Low Energy Availability & RED-S

If you train regularly, eat “well,” and still feel like your body isn’t responding the way it should, this is something that should be on your radar.

Relative Energy Deficiency in Sport (RED-S) is a common thing I see in clinic and even though it’s more well known now than years ago, most people aren’t sure how to assess if it’s something that is affecting them. It can be tricky as it’s not extreme, not always intentional and it can affect the weekend warrior just as easily as it can affect an elite athlete.

What is RED-S?

RED-S occurs when the body doesn’t have enough energy available to support both exercise and normal physiological function.

It was once known as the Female Athlete Triad, which focused on:

Low energy intake
Loss of menstrual cycle
Reduced bone density

We now know this is only part of the picture. RED-S is now recognised as a clinical syndrome that affects:

Both men and women
Multiple systems across the body
Athletes at all levels including recreational sports people to elites

How common is it?

Low energy availability is common in athletes, especially in endurance, aesthetic, and weight-sensitive sports, and it can affect both women and men:

In runners:

Up to 64% of younger female runners are at risk
Around 30% of master’s runners are still affected

It’s not rare and it’s often missed.

Who is most at risk?

Endurance athletes (running, cycling, triathlon) are the highest risk group, not because they’re doing anything wrong, but because their energy output is high and often underestimated.

Higher rates are also seen in:

Gymnastics, dance, bodybuilding
Competitive or high-performance environments
Athletes managing body composition closely

It affects men as well, it’s just less commonly identified.

Why it happens?

At its core, RED-S is about low energy availability.

There simply isn’t enough energy left over for the body once training is accounted for.

It’s often not dieting, it’s underfueling without realising it.

Common patterns I see:

Training increases, but food doesn’t adjust with it
Meals get skipped, especially earlier in the day & trying to play catch up
Diets are “clean” or “healthy” but low in energy and often low in carbohydrate
Appetite is suppressed from stress, caffeine or being busy
Relying on hunger cues that don’t reflect true energy needs
Underestimating how much fuel training requires

In some cases, it is more intentional and often driven by goals around body composition, performance, or simply trying to feel more in control of health.

It’s rarely extreme. More often, it’s subtle: eating a little less, being a bit more cautious with certain foods, or trying to “eat light” while still training hard.

The challenge is that even small, consistent reductions in intake can create a large deficit over time.

What’s happening in the body?

When energy availability is too low, the body starts to adapt to being provided with insufficient fuel. It starts to down regulate non-essential processes to conserve energy.

This is why RED-S is considered a clinical syndrome because it affects multiple systems at once.

It can impact:

Hormonal function, including reproductive and metabolic hormones
Menstrual function (Irregular cycles or loss of cycle)
Bone health (Increased risk of stress injuries/fractures, osteopaenia, osteoporosis)
Metabolic function (altered metabolic rate and energy conservation)
Immune system (increased illness frequency)
Gastrointestinal function (bloating, slower digestion)
Cardiovascular system (reduced performance capacity)
Haematological function (iron-related issues and oxygen delivery)
Psychological health (anxiety, low mood, reduced resilience)

This is why it can get missed, sometimes symptoms don’t seem connected, but they can all potentially link back to low energy availability.

Signs to look out for that could mean you need to check your fuel intake

The more obvious signs:

Fatigue that doesn’t match your training
Poor recovery
Getting sick more often
Loss of menstrual cycle or irregular cycles
Stress injuries/fractures

The more subtle ones:

Feeling wired but tired
Poor sleep despite being tired
Increased anxiety or low mood
Feeling cold more often
Digestive changes (bloating, slower motility)
Plateaued or stubborn body composition
Ongoing niggles or injuries

In men, due to no menstrual cycle as a marker.

Reduced libido
Lower testosterone
Decline in performance

How do you know if you’re under fueled?

There is a clinical equation called energy availability where I look at your energy intake, exercise expenditure and your fat free mass. But in practice, you don’t always need to calculate it.

Red flags I look for:

Regular fasted training
Long gaps between meals
Low carbohydrate intake relative to training
Food groups removed from diet or referencing to “good/bad” foods
Increasing training without increasing food
Persistent fatigue + poor recovery
Loss of interest in training

The most common pattern? Chronic, low-level under fueling over time.

Why fixing RED-S isn’t just “eat more” and requires a multifaceted approach.

We need to consider:

Nutritional – Increasing total intake and improving timing (especially around training)
Training load – working with trainers and adjusting volume or intensity where needed
Hormonal / physiological – Supporting recovery of normal function
Psychological – addressing beliefs around food, body composition or performance

The approach will be slightly different for everyone.

You don’t need to overhaul everything at once.

Start with the foundations:

Eat consistently (as an athlete don’t rely on appetite alone)
Fuel before and after training
Include carbohydrates regularly, particularly around training sessions
Avoid long gaps without food
Match intake to output

From there, it becomes more individual.

Final Thoughts

If you’re training regularly and just not feeling right, it’s worth looking at how well you’re fueling, not just how hard you’re training.

If you’re not sure whether you’re fueling enough for your training, this is something we can work through together because often it’s not about doing more, it’s about supporting your body better.

BOOK CONSULTATION HERE

Reference List

Australian Sports Commission. (2025). Energy availability. https://www.ausport.gov.au/ais/nutrition/energy-availability
Coast Sport. (2025). Relative energy deficiency in sport (RED-S). https://www.coastsport.com.au/conditions/relative-energy-deficiency-in-sport-red-s/
Parkview Health. (2024, January 17). How to identify and address low energy availability. https://www.parkview.com/blog/how-to-identify-and-address-low-energy-availability
Physio Network. (2024, July 13). Understanding relative energy deficiency in sport (RED-S). https://www.physio-network.com/blog/relative-energy-deficiency-in-sport/
PubMed. (2022, April 2). Is RED-S in athletes just another face of malnutrition? https://pubmed.ncbi.nlm.nih.gov/35331505/
PubMed. (2023, March 29). Prevalence of relative energy deficiency in sport (RED-S) risk in Malaysian national athletes. https://pmc.ncbi.nlm.nih.gov/articles/PMC10096906/
Swimming Australia. (n.d.). RED-S: Relative energy deficiency in sport. https://www.swimming.org.au/performance/athlete-resources/ignite-athlete-program/red-s-relative-energy-deficiency-in-sport