I’m a Health Writer who lost her period due to underfuelling - 9 sure-fire signs you’re not eating enough

I think we can all agree that the world of nutrition has never been busier. Walk into any health food shop, scroll TikTok for 30 seconds, or listen to the latest wellness podcast, and you’ll be hit with a barrage of advice about adding more protein, eating for hormone balance, cutting out seed oils, or diversifying your gut microbiome. There's also noise on intermittent fasting paired with doctors warning you not to skip breakfast - confusing, right?

Though it's often well-intentioned, these messages make for a confusing landscape - and one which, if not trodden carefully, can lead you down a path of unintentional restriction.

As a Health Writer with a former career as a track and field athlete, you’d think I’d be across it all: fuelling my body “correctly”, ticking every so-called wellness box and training regularly. But the reality was quite different. You see, after years of underfuelling, my training caused me to lose my period at age fifteen. And over a decade later, I’m still battling to get it back.

My lost period was caused by a condition known as Relative Energy Deficiency in Sport, or RED-S. Also known as low energy availability (LEA), it occurs when the energy you burn through exercise outweighs the energy you take in through food. It’s a condition typically associated with professional athletes, but as dietitian Emily Vasquez, reminds us, “it can actually occur in anyone whose energy intake doesn’t meet the requirements of their energy expenditure.” With an ever-growing number of women taking up serious training (and yes, a marathon or HYROX competition is considered serious training), the risk of underfuelling has never been more of a mainstream issue.

While missing a period might seem like a convenient side effect of your hard training or busy schedule, it’s actually a major red flag from your body, and one with serious long-term implications. Research has shown that prolonged low energy availability can disrupt everything from metabolic rate and immunity to bone health and cardiovascular function, whilst the hormonal disruption caused by underfuelling has been shown in numerous studies to impact fertility, mood, and even gut health.

It’s very easy to fall into the trap of underfuelling, especially with so many confusing nutritional messages flying around, and you don’t have to be skipping meals or intentionally restricting either. While RED-S is often linked with disordered eating, it can also be due to not knowing how much you should be eating, not eating at the right time, being too busy to fit in a proper meal, or missing out on certain food groups.

Worried that this could be you? Read on to discover the signs that you’re not eating enough, plus what you can do to solve it, from RED-S specialist nutritionists and dietitians. After more expert nutritional advice? Check out this guide to the common nutrition myths a dietitian recommends avoiding, plus balanced nutrition tips they swear by. We've also got explainers on what to eat before a workout, the benefits of mindful eating, and intuitive eating. Recipe hunting? Scroll through expert-approved Em The Nutritionist's recipes, healthy breakfast ideas, healthy snack ideas, and healthy smoothie recipes, here.

9 signs you're not eating enough, according to top wellness experts, plus how it might be affecting you

1. Poor recovery, fatigue and frequent injury

“When we are under-fuelled, the body has evolved to prioritise sending energy to the brain, lungs, heart and movement in order to keep us alive and functioning,” explains Sports Dietitian, Renee McGregor. “As a result, the body starts to switch off processes that it doesn’t deem essential for survival. One of the first signs of this is poor recovery between training sessions, which you may notice as increased muscle soreness, greater general fatigue, and a higher resting heart rate.”

McGregor goes on to explain that poor recovery between sessions can have a knock-on effect. “By going into each session fatigued, you increase your risk of injury and put your nervous system into a state of high stress.”

“Persistent tiredness can be another strong sign that your body is under-fuelled”, says Hannah Cartwright, nutritionist and co-founder of The Nourishu Method. “If the body doesn’t even have enough energy (from glucose, fats and protein) to power basic cellular functions, it will be challenging for you to exercise, work, or even think clearly.”

So, how might this show up in our lives? Well, as Cartwright explains, “this can feel like brain fog, sluggishness, or needing to nap often. Increased energy is often one of the first things people notice when they begin fuelling adequately again, with even small things like walking up the stairs starting to not feel like such a massive mission."

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2. Performance plateaus

It’s not only recovery that’s affected by underfuelling, your performance can suffer too. “Think of it like driving a car with an empty tank,” explains Jonny Summer, specialist RED-S physiotherapist and clinical director at LDN Physio. “Athletes may notice intense fatigue mid-workout, dizziness, or an inability to hit usual performance markers.”

The danger, Summers explains, is that this drop in performance can cause us to train harder. “When you hit a new, poor performance baseline, it’s easy to feel pressure to start training more in the hope of improving performance, but this only causes further sinking into low energy availability. Instead, we need to focus on increasing calories, especially carbohydrates, to restore glycogen, whilst protein and fats will be critical for repair.”

McGregor also points out that you might not notice the body composition gains you were expecting from your training.“Instead of putting down more lean muscle mass as you would expect from a high training load, individuals with LEA may actually notice that their body actually holds onto more body fat because it is prioritising survival.”

