RED-S: When Athletes Underfuel Their Training

Training volume goes up and fitness stalls or slides backwards. Sleep is poor, injuries arrive without warning, libido has gone and a head cold turns up every other week. Before adding another volume block, there is something simpler to check: whether the body is getting as much energy as it spends. That is the RED-S question.

What RED-S is

RED-S, or Relative Energy Deficiency in Sport, is a model described by an expert panel of the International Olympic Committee. It was first proposed in 2014, and the consensus statement was updated in 2018 and 2023.

The core of it is this. When energy from food does not cover the sum of training expenditure and basic bodily functions, the body starts economising. It cuts where it can survive the cut: hormones, bone, immunity, recovery. The issue is not looking lean. It is energy availability, meaning how much fuel is left over to run the body once training has been paid for.

RED-S is not a women’s problem. The earlier female athlete triad concept was broadened precisely because the same mechanism hits men. Estimates put the risk at 15% to 70% of male athletes and 23% to 80% of female athletes across various sports. Triathlon, with its high energy expenditure, sits on the front line.

The threshold worth knowing: 30 kcal/kg

The most commonly used threshold for low energy availability in the literature is 30 kcal per kilogram of fat free mass per day. Below that value the risk of physiological disruption rises.

The arithmetic is simple. Take daily calorie intake, subtract the energy spent on training, and divide the result by fat free mass. Take a triathlete with 60 kg of fat free mass who burns 800 kcal in a session and eats 2400 kcal. That leaves (2400 minus 800) / 60, so about 27 kcal/kg. Already below the threshold, and it required no starving. A few consecutive days of underfuelling after long sessions is enough.

How to spot a deficit

RED-S hits several systems at once, which is why the symptoms can mislead. According to the IOC statement, energy deficiency impairs metabolic rate, menstrual function, bone health, immunity, protein synthesis and the cardiovascular system, among others.

Signals that should raise a flag:

• In women, absent or irregular menstruation. In a cross sectional study of Czech endurance athletes at risk of low energy availability, amenorrhoea affected 33.3% of those studied.
• More frequent infections. The IOC recorded more upper respiratory tract infections and gastrointestinal complaints with low energy availability.
• Overuse injuries, stress fractures in particular, as the effect of weakened bone.
• Falling libido, worse mood, trouble sleeping. These are signs of hormonal disruption.
• Stagnation or regression in fitness despite rising training volume.

Where several of those points appear together, adding training will only deepen the hole.

How to protect against it

First, eat around training rather than only in the evening. The dangerous pattern is the multi-hour window in which a lot is burned and nothing is replaced. After a long run or ride, the meal should not be pushed back.

Second, plan carbohydrate against volume. The longer and heavier the week, the more fuel has to cover it. An energy deficit is not a strategy for better form, it is a direct route to RED-S.

Third, treat recurring infections, injuries and cycle disruption as data rather than bad luck. They are often the first measurable signs that the energy balance is negative.

Fourth, and this matters most: anyone who suspects RED-S should consult a sports physician or a dietitian. This is not something to diagnose alone off the internet. Only blood work and a specialist assessment show how deep the problem runs and how to get out of it.

More on this: How to Raise VO2max and What to Expect. Racelify lists triathlons worldwide with dates and organiser links.