The “calories in minus calories out” model says that to maintain our weight, the number of calories we take in (i.e. eat) should match the number we use up (by keeping the basic body metabolism going, digesting food and carrying out physical activity).
However, not all of the calories in a food might be absorbed, digested and available for the body to use. Some foods are easier to digest than others. Different people can digest food in different ways. Cooking food tends to make the food more easy to digest. Some of the calories in foods get absorbed by the bacteria inside us.
So the “calories in minus calories out” idea is correct, if we define “calories in” as the calories our bodies actually absorb. The differences may be small, perhaps within the “accuracy range” of daily Calorie requirements, The study below, however, seems to indicate that a higher fibre diet could reduce the number of calories absorbed when compared to a diet with less fibre.
https://blogs.scientificamerican.com/guest-blog/the-hidden-truths-about-calories/
https://www.ncbi.nlm.nih.gov/pubmed/2841437
Here are the UK Governement Regulations on Nutritional labelling:
https://www.food.gov.uk/business-guidance/nutrition-labelling
Which says that:
“The legislation allows for different methods of calculating the nutrient values. It does not necessarily require laboratory analysis and it may be possible for a food business operator to calculate the values themselves depending on the type of product. Declared values must be based on:
- manufacturer’s analysis of the food
- calculation from the known or actual average values of the ingredients used
- calculation from generally established and accepted data
The declared values in the nutrition table are average values to take into account of the natural variation in foods.”
And here is the current EU guidance :
htttps://ec.europa.eu/food/safety/labelling_nutrition/labelling_legislation_en
This 2002 scientific report on how Calories are actually calculated and says:
“The total combustible energy content (or theoretical maximum energy content) of a food can be measured using bomb calorimetry. Not all combustible energy is available to the human for maintaining energy balance (constant weight) and meeting the needs of growth, pregnancy and lactation. First, foods are not completely digested and absorbed, and consequently food energy is lost in the faeces. The degree of incomplete absorption is a function of the food itself (its matrix and the amounts and types of protein, fat and carbohydrate), how the food has been prepared, and – in some instances (e.g. infancy, illness) – the physiological state of the individual consuming the food. Second, compounds derived from incomplete catabolism of protein are lost in the urine. Third, the capture of energy (conversion to adenosine triphosphate [ATP]) from food is less than completely efficient in intermediary metabolism (Flatt and Tremblay, 1997). Conceptually, food energy conversion factors should reflect the amount of energy in food components (protein, fat, carbohydrate, alcohol, novel compounds, polyols and organic acids) that can ultimately be utilized by the human organism, thereby representing the input factor in the energy balance equation. ”
http://www.fao.org/uploads/media/FAO_2003_Food_Energy_02.pdf