What is RED-S (relative energy deficiency in sport)?

What is RED-S, in plain words?

The gap between energy eaten and energy needed is called low energy availability, and RED-S is the wider set of effects that gap can have on periods, bone, hormones, mood and performance. The deficiency is relative: it is measured against what a body is being asked to do, not against a fixed amount of food. You may also see it written as RED-S syndrome. US sources write under-fueling and call missing periods amenorrhea; UK sources write under-fuelling and amenorrhoea.

RED-S symptoms can each have other explanations, and a doctor can help sort them out one at a time. If a changing cycle is the main thing you have noticed, our page on irregular periods covers the wider list of possible causes.

What does the evidence say about RED-S?

A good place to start is the reassuring part. The 2023 IOC consensus statement on RED-S (Mountjoy et al., British Journal of Sports Medicine), the main reference for this condition, frames low energy availability as a spectrum, from adaptable (mild or brief, with benign effects) to problematic. RED-S is the concern when an athlete stays in the problematic range for too long or is too severely exposed, so a short, mild dip sits at the adaptable end.

The same statement is clear about periods. The evidence shows that missing periods while training hard is a sign of under-fueling, not a sign of fitness, and that a period is not something training is supposed to cost you. If you were once told that a lost period simply comes with serious sport, that advice has been overtaken.

US obstetrics and gynecology guidance on teenage periods points the same way: it treats the menstrual cycle in girls and adolescents as a vital sign, a piece of health information in its own right.

Bone is the reason the cycle gets this much attention. The teen years are when bone should be building fastest, and the consensus names bone as one of the things under-fueling affects at that age. For women past their teens, a period that goes missing during hard training is read the same way, as a prompt to look after bone. Bone changes cannot be felt day to day, which is what makes the cycle such a useful signal.

RED-S reaches beyond periods and bone. The same evidence links low energy availability to hormones and performance, which means eating less than training needs tends to work against the training itself. Fueling well and training well are on the same side.

It is natural to wonder whether a lighter or absent period matters. The research answer is that it is worth a gentle look, and that looking is about protecting strength, not about finding fault with how anyone trains.

How much energy is enough, and how often is it worth checking in?

Enough energy to keep regular periods is the guide in the evidence, and it is a guide you can read without counting anything. A period that arrives on its usual pattern is a visible sign that fueling is keeping up with training.

That marker earns extra attention when training steps up. A new season, a harder block or a second sport all draw more from the budget, and many women find that meals and snacks need to grow with the workload rather than stay where they were. Strength, health and how you feel are the aims here.

Fueling is more than one nutrient. When you train, a question many women bring is how much protein do I need, and that page sets out the research; protein sits alongside eating enough overall, not in place of it.

Each cycle is a natural moment to check in, with three simple things to notice: whether the period arrives, whether it has changed, and whether training load shifted around the same time. Many women find it helpful to jot these down month by month, and it's worth remembering that a change is information, not a verdict.

What do the studies show about RED-S, and what do they not?

The 2023 IOC statement is a consensus, meaning experts weighed the research and agreed on a model and a clinical approach; it is not one trial following athletes over time. It replaced the 2018 framework with a physiological model of how low energy availability affects the body, and with a revised Clinical Assessment Tool, the IOC REDs CAT2, which doctors use for diagnosis and for sorting levels of risk. The tool belongs to the appointment and is not something to fill in at home.

What the studies do not give is a personal prescription. They describe which way the effects run, on bone, hormones and performance, without setting a number that fits every body. The warning signs work as prompts for an individual assessment, not as a checklist to score yourself against, which is why two women with the same training week can leave an appointment with different plans.

Next steps

If your period has gone missing or turned irregular while you train, a conversation with a doctor or sports doctor (a GP in the UK) is a good place to start. A stress fracture, unexplained weight loss, or symptoms that are getting worse are reasons to make that appointment sooner rather than later. From there, some girls and women will find one conversation is enough, and others will want a fuller assessment over time. Take what fits your body, your training and your life, because this is not one size fits all.