Different foodstuffs on burning give different amounts of energy. How much heat will be obtained by burning a particular foodstuff is expressed by the term Caloric value. Caloric value is defined as amount of heat-energy obtained by burning 1.0 gm of the food stuff completely in the presence of O2. Caloric value of different foodstuffs is determined in vitro in a special apparatus called bomb calorimeter
Principle and Procedure
1. A weighed amount of the sample is burnt in an atmosphere of O2 by an electrically heated platinum wire.
2. The heat evolved is absorbed in a weighed amount of water which surrounds the burning chamber.
3. The rise of temperature is recorded with the help of a sensitive thermometer.
Calculation: From the above data, heat evolved by 1.0 gm of food can be calculated with the following formula:
W (T2 – T1)
H = __________ ______ Kcal/gm
M (1000)
Where,
W = Water equivalent of calorimeter and its water.
T2–T1 = Rise in temperature in centigrade.
M = Denotes amount of food burnt in gm.
The unit of energy is calorie (c). It is defined as the amount of heat required to raise the temperature of 1.0 gm of water by 1oC (specifically from 15oC to 16oC). This is the ordinary calorie and is found too small a unit for measuring the energy value of foods.
A unit thousand times of the ordinary calorie is called kilo-calorie or simply calorie (by capital ‘C’) is used for this purpose. Calorie in biological science always means a “kilocalorie” (“C”). Food materials undergo combustion in the animal body and liberate energy in the same way as in a bomb calorimeter, but in a graded and continuous step wise manner instead of in an explosive way.
• Glucose = 3.6
• Starch = 4.2
• Animal fats = 9.5
• Animal proteins = 5.6
(vegetable proteins slightly lower)
In the body, proteins do not undergo complete oxidation, a portion of its amino groups being converted to and excreted as NH3 and urea (in vitro it is further oxidisable). This involves a loss of about 1.3 cal/gm, leaving 4.3 C produced/gram of protein metabolised. Taking into consideration, the variations in caloric value of individual carbohydrate/fat/protein, their average energy value when metabolised may be represented as follows in C/gm:
• Carbohydrates = 4.1
• Fats = 9.3
• Proteins = 4.1
When these corrected figures are applied, it is found that the amount of energy (calories) produced by a given quantity of these foodstuffs in the body is the same as that produced by their combustion outside the body. On accounts of losses in digestion and absorption and other unaccountable factors, the caloric value are usually rounded off and said to be 4.0 calories/gm of carb and proteins and 9.0 calories/gm for fats
Caloric requirement
To maintain caloric balance in an adult, it is necessary to supply enough foods to replace the calories expended per day.
These include the following:
• A supply of “basal” requirements (BMR)
• Supplies of calories to meet the extra requirement caused by specific dynamic action (SDA), also called “calorigenic action” of foods.
• Supply for physical activity over and above the basal requirements (most ‘Variable’ under normal conditions).
• During periods of growth and/or convalescence, extra provision has to be made to meet the synthesis of tissues and weight gain.
