The calories of food

Calories calculation of foods

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The most accurate method to measure the energy contained in various foods is to burn them in an instrument called a calorimetric bomb.

PRINCIPLE OF OPERATION OF THE CALORIMETRIC BOMB:

A known amount of fuel (food) is inserted into an oxygen-saturated chamber. The combustion process is triggered by an internal device and the heat developed is transferred to the surrounding water. Everything is placed inside an insulating container to avoid heat exchange with the external environment.

By recording the variation in water temperature it is possible to calculate the caloric value of the fuel.

To quantify the calorific value, in Italy Kilocaloria (Kcal) is mainly used, which is commonly called calorie (in reality an error of assessment is made because a kilocalorie corresponds to 1000 calories).

kilocalorie:

represents the amount of heat needed to raise the temperature of 1 kg of distilled water from 14.5 ° C to 15.5 ° C.

According to the international measurement system, the calorific power of a food is expressed in Kilojoule (Kj).

A calorie is equal to 4.188 joules so to convert Kilocalories into kilojoules it is enough to multiply them by 4.188

Deepening: what is the use of a calorimetric bomb to evaluate the energy of food, given that the human body does not burn the nutrients with flame?

Calories of Carbohydrates

Burning a gram of carbohydrates develops an average heat of 4.2 Kcal per gram.

Normally 97% of the carbohydrates introduced with the diet are absorbed.

It follows that carbohydrates supply our body on average 4 Kcal per gram .

Calories of Fats

Burning a gram of fat develops an average heat of 9.45 Kcal per gram.

Normally 95% of the fat introduced with the diet is absorbed.

It follows that fats provide our body with an average of 9 Kcal per gram .

Calories of Proteins

Burning a gram of protein develops an average heat of 5.65 Kcal per gram. However, since our body is unable to use the nitrogen contained in them, their energy power is reduced to 4.35 Kcal per gram.

Normally 92% of the proteins introduced with the diet are absorbed (97% of the animal ones and 78% of the vegetable ones).

It follows the proteins they supply to our body on average 4 Kcal per gram .

These caloric values are called Atwater, from the name of the scientist who first measured the energy released by food in the body.

Calorie Calculation Limits

In reality the caloric values reported above, even if corrected for the absorption coefficient, represent an average value. For example, 1 gram of carbohydrate from a dessert does not develop the same calories as 1 gram of carbohydrate from an apple. In some cases the variations between the various foods can reach 10%.

Let's look at some examples:

The energy value of 1 g of fat from pork is 9.5 kcal while that deriving from 1 g of vegetable fat is 9.3 kcal.

The energy value of 1 g of glucose is equal to 3.74 kcal while that of 1 g of starch is 4.2 Kcal.

Calculating exactly the caloric quantity of a dish is therefore an arduous task, practicable only in specialized laboratories.

Even if the values reported in the nutritional tables are used, it must be considered that the variations in the composition of the individual foods cause errors of evaluation (for example an apple can be more or less mature, a beef fillet can be more or less fat).