Whenever the discussion of weight loss comes up, one particular subject always follows…calories.


The fact that ⅔ people in the UK are overweight or obese tells us that people find managing calories difficult (1).


So if you want to lose weight, transform your body, health and confidence then ultimately, calories are king.


But what exactly is a calorie? How do they affect your weight loss? How many do you need? Where should your calories come from? How accurate are calorie calculators? What are the best ways for counting calories, and ultimately get results?


We’ll answer these questions and much more below…

What is a calorie?

A calorie (also known as a kilocalorie, or kcal), is not a ‘thing’, but rather it’s a measurement. To be more specific, a calorie is a measure of the energy needed to increase the temperature of 1kg of water by 1°C. Which is where we get the phrase ‘burning calories’ from.


The energy from calories is released when we eat through digestion. The energy gets stored in our body until we need it.


There are 3 main reasons we use energy: 


– Digestion: This accounts for 10% of the total energy we use 

– Physical activity: This uses up around 20% of our total energy

– Basic functions: This takes up most of our energy, accounting for 70%. This includes everyday tasks, that you won’t even think about such as, well, thinking! Other functions include growing and breathing.  



How do calories affect your weight loss?

When you don’t use the energy that gets stored in our bodies from the food and drinks you consume, a buildup of excess energy can happen, leading to weight gain.


Energy balance - weight gain
To reduce the excess weight, we need to reduce the amount of energy that is stored in the body (as fat) by either using more energy through physical activity or we need to decrease the amount of energy we consume by consuming less high calorie, poor quality foods and drinks.


And eating more lower-calorie, high-quality foods and drinks.

Energy balance - weight loss

That means less of this…

And more of this…
To maintain our weight we need to find the right balance between consuming and using enough energy. But obviously, this is not always straightforward.
Energy balance - no change

How many calories do you need?


Ok so let’s find out how many calories you need!


To keep things simple we’re going to use your current weight, height, age, gender and physical activity levels using the calculator below to calculate how many calories you need if you want to lose weight.

Where should your calories come from?


The calories we eat, digest and absorb come from one of three different macronutrients: 


– Carbohydrates

– Proteins and 

– Fats


A diet without a balance of these nutrients will lead to detrimental health effects, no matter what the number of calories is. How much you need of each one depends on your current weight and activity levels. 


You can find out exactly how many carbohydrates you need to lose weight here, how much protein you need to lose weight here and how many fats you need, here.


You can learn more about the best nutrition for losing fat in our FREE Weight Loss Kickstarter which you can get instant access to here.


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If you have tried dieting and are confused or overwhelmed by the huge amount of weight loss misinformation on the web, then try our weight-loss scorecard below.


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How accurate is calorie counting?


Unfortunately, Kcals on food labels can be misleading. 


The calorie numbers are based on averages, for example, an apple can have a variance as much as 29%

calories in an apple image
So that can mean the 150kcal sandwich you had was actually 193kcals!

We know! Sorry…


Even when the calories we eat are accurate, we don’t absorb all of the calories we consume. For example, only 68% of the calories from almond nuts are absorbed (2). The rest go through our body system (as fibre) which is great for digestion but isn’t actually absorbed for energy.

calories absorbed from almond nuts

How we cook food can also affect the accuracy of calorie counting. Certain foods can increase the number of calories that can be absorbed, like potatoes which can have a difference of up to 100kcals (3-4).

calories absorbed from potatoes

So is calorie counting a waste of time?


Absolutely not.


When setting out on a journey it’s useful to have some sort of map rather than no map at all, even if it isn’t 100% accurate it will help you set out in the right direction.

sign post
Although calorie counting is not completely reliable, it can still be useful.


The key is therefore to be outcome-based and understand how to adjust your course if you aren’t heading in the direction you want.


Despite its limitations, knowing how much energy you need and how much energy is in certain foods is absolutely key, even if they aren’t perfect, they are a great start for helping you head in the right direction.


But it also exemplifies why it’s so important to stay consistent with your nutrition. Because when you are consistent you can be outcome-based.


For example, if you think you should be eating about 2500 calories a day to lose weight, but you aren’t, then you can try reducing your calorie intake by another 500 calories and then see what happens. 


If you begin losing weight, then great!


If not, then you can try reducing your calories again by 500 calories until the weight slowly and sustainable starts to fall off.


We say only reduce by 500 calories because the body doesn’t like rapid change. 


If you go from 2500kcals to 1000kcals in a week, your body will react by desperately trying to hold on to fat and increasing your hunger. 


Changing calories has to be a gradual change like we’re trying to sneak our calories intake without the body knowing…

Just like our clients who use this approach have been able to lose weight sustainably, despite life getting in the way occasionally.


This is what science-based sustainable nutrition is all about.

Image of weight loss client Beth
“I’ve been working with Jamie since Feb 2019 and moved to Health by Science with him. I love working out at their gym, everyone’s super friendly.”



Over a 12 month period, David managed to lose over 10kg and record his lowest and healthiest weight in the last 25 years.



Image of weight loss client Lucy

Lucy used to think that people like her weren’t meant to exercise. Lucy has now achieved results she never thought were possible.



What are the best ways of counting calories?


The most accurate way to track your calories is to use My Fitness Pal, which syncs directly with our Health by Science app when you sign up for our Free Weight Loss Kickstarter.


A slightly easier way to track your calories is to use a simple food diary and take pictures of your portion sizes.


Although not as objective, this mindful approach allows you to record the rough calorie amounts in foods and make objective decisions about what the best changes for you are in order to lose weight sustainably.


To learn more about how to do this and work directly with our expert coaches then sign up below for our Free Weight Loss Kickstarter.


image of weight loss scales


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1) The prevalence of obesity. (n.d.). Retrieved from


2) Durham, P. B., Emma, 2.0, E., Axel, Autio, A., John, & Morisset, S. (2018, March 23). Going Nuts for Calories! Retrieved from


3) Tamanna, N., & Mahmood, N. (2015). Food Processing and Maillard Reaction Products: Effect on Human Health and Nutrition. Retrieved from


4) B;, R. P. (n.d.). Resistant starch in food: A review. Retrieved from