Is My Swimmer Eating Enough? Under-Fuelling and RED-S, Explained for Swim Parents

Swimpros runs Europe's most popular performance swim camp right now, and if I could get parents to worry about one nutrition thing, it would not be sugar or supplements. It would be this: is my swimmer actually eating enough? For a growing teenager training 15 to 20 hours a week, the honest answer is often no.

I am David Karasek, an Olympic finalist and Swiss record holder in the 200m IM. What follows is general guidance, not medical advice. If you recognise the warning signs below, please talk to a sports physician or a registered dietitian.

How much a training swimmer really needs

The energy demand of competitive swimming is genuinely enormous. Studies of heavily training adolescent swimmers have measured daily energy needs running into several thousand calories, and swimmers are one of the sports most prone to unknowingly running a big energy deficit. To fuel that load, sports nutrition guidance points to roughly 6 to 10 grams of carbohydrate per kilogram of body weight per day, alongside protein spread through the day and enough total food to also cover growth. The tank has to fill training, recovery, and a still-growing body.

What RED-S is, in plain language

When a swimmer repeatedly does not eat enough to cover both their training and their basic biological needs, the body starts shutting down what it treats as non-essential. Sports scientists call this low energy availability, and the wider syndrome is Relative Energy Deficiency in Sport (RED-S), defined in the International Olympic Committee consensus statement. It is not about an eating disorder (though it can overlap); most of the time it is simply an active kid who is not eating enough to keep up with their own training.

Under-fuelling does not look like a nutrition problem at first. It looks like a swimmer who just stopped improving.

Warning signs a parent can watch for

None of these on its own means RED-S, but a cluster is worth acting on:

• Performance: times stall or slide, recovery drags, constant fatigue in workouts.
• Health: getting ill often, injuries that heal slowly, and stress fractures in particular.
• In girls: periods that become irregular, stop, or never start by the mid-teens. This is a significant flag, not a normal part of training hard.
• In boys: the signs are subtler, but drops in strength, mood and drive can point the same way.
• Mind and sleep: low mood, irritability, disrupted sleep, or an increasingly rigid preoccupation with food or weight.

The long-term stakes are why this matters so much in the teenage years: chronic under-fuelling can blunt growth, weaken developing bones (raising the risk of low bone density later), disrupt hormones, and, of course, cap performance. Guidance here comes from the IOC RED-S consensus, the US Olympic and Paralympic Committee, and Sports Dietitians Australia, among others.

What to do about it

• Assume the default is under-eating, and add food first. More carbohydrate through the day, a real breakfast before morning practice, and a carb-plus-protein snack soon after hard sessions.
• Frame it as fuelling an athlete, never as dieting. A growing competitive swimmer is the wrong person to be cutting carbs or calories.
• If you see a cluster of warning signs, get help. A sports physician or registered dietitian can assess properly. This is not a wait-and-see situation.

We talk about daily fuelling and recovery habits with parents in our free Skool community, and you can watch how we build them into a swimmer's routine on our YouTube channel.

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