ENERGY VALUE OF FOODS
The energy in various foods is measured by calorimetry. Calorimetry is the measurement of heat loss.
An instrument for measuring heat output of the body or the energy value of foods is called a Calorimeter. In measuring the calorie value of foods, Bomb calorimeter is used. The maximum amount of energy that the sample is capable of yielding when it is completely burnt or oxidized is the energy value of that food, also known as heat of combustion.
The energy measured using a Bomb Calorimeter is as follows
1g of Carbohydrate - 4.1 kcal
1g of fat - 9.45 kcal
1g of protein - 5.65 kcal
When samples of Carbohydrate, Fat, Protein are burned, the amount of heat produced is always the same for each of these nutrients.
In the bomb calorimeter carbohydrates, fats and proteins are completely oxidized whereas in the human body the process of digestion and absorption does not proceed with 100 percent efficiency. The extent of digestion varies from one nutrient to another.
The Coefficient of digestibility is used to express the proportion of an ingested nutrient that ultimately becomes available to the body cells.
The coefficient of digestibility for carbohydrate, fat and protein are 0.98, 0.95 and 0.92 respectively. It is observed that carbohydrate and fat are metabolized almost completely, Whereas protein metabolism is incomplete due to the presence of nitrogen. The physiological energy value of carbohydrate, fat and protein are 4, 9 and 4.
These values are known as Atwater Bryant factors or physiological fuel values as given in table-20A.