Underfuelling doesn’t just leave you feeling hungry. It can affect your strength, sleep, mood, recovery and even your immune system. Here’s how to recognise the warning signs before they derail your training
Ever walked on the gym floor and felt a little off? Lethargic, perhaps, or just struggling to focus? The signs of underfuelling — not giving yourself enough energy ahead of the task at hand — can be myriad and, in many cases, often end up being misinterpreted for something else entirely.
But when you’re struggling to concentrate, or simply don’t feel strong enough to lift your usual 5RM, for example, injuries can be just around the corner. Below, Men’s Fitness shares five key signs that you’re underfuelling your workouts and, crucially, the tactics to deploy when you need a top-up.
Low energy levels
Let’s start with the low-hanging fruit. If your energy levels are lacking, and you’re dragging yourself to the gym floor even after a good night’s rest, it could be a sign you’ve not taken on enough calories beforehand.
This is particularly common for those operating on a calorie deficit — eating fewer calories than you’re burning, a key part of an effective fat-loss strategy — but when you’re not taking enough calories on board, you’re neglecting your body of the fuel it needs to progress.
As a guideline, aim to take on one to four grams of carbohydrates per kilogram of body weight before your session. If you weigh 80 kg, for example, you should be consuming 80-320 grams of carbohydrates before your workout — depending, of course, on how strenuous the session is.
Your mood is off
If you’re feeling irritable and frustrated at how the weights are (or aren’t) moving during your workout, it’s another tell-tale sign that you’ve not taken enough nutrients on board. It’s an issue that the International Olympic Committee (IOC) correlated with a risk of depression, both as a consequence and a risk factor, creating a feedback loop as long as the habit continues.
A narrative review on the psychological effects of low energy availability found that the early signs aren’t dramatic — mood shifts, fatigue, a low-grade sense of being off — but that prolonged underfuelling tips into something more serious: reduced wellbeing, elevated anxiety and clinical depressive symptoms.
Run your body at an energy deficit for long enough, effectively, and cortisol climbs while testosterone drops. It’s a reliable formula for feeling terrible, you’ll agree.
RED-S alert
Relative Energy Deficiency in Sport (RED-S) sounds like something that only happens to elite marathon runners, but the research tells a less comfortable story.
Drop your calories too aggressively and bone formation begins to decline within five days. Keep that deficit running and the consequences stack up, with daily signs including persistent injury, stalled recovery and performance that ticks downward no matter how hard you train. It’s the kind of thing that gets mistaken for overtraining, but often, it’s just not eating enough.
You’re struggling with sleep
Can’t catch 40 winks? Before you reach for over-the-counter sleep supplements, consider taking a closer look at your diet. A study tracking 42 male rugby players using portable polysomnography devices — a medical monitor used to produce comprehensive sleep data — found that those with low energy availability had significantly worse sleep efficiency, spent less time in deep sleep and woke more frequently through the night compared to those eating adequately.
Nearly half the players were underfuelling without apparently knowing it. Poor sleep compounds everything else — including recovery, mood, performance and, testosterone — so underfuelling often makes itself look like several different problems at once.
Your immunity takes a hit
Getting ill more than usual — colds that linger, or bugs that floor you for a week when they’d normally clear in a few days — is one of the less obvious signs that your body isn’t getting enough fuel.
A 2024 randomised crossover study found that just 14 days of low energy availability was enough to significantly disrupt immune cell function, ramping up oxidative stress and altering the body’s inflammatory response. Three days of eating adequately afterwards wasn’t enough to undo the damage, either.
The caveat, however, is that underfuelling rarely happens in isolation. A combination of poor sleep, high training loads and chronic stress all hit immunity too, and they often show up together. But the direction of the evidence is consistent: chronically under-eat and your immune system will be one of the first things to feel it.
