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Updated: Jun 27, 2023

Looking at the label of any food on sale in the supermarket, you immediately notice a table showing the nutritional values, i.e. the composition of the food.

Under the heading Energy Value there are usually two values represented by the acronyms Kcal and Kj.

Although the acronym Kcal is used to indicate calories on food labels, this definition is not correct. Indeed, a calorie should be shown with the acronym Cal that represents a small calorie, also called gram calorie, i.e. how much energy is needed to raise the temperature of one gram of water by one degree Celsius (1°C). Kcal, on the other hand, indicates the kilocalorie, i.e. 1000 small calories and represents the amount of energy needed to raise the temperature of one liter of water by 1°C.

The acronym Kj is the kilojoule. The Joule (J) is the unit of measurement for energy, work and heat adopted by the International System of Units.

The value of one Kcal is 4.184 Kj. Therefore, a 600 Kcal dish of pasta will be made of 2510.4 Kj:

600 Kcal x 4,184 J = 2510.4 Kj

Kcal and macronutrients

Carbohydrates, proteins and fats (lipids) are all macronutrients.

1g of fat is equivalent of about 9 Kcal, while 1g of carbohydrates and 1g of protein both are equivalent of about 4 Kcal.

These values come from the burning of macronutrients. Our body "burns" carbohydrates, proteins and fats, and each time it needs a heat of combustion of about 4.2 Kcal for each gram of carbohydrates, about 5.65 Kcal for each gram of protein and about 9.4 Kcal for each gram of fat.

However, according to this data the protein values does not match what was previously written. This is because the values are gross and do not represent the net energy available to the body. Let's have a look at the proteins: the body is unable to oxidize the nitrogenous components that are eliminated in the form of urea (containing hydrogen and nitrogen) by the kidneys through the urine. The loss of hydrogen reduces the caloric value of the proteins to about 4.6 calories per gram.

Carbohydrates and lipids, on the other hand, do not contain nitrogen in their molecules, therefore the caloric value remains unchanged.

Coefficient of digestibility

Another factor to consider is the coefficient of digestibility, i.e. the percentage actually ingested and absorbed of a food. In general, carbohydrates have a digestibility coefficient of 97%, fats 95% and proteins 92%. These percentages vary greatly according to the category of food to which reference is made, but for convenience, the average values of net energy are rounded according to Atwater's 4-4-9 rule. We will thus have values of 4 Kcal for each gram of carbohydrates and proteins, and 9 Kcal for each gram of fat.

Energy for training

The human body gets energy from food through cellular respiration and forms adenosine triphosphate, also called ATP. This is then used to produce energy to perform various functions, including: mechanical work linked to muscle activity (for example, when there is muscle contraction during training); chemical work, i.e. the process of cell regeneration (for example, when hypertrophic signals are activated following resistance training, therefore the muscle grows); transport work, i.e. when cellular substances are transported from one area of the body to another.

The human body needs an adequate quantity of fuel, otherwise it will have limited energy and it will not be possible to make the most of one's potential. A lack of energy would affect one's ability to achieve the desired results and also increase the risk of injuries. In order to give your best during training, you need to feed yourself, and if you are not sure what, how much and when to eat, or you are afraid of gaining weight, the best thing to do is to consult a nutritionist.

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