Written by Lucy Walton
We’re constantly bombarded with messages to limit the number of calories we eat to be healthier, manage weight and reduce disease risk such as diabetes. We see calories listed on food labels and front of packages often in statements such as “less than 90 calories per serve.” They’re mentioned everywhere we look. But what do statements like this even mean? Is health all about calories? Does eating less equal better health?
What is a calorie?
A calorie, put simply is a unit (measurement) of energy. You might see them displayed as kcal or kJ (one calorie equals 4.2 kilojoules). More specifically, a calorie is the amount of energy required to increase the temperature of 1 litre of water by 1 degree centigrade at sea level.
Calories (kcal) are measured empirically using a unique piece of equipment called a bomb calorimeter. A sample of food is placed into a ‘bomb’, a sealed container that is pressurised with oxygen. This container is added to a known volume of water which is heated and burns the food. The change in water temperature is measured which tells us how many calories there are in that food.
Are calories accurate?
The bomb calorimeter can tell us about the actual calories in the food which is great, however it’s not that straightforward. We eat food, not calories. We are humans, not machines (or bomb calorimeters). Calories from the food we eat are absorbed and used by humans. How many calories are absorbed depends on several factors, for example, what we are eating (carbohydrates, fats, protein or fibre) or digestion and excretion (removal of waste products).
Back in the 19th century, a scientist named Wilbur Olin Atwater proved that different types of foods contain different amounts of energy. He created the Atwater Factor, which simplifies the amount of calories in the main macronutrients. He found that (1):
Fats contain 9 calories for every 1 gram of fat
Carbohydrates contain 4 calories for every 1 gram of carbohydrates
Proteins contain 4 calories for every 1 gram of protein
Alcohol contains 7 calories for every 1 gram of alcohol
This is how the calorie values of foods are calculated today. However, there are several issues with this method, including:
Variability in how the percentage of protein in food is calculated
The method is outdated (more than 100 years ago)
The Atwater factor is over-simplified as the number of calories on the side of a packet is not equal to the number of calories we digest or absorb
Are all calories equal?
Let’s consider this… 200 calories of chocolate is twice the amount of 100 calories of chocolate, in the same way 200g of chocolate is twice the portion of 100g. But we would not compare 100g of chocolate to 100g of carrots, because they are different foods and made up of different nutrients. This is exactly the same with calories. Our body processes foods differently, depending on what the food is and the nutrients it contains (2). There are a number of factors which may impact this, let’s take a look.
The type of food and nutrients we eat
There are two nutrients which stand out when we think of the effect they have on digestion and calories:
High protein diets are often recommended because we feel fuller for longer when we consume protein. Protein is the most complex macronutrient because it takes the longest to digest. This means hormones are released which signal to the brain to say it’s full. The complexity of protein also means there’s more work to be done to metabolise the nutrient. For example, if we eat 100 calories of protein, we spend around 30 calories metabolising it, so we are already 30% down on the calories listed.
Fibre is a type of non-digestible carbohydrate, meaning that the body cannot break it down. Instead, it passes through our gut into the large intestine and into our stools. As a result, we excrete some of the energy from fibre in our stool.
Food processing can affect the calories available from that food. When we eat a wholefood, our body needs to digest and absorb every part of the food, including the parts we might not digest, like fibre. With a processed food, some of the work is already done, so our body has less to do. This means more of the energy is absorbed from the food. However, if we look at the food label of two foods that are made of the same food but processed differently, for example a corn on the cob and corn tortilla, both food packets will indicate the same calorie value per100g of corn even though we absorb less calories from the corn on the cob.
How food is prepared has an impact on how many calories are available. Cooking helps to breakdown the cell structure of food which allows us to absorb more of the nutrients and energy, acting as an extension of our digestive system. Therefore, when we cook some foods, more energy becomes available to us, which may not be accounted for on the food label.
Should we be counting calories?
The short answer is that we don't need to count calories to live a healthy lifestyle and nourish our bodies. It is important to understand why calorie counting isn't recommended for everyone.
Inaccuracy of food labels
One study found that calories in packaged foods differ from the label by around 25%, suggesting food labels may be inadequate sources of caloric monitoring (5). One reason for this is the mass production of food with the value indicated on packaging being an average of the product, each will weigh a slightly different amount and have a different percentage of ingredients and therefore nutrients. Sometimes food labels might not be updated with a small recipe tweak immediately, and you can guarantee chefs are not weighing out every plate of meal they serve. If everyone at a table ordered the same meal, they would all have different calorie values, reinforcing that the calories listed on restaurant menus are only an estimate.
2. We are humans, not machines
It’s easy to see that the accuracy of calculating calorie requirements and intake is not straightforward. Calorie requirements vary day to day depending on numerous factors including (but not limited to) genetics, daily activity, age, stress, hormones, sex, lifestyle, and illness. An app, equation or website cannot decipher this for you and does not know you as well as your body does. In addition, due to our genetics and unique (and forever changing) types and amounts of bacteria in our guts, we all digest food differently! Give 10 people the exact same meal and each person will extract the nutrients differently.
3. Counting calories can create rigid food rules
Food labels and calorie counts cannot measure your relationship with food and can interfere with intuitive eating. Making decisions of what you would like to eat based on your own internal knowledge and desire, rather than external cues such as calorie counts, helps us to develop a trusting relationship with our body. This removes the pressure of eating the ‘right’ number of calories each day or eating ‘perfectly’, which doesn’t really exist.
4. Quality over quantity
We’re sold the notion that less calories is always the healthier option however lower fat options could contain more additives to make the food taste good, and be low in important nutrients such as fat soluble vitamins. Low fat foods also generally aren't as satiating so you can end up eating more anyway!
Calories are equal if we think of them as a unit of energy, but when it comes to thinking about calories as energy from the food we eat they aren't equal. Calorie counting might work as a short-term strategy for people who have a healthy relationship with food to create calorie awareness, but we would never recommend it long-term or for anyone that has a disordered relationship with food or an eating disorder due to the risk of it becoming a rigid and obsessive behaviour. The process of counting calories is not a normal way of eating and the ultimate aim in our eyes is to work on being an intuitive and flexible eater.
Lucy is a Registered Associate Nutritionist and intern at TCN! With Lucy's background in nutrition and psychology, her aim is to help you become more confident in your food choices and enrich your mindset. You can find Lucy on instagram @lutritionw and her website here
 Fao.org. 2021. CHAPTER 3: CALCULATION OF THE ENERGY CONTENT OF FOODS - ENERGY CONVERSION FACTORS. [online] Available at: <https://www.fao.org/3/y5022e/y5022e04.htm> [Accessed 17 October 2021].
 BBC Scienceand Technology, 2021. Calories, with Dr Giles Yeo. Instant Genius.
 MyNutriWeb Webinar – Why Calories Don’t Count, Dr Giles Yeo
 Yeo, G., 2018. Gene eating. Great Britan: Seven Dials.
 D B, A., S, H. and S B, H., 1993. Counting calories--caveat emptor. PubMed, 270(12), pp.1454-6.
Talia Cecchele Nutrition is a team of registered dietitians specialising in eating disorders and disordered eating. We aim to bring balance back to nutrition, help you to break free from food rules and find food freedom. We offer virtual consultations and group programs so whether you are based in London, the UK or around the world we would love to support you. To enquire about a private consultation please fill out a contact form.