3. Irregular or absent menstrual cycle

As someone who has lost her period, I’m always curious to understand what’s actually going on internally to have made them stop, which McGregor explains. “Placing the body under prolonged high stress, such as in the case of underfuelling, causes a communication network, known as the hypothalamic pituitary axis, to become down-regulated. This system is responsible for controlling our hormonal system, which is why we often see women experience changes to their menstrual cycle as a result of underfuelling.”

McGregor reminds us that it’s not just a total loss of periods which could indicate underfuelling. Any significant changes, such as more erratic cycles, lighter periods, as well as lengthened or shortened gaps between periods, could all indicate low energy availability.

“Missing just three consecutive periods can start to have a negative impact on bone health and thus puts individuals at higher risk of bone stress injuries, stress fractures, as well as recurrent ligament and tendon injuries,” she adds.

And as Vasquez points out, “it’s not just our bone health which is affected by missing periods. There’s also a number of other long term health effects, such as an increased risk of cardiovascular disease and infertility,” which is why it’s so important not to ignore the problem, and speak to a professional as soon as possible.

4. GI distress

According to McGregor, gut permeability (also colloquially known as a leaky gut) can be a consequence of low energy availability. “It’s common for individuals with low energy availability to experience more frequent bouts of gastric distress during activities, such as running. You may also notice IBS-like symptoms day-to-day, but this is actually caused by a phenomenon known as gastroparesis, or slow digestion. ”

Cartwright concurs. “It can feel ironic that someone who is under-eating would experience gut discomfort, and yet, slower metabolism means food can linger in the gut, causing bloating or constipation. Sometimes we cut out food, such as bread or gluten, because we’re struggling with IBS symptoms, when actually the problems are caused by under-eating, overexercise, high stress or food fears. Before anyone decides to eliminate a food group, I would always ask yourself if you could do with eating more, or eating more consistently.”

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5. Sleep disturbance

Good sleep is important for anyone, but especially those juggling exercise, work and busy social lives. And yet, “when someone is underfuelling, their sleep patterns are often disrupted,” says Cartwright. “A lack of essential nutrients and calories can make it harder to fall asleep, stay asleep, or sleep through the early morning hours.”

According to Cartwright, carbohydrates are particularly important when it comes to sleep because they “play a key role in the production of melatonin, our primary sleep hormone. For someone not eating enough carbohydrates, melatonin synthesis can become impaired, which in turn affects sleep quality.”

6. Mood swings, anxiety and low mood

“The brain is an energy-hungry organ, and requires so many omega-3 and omega-6 fats to regulate thoughts and feelings, to concentrate and to memorise,” says Cartwright. “Without enough fuel, neurotransmitters like serotonin and dopamine can’t be created effectively. You might feel anxious, irritable, or low for no obvious reason - this is your brain's chemistry being thrown off by lack of nutrients.”

Summers agrees. “One of the first signs that the body is under-fuelled is a shift in mood. We start to notice heightened irritability, as well as brain fog, as the brain scrambles for energy.”

7. Food fixation

We all know the friend who’s excited for breakfast before they’ve even gone to bed at night, but, as Cartwright explains, there’s a big difference between a love of food and food restriction. “If you find yourself constantly preoccupied with thoughts about food, recipes, or what you’ll eat next, this can be a biological response to restriction. The body’s hunger hormones ramp up, and your brain becomes hyper-focused on finding energy. This is a common sign of energy deficiency, and isn’t a lack of willpower.”

She reminds us that “when our body feels safe and knows food is coming consistently, our brain will naturally not need to constantly think about it, plan out meals, seek out food and obsess over calories.”

8. Feeling cold all the time

“Food provides energy not just for movement but for maintaining body temperature”, explains Cartwright. “When your body recognises it is underfuelled, your core temperature may drop as part of the prioritisation of survival. At the same time, we see the thyroid slow down in response to low energy availability, which in turn reduces metabolic rate and heat production.”

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9. Hair thinning or loss

And finally, if you notice hair thinning or loss, it could be a sign that you're under-fuelling. “Hair isn't vital for survival, so in times of energy deficit, nutrients like protein, iron, zinc, and biotin are diverted away from hair growth, causing shedding or thinning over time”, explains Cartwright.

Bottom line? Remember, even if you haven’t started to notice any of these signs, but still think you may be underfuelling, the best course of action is always to go and speak to your GP or, if possible, to a sports medical professional such as a RED-S specialist nutritionist or dietitian. This is because a lot of the signs of low energy availability take time to present themselves, as McGregor explains. “In a lot of cases, individuals actually often produce very good performances in the short term, but the issue is that the body cannot sustain this state, and as a result, the negative consequences of underfuelling start to occur after several weeks and months of low energy availability.”

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