Athletes are often taught to think about food as fuel for training. Yet in sports where body weight matters, that message can become distorted.

Eating less may initially make an athlete lighter and even appear to improve performance, but over time the body can begin sacrificing essential functions simply to keep up with exercise. This condition is known as Relative Energy Deficiency in Sport, or RED-S. It can affect athletes of any gender and at almost any level.

The Body Runs Out Of Energy

RED-S develops when energy intake consistently fails to match the demands of training and everyday life.

The body responds by prioritizing immediate survival and movement. Processes that seem less urgent, including reproductive hormones and skeletal maintenance, may begin to slow down.

Hormone-health specialist Dr Nicky Keay has explained that healthy hormone systems require a reliable energy supply. When the body repeatedly experiences an energy deficit, some hormonal pathways can reduce their activity or effectively shut down.

The consequences may include fatigue, poor sleep, low mood, repeated illness and stress fractures.

Men Can Miss The Warning Signs

RED-S was once discussed mainly in relation to female athletes because disrupted menstrual cycles provide a relatively visible sign of low energy availability.

In men, the warning signs are often less obvious.

Testosterone levels may fall, libido can decrease and fatigue may become persistent. By the time the problem is recognized, an athlete may already have developed serious skeletal injuries or experienced a major decline in performance.

Recent research involving elite and amateur athletes has suggested that low energy availability may be at least as common among men as women.

This makes the idea that RED-S is primarily a female problem particularly misleading.

Being Lighter Is Not Always Faster

Endurance sports have long carried the belief that a lower body weight automatically improves performance.

Running, cycling, climbing and other weight-sensitive sports can create pressure to become lighter, especially when small differences in speed or power matter.

British runner Jake Smith experienced this directly. After years of trying to reduce his weight while maintaining an intense training schedule, he developed repeated stress fractures and a severely weakened skeletal system.

His experience illustrates why early success can be deceptive. Athletes may continue performing well for a time even while their health is deteriorating underneath.

Keay has described this period as a misleading phase in which the apparent performance benefit eventually gives way to injuries and declining results.

Recovery Is Possible

The encouraging part is that RED-S can often be reversed.

Research involving competitive male cyclists found that athletes who received structured nutrition and training advice improved both skeletal health and race performance. Those who continued their previous habits lost further skeletal mineral density.

The goal is not simply to eat more at random. Recovery may involve increasing overall energy intake, restoring adequate carbohydrate consumption, adjusting training load and addressing any disordered eating patterns.

Medical and nutritional supervision can be important when symptoms are severe.

Sports Culture Is Starting To Change

Some sporting organizations are moving away from conversations focused primarily on weight.

British Cycling has increasingly framed nutrition as a performance issue, emphasizing that under-fuelled athletes cannot train, recover or compete at their best.

World Climbing has gone further by introducing health screening designed to identify signs of low energy availability before athletes compete at major events.

These approaches reflect a broader change: being lighter is no longer automatically treated as being better.

Health Has To Come First

RED-S challenges one of the most persistent assumptions in competitive sport: that suffering and restriction are simply part of becoming faster.

The body can tolerate an energy deficit for a while, but it eventually demands a price.

Athletes who experience repeated stress fractures, persistent fatigue, hormonal changes or unexplained performance decline may need to consider whether they are fuelling enough for their training load.

Strong performance depends on a body that has enough energy not only to move, but also to repair, regulate hormones and stay healthy between sessions.