BSc Sports and Exercise Science 

Personal Trainer and

Nutrition Coach 

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Does Your Metabolism “Crash” and Can Eating Too Little Actually Damage Your Metabolism?

December 4, 2018


2 minute read

*sorry due to a technical hitch I have had to repost this from yesterday*


Let’s explore the truth behind the phrase “my metabolism crashed”. There is a lot of talk of quick crash diets to get you those holiday results fast, but do these actually cause metabolic damage? 


Sound familiar? 


“I have hit a plateau - I’m eating the same and working out as hard but I can't seem to shift the last bit of weight.”


“I was losing weight and now I have stopped, even though I am eating even less and doing more exercise.”


Can months of dieting on a very low-calorie diet cause a change in the way our body processes and uses food?


Not exactly, but there are many factors in the mix that you need to consider. That can cause metabolic adaptations, reducing the amount of energy you use day to day. 


In theory: When you are eating less energy (calories) than your burning you should lose weight and if you eat more energy (calories) than you burn you should gain weight. 


This may determine your weight, however, it doesn’t tell us much about body composition (fat, muscle, bone etc). This is influenced by things like macronutrient intake (especially protein) sex hormone levels, exercise style, frequency and intensity, age medication use, genetic predisposition and a lot more. 


Don’t go around thinking your body defies the laws of physics and you are eating less than your burning but not losing weight. Actually, the equation is just a little more complicated;


Energy consumed is more complicated than you realise:


1. The amount of energy (calories) food contains is not necessarily the amount of energy you absorb, store or use. 


  • We absorb less energy from minimally processed foods; carbohydrates and fats because they are harder to digest. 

  • We absorb more energy from highly processed foods; carbohydrates and fats because they are easier to digest as some of the digestion work has already been done for you,        during the processing methods. 

  • We can also absorb more energy from foods that are cooked, chopped, soaked or blended, because some of their cells are already broken down, making digestion easier.

  • Cooking then cooling starchy foods (like sweet potatoes) decreases the calories we absorb from them, most likely due to the fermentation of resistant starches. 


2. We may absorb more or less energy depending on the types of bacteria in your gut. 

  • Some people have higher levels of a bacteria (Bacteroidetes) which are better at extracting calories from tough plant cell walls than other bacteria.

  • Studies done have found that people were absorbing 80% of the calories in almonds and 95% of the calories found in pistachios and large individual differences were found between people.

By eating a diet rich in whole foods and low in processed foods, the number of calories you absorb from the foods will be less than what you expect and they require more calories to digest. You will absorb more calories from eating more highly processed foods and burn fewer calories digesting the foods - these foods are also less filling, which will make you more likely to overeat. 


3. Over and above this all, the number of calories on the label don’t march the number in the meal- they can be off by 20-25%.


Energy burn is also more complex than you think:


1. Resting metabolic rate is the number of calories you burn each day at rest, this can depend on your body composition, age, sex, genetic predisposition and possibly bacteria in your gut.


  • When you lose weight this can change your body composition, changing your resting metabolic rate.

  • A larger body or a body with higher muscle mass has a higher resting metabolic rate (muscle is more metabolically active than fat).

2. Thermic effect of eating, it takes energy to digest your food. You will burn more calories digesting and absorbing protein and less energy digesting processed foods.


3. Physical activity, walking, running, bike riding, gardening, going to the gym and many more activities burns calories as you increase your heart rate. 


4. Non-exercise thermogenesis (NEAT). This is the calories you burn though moving, staying upright, fidgeting and doing little things that may not increase your heart rate but do make a difference. 


If you try and lower calories to lose weight your body tries to outsmart you; when energy in decreases, your body decreases energy out to match it. Your body has derived this mechanism to prevent starvation:


  • Thermic effect of eating goes down.

  • Resting metabolic rate goes down as you weigh less. 

  • Non-exercise thermogenesis goes down as you eat less. 

  • You absorb more of what you eat. 

The net effect can have much more of an effect than you would think on your weight loss. 


Over-exercising can make many of the above adaptations worse.


In a similar way, when energy in goes up, energy out also goes up, you will burn more calories in response to eating more.


So does dieting damage your metabolism?


No, crash dieting won't damage your metabolism, however, the adaptations your body undergoes in response to weight loss will mean you will burn fewer calories than before you lost the weight or burn less than someone who was always lean. The change can be between 5-15% fewer calories burned per day. 


BUT all is not lost, I will be doing an article in a couple of weeks with some steps to still achieve continued fat loss despite the changes in how much energy your body burns. If you have hit a plateau this will be one read. 


Kasia :) 


BSc Sports Science, Women’s Fitness Specialist and Personal Trainer 

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