Are your workouts feeling harder than usual? Do you find yourself exhausted even on days you should have plenty of energy? If you’re active and health-conscious but running on fumes, you might be underfuelling. Underfuelling means not eating enough (or not eating the right nutrients) to meet the energy needs of your exercise. In other words, your body isn’t getting the fuel it requires to power your training and recovery.

This can happen unintentionally – for example, if you’re increasing your training volume but not your food intake, or if you’re dieting without accounting for the calories you burn through exercise. The result: you’re left feeling drained, sluggish, and not quite yourself.

Underfuelling is more common than you might think, and it can affect anyone from casual gym-goers to endurance enthusiasts. It’s often a blind spot in our fitness journey – we focus on working out harder, but forget that food is fuel. Below, we’ll explore the key signs that you may be underfuelling, why this state can disrupt your performance (and health), and how to fix it with smarter nutrition.

1. You’re Always Tired

Persistent fatigue is one of the clearest signs your body isn’t getting enough fuel. This isn’t just post-workout tiredness — it’s a constant sense of low energy that affects your whole day.

When you’re not eating enough to support your training, your body doesn’t have the energy to recover or function optimally.

How to avoid it:
Make sure you’re eating regularly and enough, especially on training days. Carbohydrates are key here — they’re your muscles’ primary energy source. Skipping them can leave you drained before you even begin.

2. Your Performance Is Declining

Have your workouts started feeling harder? Is your endurance dropping, or your strength progress stalling?

Low energy availability can lead to depleted muscle glycogen stores — the fuel your body relies on during exercise — resulting in reduced performance, slower pace, and stalled gains.

How to avoid it:
Fuel up before and after workouts with a combination of carbs and protein. Your pre-workout meal helps energise you, while your post-workout meal helps repair and rebuild muscle.

 

3. Your Recovery Takes Longer Than It Should

Soreness that lingers for days. Muscles that don’t bounce back. A body that just feels off.

Without adequate fuel — especially post-exercise protein and carbohydrates — your body struggles to repair muscle tissue and recover efficiently.

How to avoid it:
A quick and effective way to recover is to include a quality post-workout shake. Form Performance Protein delivers 30g of plant-based protein with added curcumin and digestive enzymes to support recovery and muscle repair.

4. Your Mood, Sleep, and Immunity Are Off

Underfuelling doesn’t just affect your workouts — it can mess with your mood, disrupt your sleep, and leave you more susceptible to illness.

When your energy intake is too low, your body shifts into survival mode, reducing non-essential functions like hormone production, immune defence, and mood regulation.

How to avoid it:
Listen to your hunger cues. Eat nutrient-dense meals, including enough calories to support full-body function, not just exercise.

5. You Notice Physical Red Flags

Think brittle nails, dry skin, or unexpected hair loss. These signs may seem minor but can point to chronic nutrient deficits and hormonal disruptions caused by underfuelling.

How to avoid it:
Avoid extreme or restrictive diets. Aim for a well-balanced intake across all food groups, and supplement when necessary to cover any gaps.

Why Underfuelling Is So Damaging

Underfuelling sets off a cascade of physiological consequences. At its core, being underfuelled means your energy availability – the calories left for your body after accounting for exercise – is too low to cover all your needs. This forces the body to make trade-offs, prioritising basic survival functions and sacrificing areas like immune support, hormone production, and muscle repair.

Elevated cortisol, impaired thyroid hormones, reduced testosterone, and disrupted menstrual cycles are all documented effects of low energy availability, particularly among active individuals (Mountjoy et al., 2014). Performance also suffers: muscle glycogen becomes depleted, endurance drops, and strength gains plateau or reverse.

A study by Vanheest et al. (2014) demonstrated that sustained energy deficits in female endurance athletes resulted in reduced performance and compromised immune response. Even over just a few weeks, these effects can build up into fatigue, illness, or injury – all signs that your body is not recovering properly.

Muscle breakdown is another consequence. Without enough protein and carbohydrates post-exercise, the body lacks the resources to rebuild muscle tissue. Over time, this can lead to muscle loss and reduced metabolic function – ironically making it harder to maintain a lean, strong physique.

Carbs and Protein: Fuel for Performance and Recovery

Carbohydrates are your muscles’ primary fuel during exercise. They’re stored as glycogen, which is depleted during workouts – particularly in endurance and high-intensity training. Without sufficient carbohydrates, performance suffers. A review by Burke et al. (2011) highlights how low glycogen impairs training intensity and delays recovery.

Protein, meanwhile, is critical for muscle repair and adaptation. Consuming high-quality protein post-exercise enhances muscle protein synthesis and supports lean mass. Research by Phillips and Van Loon (2011) shows that timing protein intake around workouts is effective, but total daily protein intake is even more important for active individuals.

Together, these two macronutrients are the pillars of recovery. If your post-workout routine is lacking in carbs and protein, you’re likely missing the window to rebuild and restore.

How to Stay Properly Fuelled

Here are six practical strategies to support your training with better nutrition:

1. Plan meals around training: Include protein and carbs before and after exercise.
2. Use convenient fuel: Shakes and quick snacks can help fill gaps.
3. Eat enough carbs: They’re essential for energy and recovery.
4. Respond to your body: Hunger, cravings, and fatigue are all signs worth listening to.
5. Watch out for restriction: Diet culture can promote habits that do more harm than good.
6. Use quality supplements: Form Performance Protein is a great way to ensure you’re getting what your body needs post-workout.

Final Thoughts: Fuel to Feel and Perform Your Best

Underfuelling can sneak up on even the most dedicated athletes and fitness enthusiasts. But by recognising the signs early and making some smart changes to your nutrition, you can protect your energy, performance, and long-term health.

Remember: food is more than fuel — it’s your training partner. Prioritise recovery, respect your body’s signals, and support your goals with the nourishment they truly require.

Train smart. Fuel smarter. Feel stronger.

References:

• Burke, L.M., et al. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(sup1), pp.S17–S27.
• Mountjoy, M., et al. (2014). Relative Energy Deficiency in Sport (RED-S). British Journal of Sports Medicine, 48(7), pp.491–497.
• Phillips, S.M. and Van Loon, L.J. (2011). Dietary protein for athletes. Journal of Sports Sciences, 29(sup1), pp.S29–S38.
• Vanheest, J.L., et al. (2014). Ovarian suppression impairs sport performance. Medicine and Science in Sports and Exercise, 46(1), pp.156–166.