Caloric Value of Food

Value of food is expressed in calories per gram. A calorie (written with a small c) is the amount of heat required to raise temperature of 1 gram of water by 1oC (more specifically from 15oC to 16oC). The large calories (written with a capital C) or Kilocalarie is equal 1000 calories. In the discussion of energy metabolism it is kilocalorie or Calorie (abbreviated as Cal) which is used.
In order to determine amount of energy contained in various foodstuffs a sample known weight is blace in a bomb calorimeter which is a metabolic chamber having an arrangement for producing electric sparks. The bomb calorimeter is surrounded by a known amount of water whose temperature is noted down. A spark is produced and the foodstuff is ignited. The combustion of the foodstuff produces heat which serves to raise the temperature of the surrounding water. The temperature of water is again noted and rise in its temperature found out. From the rise in the temperature of the water the total heat given off in this process can be easily found out. Average values of the heat combustion for different food stuffs are given below:
Carbohydrate       ------           4.1  Cals/gram
Fat                              ------           9.4 Cals/gram
Protein                     ------            5.6 Cals/gram
The heats of combustion of carbohydrates and fats as determined by this method are found to the same as that produced within the human body. This is because in the body these foodstuffs are completely oxidized to CO2 and water just outside the body. 1.3 Cal/gram less than its heat of combustion. This is because unlike burning outside the body, the proteins are not completely oxidized in the body. The main product of protein catabolism is urea which can further be oxidized in the bomb calorimeter but not in the body.
For the sake of convenience the caloric value of foodstuffs are rounded off and the following values are used

Carbohydrates        ----         4 Cals/gram
Protein                        ----         4 Cals/gram
Fat                                 ----         9 Cals/gram

The errors thus produced are quite small and are of no significance.