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Dieting and Fatigue – The Relationship

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You’ve been working hard lately to overhaul your lifestyle. The weight has been coming off nicely and you’re starting to shape up pretty well.

But there’s one side effect that’s just beginning to take its toll – fatigue. You’re feeling more and more tired and the gym is starting to feel much more difficult to get to, let alone finish your training sessions.

What exactly is the relationship between dieting and fatigue? And is it something you can work around?

Or do you just have to suck it up and get on with it?

In this article we take a look. Here’s what we’ll cover…

  • Calories and fat loss
  • Why does dieting cause fatigue? – the relationship
  • How could fatigue slow down fat loss?
  • Ways to get back on track when fatigued

Calorie Restriction and Dieting

The key to any successful diet is an energy deficit – the number of calories you take into your body, minus the number you burn off.

It doesn’t matter whether you’re carb cycling, intermittent fasting, following keto or just clean eating, if you’re not in a deficit you won’t lose fat.

When you’re in a deficit, you’re forcing your body to use non-food sources of energy to make up the shortfall. In other words, you turn to stored energy from the fat on your body fat to make up the difference.

How does this happen?

Inside your fat cells are molecules called triglycerides; these are made up of glycerol and three fatty acids.

During a deficit, a process called lipolysis takes place. The fatty acids split from the glycerol component and travel in the blood to the mitochondria where they can be burned for energy. When this happens, the fat cell shrinks – a bit like a water balloon if you let a bit of water out. And the result is you lose fat and lean out.

Calories in vs calories out

It’s worth noting that although ‘calories in’ is fairly simple (it’s a combination of energy from carbs, fats and proteins that you eat), calories out is a little more complex.

There are a number of components that make up the calories you burn each day – or your total daily energy expenditure (TDEE).

  • Basal metabolic rate (BMR) – This is the calories you burn just to stay alive. You expend around 60-70% of your TDEE just to make your brain work, your heart beat and all of the chemical reactions to occur in your body.
  • Non-exercise activity thermogenesis (NEAT) – All unintentional activity such as walking, fidgeting, cleaning and tidying and so on. It accounts for up to 30% of TDEE.
  • Exercise activity thermogenesis (EAT) – this refers to your gym sessions, your sports competitions and your jogs around the park. Conscious exercise accounts for a relatively small 15% of TDEE
  • Thermal effect of food (TEF) – this rather tiny component of TDEE refers to the energy you use digesting food. It accounts for 5-10%.

Athletic woman in sportwear jogging on the spot on grey background

Long-Term Dieting Can Cause Fatigue and Tiredness

Less food means less nutrient going into your body and less energy availability.

You’re stuck between a rock and a hard place in a way because you need to sacrifice calories in order to liberate fatty acids from the fat cell. But the consequence is that you might not have as much energy as you usually have.

The result is that you can feel tired and lack that ‘get up and go’ attitude you normally have.

Fatigue can cause a drop in activity levels

When you’re in a deficit your body has to prioritise what to do with the calories that are coming in.

It makes sense that you feed your internal organs first – your heart, brain, liver and so on come first – your BMR. And then from there you use whatever energy is left to get you through your day.

And if you haven’t got enough energy coming in you’ll sacrifice all of the more adaptable components of TDEE – namely your EAT and NEAT.

The result of this is that you move around less. And this can cause your fat loss to slow down…

Why Does Fat Loss Slow Down?

You’ve been tracking your calories and you know you’re eating the right amount. But for some reason you’re just not losing fat.

Why is that? 

Many people believe you get to a point on a diet where you hit starvation mode – where your body gets so concerned about how few calories are coming in that it slows down your metabolism to reduce calorie output. All of a sudden you stop losing weight because your body is ‘clinging’ onto whatever energy it has left.

Many people believe that starvation mode can cause your metabolic rate to plummet, ruining your diet. And some even say that starvation mode can cause you to gain weight in a deficit as well.

Starvation mode is a myth…

The problem with the starvation mode theory is that this goes against everything we know about the thermodynamics of fat loss.

Here’s the key point… your weight loss won’t stall because of starvation mode – it stalls because your body isn’t being as active because of fatigue.

On top of that, as you lose fat and become leaner. You’re then decreasing your body size and the result is your calorie needs become lower. 

It’s a simply matter of maths, not some underlying evolutionary safety switch.

Studies back this up to. For example, Martin et al [1] published a study in Obesity that found weight loss during dieting slowed down not just because of a drop in BMR, but because the volunteers moved around less.

And a similar study in The New England Journal of Medicine [2] reported that “maintenance of a reduced or elevated body weight is associated with compensatory changes in energy expenditure, which oppose the maintenance of a body weight that is different from the usual weight”.

…but fatigue does reduce calorie output

Remember earlier, when we said that fatigue can cause your activity levels to drop. That’s all that’s happening when weight loss slows during a diet – fatigue has resulted in a drop in activity levels and that has caused a smaller-than-expected calorie expenditure.

This so called adaptive thermogenesis is a side effect of having long-term low energy intake.

“Adaptive thermogenesis refers to underfeeding-associated fall in resting and non-resting TDEE; this is independent of body weight and body composition” [3].

It is highly associated with fatigue and tiredness when dieting.

Tired, fatigued athlete sat on the leg extension with her head in her hands

Key Points:

  • Successful dieting requires a calorie deficit, resulting in less available energy for NEAT.
  • Low energy availability results in fatigue and a decrease in subsequent energy and activity levels.
  • Weight loss doesn’t slow down because of starvation mode. It is simply reduced NEAT and EAT that reduced TDEE.

Can You Avoid Dieting Fatigue?

The key to ensuring you’re still burning fat while on your diet is to try and maintain lean mass and activity levels. Keep these elevated and the fat will continue to fall off.

Make sure your deficit isn’t too aggressive

If you feel so tired that even getting off the sofa is a challenge, then it might be time to take your calorie up a little. Make sure you’re tracking your energy intake properly – if your deficit is more than 20% from maintenance calories then try and increase energy intake slightly.

Having ‘refeeds‘ where you occasionally up your calorie intake up whilst cutting will help to give you that much needed energy boost – and that boost will last for a few days too. So sacrificing one day of deficit for a few days of high energy whilst in a deficit is well worth it.

Try to keep moving

You now know that the biggest adaptive component that suffers with diet fatigue is NEAT- you’ll just not feel like moving around much.

But consciously trying to increase NEAT will be the difference between success and failure. Use an activity tracker, set an activity alarm to go for a walk or even just be vigilant to be on your feet as much as possible. Whatever you need to do, don’t forget to get your activity in.

Maximize nutrients

Although you need to reduce calories to achieve a deficit, you don’t necessarily have to reduce nutrients. Focusing on fruit, vegetables and protein will help you maximize nutrition and reduce the chances of nutrient deficiencies. A good fat burner supplement will help to give you energy during your cut too.

Aim to maximize fat burning nutrients in your daily diet to switch on thermogenesis and reduce appetite are also important if you are to win the war against fat.


  1. Martin, CK et al. Effect of calorie restriction on resting metabolic rate and spontaneous physical activity. Obesity. 2007; 15(12): 2964-2973
  2. Leibel, RL et al. Changes in Energy Expenditure Resulting from Altered Body Weight. The New Engl J Med. 1995; 332: 621-628
  3. Müller, MJ et al. Adaptive Thermogenesis with Weight Loss in Humans. Obesity. 2013; 21(2): 218-228