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Calories in – calories out, myth or fact?

The theory behind calories

0First, what is a calorie? It’s a measure of energy. The principles of energy are just as well established within physics as Newton’s laws of motion. One of the laws of thermodynamics states that energy cannot be destroyed, only transformed into different shapes. This is the basis of the concept that energy in/out dictates if an organism increases or decreases in weight. Excess energy will be stored as chemical energy (like fat).

Sceptics claims the human body is too complex and the laws of physics does not apply to us. The energy in food are burned in a bomb calorimeter to assess the energy content and the body process the food in an entirely different way. However, just as we cannot escape gravity because we are more complex than a stone we cannot violate the laws of thermodynamic either. It’s the starting point (carbs, protein, fat) and the end point (carbon dioxide, water) that sets the energy released. How we go from A to B does not matter, the energy is constant regardless of the path. Our complex biology does not come to play here.

Clinical trials on isocaloric diets

The laws of thermodynamic are often interpreted as if an organism are fed two different diets with the same amount of calories (isocaloric) the weigh change will be identical. However, this is not always the case in clinical trials. So we can break the laws of thermodynamics?

No, we cannot. We also need to consider the energy consumption (energy out). The diet can influence the energy consumption which means that even though the diets are supposed to contain the same amount of energy the organism energy expenditure are influenced by the diet and therefor the calories in/out are not identical in the trials.


On top of that food science does not measure energy content on specific molecules, rather the average. Let’s look at fat. Often the energy value is given as 9 kcal/g. However, saturated fats contain more energy than unsaturated fats and short chained fats contains less energy than long chained fats. The range goes from 5 to 10 kcal/g which means that one form of fat can contain double as much energy as another form. This is not accounted for in nutritional trials. On top of that we have a microbiota in the gut that eat part of our food and this factor can also influence how much of the food energy that actually are absorbed by the body.


Even though we cannot escape the laws of thermodynamics, the energy in and out principle, clinical trials on isocaloric diets does not always yield the same weight change. Since the energy in and out are not constant the trials are in reality not identical once it comes to energy in and out. This means that all diet are not equal once it comes to weigh loss.

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