Calorie content is defined based on the energetic value of oxidation of different substances in our organism (according to “Chemical composition of food products” A. Pokrovsky). Examples are shown in table.
Typically, the product of interest is subject to chemical analysis showing how much protein, fat, carbohydrate and other substances it contains. For example, the amount of protein is determined using protein nitrogen (using a catalyst, the product is burned together with sulphuric acid and then the volume of ammonia generated is measured). Sugars are extracted with the help of ethanol. Starches are dissolved in water or in a salty solution. As you can see, the process can be quite complicated.
By knowing the chemical composition of a product one can mathematically calculate its caloric content (1 kilocalorie, or kcal, is the amount of energy needed to warm up 1 liter of water for 1 degree Celsius).
All of this is logical and quite beautiful, but the human organism is not an oven burning whatever you put in it. It is nurtured not by the calories it consumes, but by the calories it digests. Skinny people often overeat hoping to gain weight, but one can see that it does not change anything. There are also many fat people eating quite moderately, but still not losing their weight. So what does energy exchange depend on in the end? Why one diet makes you lose weight, while another one just makes you fatter? Though there are more questions than answers nowadays, let’s try to understand it.
Text: from the book of A Eddar "The treatise of nutrition".