RED-S: Are you at risk?

We all have days when our schedules get the better of us and we end up missing meals or not getting enough to eat overall. But if underfueling becomes the norm (intentionally or unintentionally), you might be at risk of developing Relative Energy Deficiency in Sport (RED-S). Let’s look at how this happens, what the warning signs are, and how an energy deficit can impact your performance, recovery and health.

Low Energy Availability (LEA) & RED-S

Finding yourself in a calorie deficit occasionally probably isn’t going to be a problem, if you recognize it and make changes. But when you consistently have insufficient energy intake or the amount of energy you are expending is far greater than your intake, or both you might have low energy availability (LEA) which could lead to RED-S. According to a consensus statement by the International Olympic Committee, “LEA, which underpins the concept of RED-S, is a mismatch between an athlete’s energy intake (diet) and the energy expended in exercise, leaving inadequate energy to support the functions required by the body to maintain optimal health and performance.”

As a result of this mismatch between how much energy your body needs and how much you’re giving it, you can begin experiencing health issues, a performance drop-off, and inadequate recovery, as your body struggles to find ways to get the energy it needs for basic functions while also fueling your training and recovery. The IOC paper defined RED-S as “impaired physiological functioning caused by relative energy deficiency, and includes but is not limited to impairments of metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health.”

What is energy availability?

One of the ways that registered dietitians and other practitioners try to identify whether someone is suffering from RED-S or a similar condition is by calculating their energy availability. Calculating food energy intake minus exercise energy expenditure equals energy availability. The leftover ‘energy availability’ must be enough energy for your body to fuel basic physiological processes.

For many athletes, they need to be consuming 45 kcal/kg FFM (fat-free mass) at baseline + recouping any energy needed for training. An energy availability of 30 kcal/kg FFM or below, is called ‘low energy availability.’ This means the amount of calories you’re consuming cannot support your daily energy needs plus the fuel your body requires during and after exercise. The deficit compromises body systems that need fuel, which start to shut down to preserve energy and keep you alive.

Calculating your exact FFM is difficult and not accessible to most people. FFM is measured or estimated by dietitians or medical professionals – so if you are concerned in this area, please reach out to your sports dietitian or physician.

Are athletes more susceptible to RED-S?

There’s a reason that the S in RED-S stands for sport. Whether you’re a runner, cyclist, triathlete, or dabble in multiple disciplines, the more active you are, the greater number of calories you’ll burn. It isn’t just about your energy needs during training either. Your body needs extra fuel to support recovery and lean muscle mass is more metabolically demanding. Plus, there are many energy-dependent processes your body and brain need to perform to keep you functioning. Overlooking these factors is why some athletes drastically underestimate their fueling needs and can slip into chronic energy deficiency. Studies on runners suggest LEA (low energy availability) is most prevalent with high training loads, high energy expenditure, and low energy intake. A sudden increase in mileage and/or intensity, changing nutrition choices or restricting new foods, lack of hunger cues or decreased appetite with higher training loads, and competing in more races could all put an athlete at greater risk for RED-S if they are not conscious of proper fueling.

Why are female athletes more at risk for developing RED-S?

While chronic energy deficiency is more prevalent among women, male athletes can suffer from RED-S (see recent Ryan Hall post). Reasons can include societal pressure to look a certain way as a runner or endurance athlete, advice from a coach that leaner improves race times, lower caloric intake to begin with, disordered eating, lack of knowledge on how to fuel your body for your training load, and an energy deficit from pursuing a new dietary pattern and restricting foods or times of nutrition.

RED-S & Eating Disorders

It’s a misconception that only people who are battling eating disorders struggle with RED-S. RED-S isn’t always paired with disordered eating, fear of consuming too many calories, or other unhealthy perspectives toward food. One of the most common causes I see among my clients is unintentional underfueling – that they simply underestimate the amount of fuel they need to power their performance and recovery, while also meeting their daily energy needs.

Why can it be hard to spot RED-S?

Sometimes, RED-S presents several warning signs that pop up like an engine light in your car. However, researchers from the University of North Carolina found that when observing male athletes with RED-S,  “participants displayed no major hormonal or bone health disturbances” in the short term. This means that it could be harder to pinpoint the condition in men than in women, who often start to experience changes in their period and/or stress fractures (more on the long-term effects of RED-S to come in part two). That being said, an article by Brown University stated that “This often unrecognized disorder can include low energy availability (inadequate caloric intake); with or without disordered eating; amenorrhea (lack of menstrual periods); and low bone mineral density.” In other words, RED-S can go undiagnosed in women too.

What are the short-term symptoms of RED-S?

When red flags do appear, they typically impact performance and recovery first, and then begin to extend into overall physical, mental, and emotional wellbeing. Common symptoms of RED-S include:

  • Poor performance (such as inability to hit or hold paces, quicker time to exhaustion, and increased perceived effort)
  • Delayed recovery 
  • Decreased power and strength output
  • Chronic fatigue
  • Feeling heavy, lethargic, and low on energy
  • Loss of bodyweight and muscle mass
  • Sleep disturbances
  • Nagging injuries
  • Disrupted menstrual cycle and/or missed period
  • Emotional volatility and irritability 
  • Change in appetite
  • Delayed recovery
  • Lower heart rate variability (HRV)

Check back soon for part two in this series, when we’ll explore what you can do to reverse RED-S (hint: it might be easier than you think, and you CAN come back from this!).

 1.Margo Mountjoy et al, “International Olympic Committee (IOC) Consensus Statement on Relative Energy Deficiency in Sport (RED-S): 2018 Update,” International Journal of Sport Nutrition and Exercise Metabolism, July 2018, available online at 2.Amy R Lane et al, “Energy Availability and RED-S Risk Factors in Competitive, Non-elite Male Endurance Athletes,” Translational Medicine and Exercise Prescription, June 2021, available online at 3. “Relative Energy Deficiency in Sport (RED-S),” Brown University, available online at

Disclaimer: The content in our blog articles provides generalized nutrition guidance. The information above may not apply to everyone. For personalized recommendations, please reach out to your sports dietitian. Individuals who may chose to implement nutrition changes agree that Featherstone Nutrition is not responsible for any injury, damage or loss related to those changes or participation.