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Can Eating Too Little Cause Metabolic Damage?

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You’ve decided that you wanted to start a fat loss plan and with your motivation levels through the roof you’ve started to eat well and hit the gym.

In the beginning you lost weight and all was going well, but despite dropping your calories and working hard you’ve stopped losing weight. You’re stuck and you don’t know what to do – and to make matters worse you’ve heard that if you drop your calories too low you’ll cause metabolic damage and put weight back on.

Can weeks and weeks of dieting and negative energy balance cause long-term damage which leads to weight gain?

In this article we’ll take a look.

Calories and Weight Loss

Weight loss can be drilled down to simple physics – thermodynamics to be more precise.

If you eat more calories than you expend you’ll create a positive energy balance and put weight on. Your body will store the excess, incoming energy as fat ready to be used later – only later never comes. On the other end of the scale, if you eat less than your body expends you’ll create a negative energy balance. Your body will break down stored energy reserves and you’ll lose fat and body weight.

This theory is referred to as calories in – calories out, or the energy balance equation. Whilst it is only a theory, there are many scientific studies that that back it up.

The amount of calories you burn in a day can be split into four different categories:

  • Basal Metabolic Rate – the energy your body uses to maintain bodily functions such as heart and breathing rate, core body temperature and brain function.
  • Physical Activity – the energy used during exercise or other structured movement such as sport.
  • Non-exercise activity thermogenesis (NEAT) – energy used during non-structured activity such as general movement, changing posture, fidgeting and household chores etc.
  • Thermic effect of food – energy used to digest macronutrients.

A calorie deficit therefore can be achieved in a number of different ways, so whilst energy balance may sound simple and easy to follow, there is a lot more to it than you might think.

Each factor can influence whether you achieve positive or negative balance. Additional factors such as lean mass and androgenic hormone levels, medications, age, gender may also affect energy balance.

Calories in – calories out is ultimately a straightforward model of physics, but with many complicated variables. 

So What is Metabolic Damage?

There are implications to large calorie deficits, or even smaller deficits over a prolonged period of time. It can be psychologically challenging and can be physically tiring too. But could you also be creating be more serious, long-term issues too or is it just a matter of willpower?

In its simplest term, metabolic damage is a dysfunction of the nervous, hormonal and endocrine systems. 

You might be familiar with the term ‘starvation mode’. It’s the concept that suggests if you diet too aggressively your body will not only fall into a deficit but will begin to cling on to any remaining stored energy, slowing your metabolism down and halting your weight loss progress. It’s a bit like a safety switch that your body turns on to protect itself.

Sometimes referred to as weight loss resistance or neuroendocrine dysfunction, the concept of metabolic damage suggests that even if you were to temporarily up your calories after damage has occurred, to have a break from dieting, your body has becomes resistant to weight loss.

This means that you may even start to gain weight even though you are in a calorie deficit, feel tired and lethargic, and much hungrier than normal.

But are these side effects, symptoms of a damaged metabolism? Are you really in starvation mode or is it simply the response to long-term dieting?

Let’s see what the research says…

Can Dieting Actually Damage Your Metabolism?

As we said earlier, weight loss comes down to thermodynamics. But the laws of physics are not necessarily static – in the case of your metabolism it is a very dynamic entity with the ability to adapt according to its environment.

When you restrict calories you body has a clever way of trying to self-regulate by reducing your involuntary NEAT and you also feel less like you want to exercise. The reasons why are complicated but all controlled by your nervous and hormonal systems.

There is Evidence that Calorie Restriction Reduces Calories Burned

A study by Johannsen et al [1] demonstrated that even if you preserve muscle mass and exercise regularly, a dramatic slowing of metabolism can occur during weight loss. They recruited a small group of obese participants in the study and put them on a 30-week weight loss program consisting of diet restriction and intense exercise. Weight loss was not linear but instead declined out of proportion to the decrease in body mass. 

basically, the more weight they were losing, the slower it became.

The researchers also claimed that the metabolic damage that occurred may have even persisted during weight maintenance and predispose the subjects to weight gain if physical activity or calorie restriction were not maintained. However this was only an estimate of what might happen next and not part of the study findings.

There is also evidence that the amount of calories you burn can change too. A study published in the American Journal of Clinical Nutrition [2] found that as weight loss occurs, decreased resting energy expenditure (REE) occurs too. This could go some way to explaining why you find it harder to lose weight over time. It doesn’t mean that your metabolism is damaged per se, rather it’s just slowed down as a reaction to weight loss.

Not all studies show a drop in metabolism though. When a group of subjects were assessed during a 12-week calorie deficit plus exercise intervention [3], researchers found that whilst weight loss varied between each subject and wasn’t as high as expected, the compensations to metabolism could largely be accounted for by physiological variables.

However, they went on to suggest that metabolic down-regulation should not be discounted and may still be an underlying cause.

But is it Really ‘Damage’?

The main question here is whether long-term damage to the metabolism has occurred or whether the total amount of calories out has reduced, leading to a weight loss plateau.

When you start a weight loss plan you’ll lose weight quickly. Most of this in the beginning is water weight and if you are excessively overweight then your body will find it quite easy to convert stored fat into energy as you’ve got lots to spare. But as you get nearer to your ideal weight the challenges of reduced NEAT and potentially REE will make it harder. This doesn’t mean that your metabolism is damaged though.

As we’ve already mentioned, one side effect of dieting is a reduction in both involuntary activity and motivation to undertake voluntary activity. Couple that with wider issues of water retention or possible miscalculation of energy expenditure and your weight loss plateau can be easily explained.

For that reason, the dynamic adaptation to your metabolism is certainly a response to low calories. Starvation mode is certainly something to be aware of.

The concept of metabolic damage though infers that the slow down in weight loss is in some way catastrophic or irreversible. A better term to explain this might be metabolic adaptation possibly as there’s no evidence to suggest that it is a long-term thing.

Taking a break from your diet is always a good thing. It allows you to have a break from the psychological and physical challenge of restriction and can be good for long-term motivation. It is however unlikely that you are allowing yourself time to ‘repair’ metabolic damage.


It is claimed that if you diet too aggressively you can cause metabolic damage – the process by which your body activates its ‘starvation mode’, leading to a plateau in weight loss and on certain occasions leads to weight gain.

There is research to suggest that your body has an ability to dynamically alter resting energy expenditure and activity levels, most likely as a way of balancing a lack of incoming energy, but there is no evidence to suggest that this process leads to long-term damage.

It is always beneficial to come off of you diet for short periods of time in order to promote motivation and allow the occasional treat, but rest sure in the knowledge that you are not damaging your body by creating a negative energy balance over longer periods of time.

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  1. Johanssen, DL et al. Metabolic slowing with massive weight loss despite preservation of fat-free mass. J Clin Endocrinol Metab. 2012; 97(7): 2489-96
  2. Pourhassan, M et al. Impact of body composition during weight change on resting energy expenditure and homeostasis model assessment index in overweight nonsmoking adults. Am J Clin Nutr. 2014; 99(4): 779-91
  3. Byrne, NM et al. Does metabolic compensation explain the majority of less-than-expected weight loss in obese adults during a short-term severe diet and exercise intervention? Int J Obes (Lond). 2012; 36(11): 1472-8