or low energy availability, and if possible get a referral to a sports dietitian who works with endurance athletes. This isn’t a fitness problem to train around; it’s a medical one that needs a proper energy-availability assessment, and in some cases bloodwork to check hormone levels and bone density. Recovery usually means a genuine, often uncomfortable increase in fuelling alongside a reduction in training load for a period — there’s no shortcut that keeps full volume and fixes the underlying deficit.

Related Reading

This article is for general information only and is not a substitute for medical advice. If you recognise these signs in yourself, please speak to your GP.