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What Is Relative Energy Deficiency (Red-S) In Sport And How Does Under-Fuelling Affect Cyclists?

If you’ve been watching the news recently, you may have come across a story on Sky Sports on the epidemic of chronic underfuelling in cyclists, featuring an extensive interview with Forth Edge nutritionist Renee McGregor. It was also discussed in a feature in Cycling Weekly magazine with Forth Edge’s Chief Medical Officer and sports medicine expert, Dr Nicky Keay.

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February 20, 2020

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It’s a topic that’s been under-studied and under-reported for years, but is now – finally – coming to light. In this article Renee McGregor explains it’s importance.


“This is more than just wanting to be thinner.” Renee explains, “RED-S stands for Relative Energy Deficiency in Sport and it’s an imbalance between the energy being consumed [as food] for the work that an athlete is doing.”

“And when I say ‘work you’re doing’, that’s because there are a lot of athletes who will focus on the energy they need for their training. But [they] don’t take into consideration other energy needs, such as the biological processes that go on inside the body every day, or the energy needed for those who are commuting by bike, have an active job or a young family. It’s quite hard sometimes to appreciate how much energy you need!”


The problem arises when an athlete is chronically under-fuelling and therefore in a continuous energy deficit. RED-S isn’t just an issue within cycling either. Athletes across every discipline of sport have been reported as suffering many of the tell-tale symptoms of the condition, and Renee has treated many herself at her clinic.


Symptoms include feeling fatigued, being more prone to illness, irregular or lack of periods (amenorrhea) in female athletes, slower recovery, stress injuries or stress fractures and a drop off in performance.

“You might start to see that you’re not hitting your numbers. That you’re not performing as well and that you’re not recovering between sessions,” explains Renee. “Your sleep may become affected too, because we know that when the body isn’t getting enough energy, it will try and stay awake to search for food. When there’s not enough carbohydrate it affects serotonin production, which again can affect your sleep.”

These initial issues are indicators of a much more serious underlying issue within the body.

“When there’s not enough energy in the system, it starts to affect every single process in the body. The body knows this and thinks ‘I can’t do all the jobs I need to do with the energy I’m getting’. While it will always prioritise movement, other processes start to slow down. Your metabolism will first take a bit of a knock, then your hormones. It can affect your immune system, your gastro-intestinal system, your cardiovascular system – pretty much every single part of your body.”


Based on her experience and research, Renee believes that RED-S is incredibly common within the athlete population, and almost certainly under diagnosed. Part of the reason for this is a lack of understanding and knowledge by both athletes and trainers which is thankfully now shifting.

For years it was taken as normal that women who trained hard would have irregular or no periods. We now know that this is simply not the case, and instead indicates a lack of energy in the system.

“Often RED-S is the result of a lack of knowledge, a sudden increase in training load alongside a lack of appreciation of how much an athlete needs to fuel alongside it,” Renee explains.

“The other difficult thing about RED-S is that often what we find is initially people might actually see an improvement in their performance. We know that if you lose a couple of kilos you might be more efficient, but it’s not sustainable and that’s the key thing. You might not notice you’ve got RED-S immediately because you might see improvement; ‘Oh, I’ve done really well so I’ll just keep going’ or ‘when I was that weight, I was at my best,’ but it’s not sustainable and performance will start to decrease. It sets up a cycle of failure.”

The composition of food that’s consumed can also play a part. The prevalence of low carb, high fat diets, for example, can be an issue for some athletes in that while they may be getting sufficient calories, the lack of carbohydrate means the calories consumed aren’t in the best form to support the body’s needs.



Renee refers to this type of RED-S, borne of unintentional under-fuelling, as involuntary RED-S. The athlete is not intentionally restricting energy intake, and often when RED-S is diagnosed and explained, the situation quickly resolves itself as the athlete adjusts their food intake.

However, there is another type that Renee refers to as Voluntary RED-S, and this is more difficult to address and a more complicated, serious issue.


“Voluntary RED-S is where someone is consciously restricting their energy intake. With this, you get psychological aspects, such as heightened anxiety about changing training or eating and worries about body image.”

“Voluntary RED-S is an eating disorder.”

In the example mentioned in the Sky Sports interview, a young cyclist was consciously restricting his food intake to try and achieve an ‘ideal’ cyclist weight and physique.

“The problem is that the culture within endurance sports such as running and cycling is that lighter makes you faster. The line between being light and optimal and being so light it affects your mental and physical health is fine.”

RED-S, Renee is keen to re-emphasise, isn’t a one- or two-day thing. It’s chronic under-fuelling, going on for weeks, months or even years, and when it comes to Voluntary RED-S, there are a number of triggers and contributing factors.

“Often individuals who have Voluntary RED-S have low self-esteem or self-worth, and the competitive nature of what they’re doing feeds into this – a constant need to prove your worth, to push through, to keep going. They may not know when enough is enough, and that’s when a coach could step in.

Blood tests are a good way to provide an evidence-based picture for the athlete on how RED-S is affecting their performance, and Renee also conducts assessments looking at their training, nutrition and mind-set to see if the person she is treating is likely to be restricting intentionally.”

She will then work with them to address the psychology behind disordered eating habits, which can be a long process. “These athletes often need a good support structure,” she explains.


The issue of RED-S isn’t just critical because it can negatively affect an athlete’s performance, the issues and consequences reach far further than that.

“There’s too much emphasis on performance and not enough emphasis on health within sport. We need to look after long term health,” Renee states, and this is an issue clearly close to her heart.

“We can’t forget about the fact that athletes are likely only going to be competing until their late thirties or forties at a push; what are they going to do afterwards if they’ve got poor bone health, problems with food and their whole identity is wrapped up with their sports performance?”


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