Trying to get fitter? You might actually need to eat more
- Published
Trying to boost your fitness? It's just as important to look at how much fuel you're getting alongside being consistent with exercise.
Professional trail runner and GP, Dr Elsy Davis, external learnt this the hard way, discovering that eating too little of the right foods was negatively impacting her ability to train.
Working with sports dietitian Renee McGregor, external, she's now found the right balance.
Why under‑fuelling harms your body
Under-fuelling is when your total energy intake isn't enough to meet your needs and perform your best.
McGregor says it most commonly refers to not having enough carbohydrates – the nutrient that's regarded as 'fuel for fitness'.
Despite following a 'healthy' diet, Davis was a victim of this. Fitting in intense early morning training around long hospital shifts made it hard to prioritise the extra nutrients she needed.
Sporadic breaks also made it hard to know when her next snack or meal might be.
This led to repeated stress fractures, tiredness and decreased performance.
She was then diagnosed with the serious medical condition RED-S (Relative Energy Deficiency in sport), external.
What RED‑S is and why it matters
"RED-S is when there's not enough energy to support all movement and biological processes," says McGregor.
"It's further impacted by internal stress on the body, such as not fuelling enough, losing weight rapidly and training at too high intensity with little or poor recovery."
It can lead to other health issues too.
"Often we'll see low iron levels in individuals who are in RED-S," says McGregor.
"Under-fuelling can also lead to disruptions in female hormones which can lead to irregular or even loss of periods.
"If you don't have regular menstrual cycles, this starts to have a negative impact on other areas of your health and can lead to injuries such as stress fractures, external."
Davis says losing your period was historically considered the norm in athletes:
"It used to be seen that if you lost your period, it meant you're training hard. Now, we know, losing your period is a key sign of under-fuelling. It's a sign you need to pull back."
Why everyday exercisers are also at risk
McGregor has seen under-fuelling, which underpins RED-S, in all types of individuals, from everyday gym users to personal trainers, football players, swimmers, ballet dancers and those that follow very restrictive diets.
Females are more vulnerable to it, primarily due to the body's requirement to protect reproductive functions.
"Some people think that if you're eating regular meals that should be enough to carry you through. The reality is, if you're training every day, you're constantly using up your glycogen stores."
Glycogen is the body's stored form of glucose, which is our main source of energy and comes from carbohydrates in food such as potatoes, rice, or bread as well as in fruits and juices. And it's not just needed for exercise.
Everything from brain function, external to adrenaline, external (crucial on race day) is reliant on us having enough glucose.
Air fryer baked potato | Save to My Food now
The carbohydrates in potatoes provide the body with glucose, which gives us energy
The warning signs you shouldn't ignore
Davis explains how her under-fuelling snuck up on her: "It's subtle and you might just put it down to tiredness from training. You need to know what's normal and when you're pushing too hard."
Signs of under-fuelling include:
Chronic fatigue and aches
Lack of progression in training
Recurrent illness, such as coughs and colds
Sleep disturbance
Low mood and motivation
Irritability
Changes in the menstrual cycle
How eating more improved performance and recovery
In a social media video, external, Davis described how gaining 3kg enhanced her health and athletic performance.
It lowered her heart rate, improved her sleep and recovery, strengthened her fitness and motivation to train.
"I made simple changes and fully focused on fuelling," says Davis. Those changes were:
Ensuring her diet was balanced, including all major food groups
Being organised with meal prep
Re-fuelling after running with a protein shake or chocolate milk within 20 minutes, external to replenish glycogen stores and provide protein for muscle repair
Stopping relying on hunger cues, because excessive exercise can suppress the hormone, external. that makes you feel hungry
Increasing meal portions and adding extra carbohydrates as part of pre- and post-running snacks
Chocolate protein milkshake recipe | Save to My Food now
This milkshake was developed with nutritionist to the Red Roses, Aimee Ellen O'Keeffe.
As well as fewer injuries, Davis "noticed positive benefits of fuelling more within weeks".
"Despite training less and gaining weight, I've noticed my fitness has flown up. I'm in a much better place mentally and have much more motivation to train."
If you are experiencing any under-fuelling symptoms, McGregor encourages people to seek help from a health professional.
If you have been affected by disordered eating, help and support is available.
Originally published April 2024. Updated February 2026.
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