The below mentioned article provides a short note on the Measurement of Energy Value of Foods.
The foodstuffs (carbohydrates, fats and proteins) on combustion by oxygen produce heat. This amount of heat can be measured in a bomb calorimeter.
Carbohydrate and fat are completely oxidized in the body to CO2 and water like that of bomb calorimeter. But proteins are not completely burned because urea, the end product of protein metabolism, still contains some energy which is not available to the body.
Therefore, the energy value of protein in the body (4.1 kcal/gm.) is less than that obtained in the bomb calorimeter. The energy value of foods in the body is customary to express in round figures. The table below shows the energy value of foodstuffs.
The Energy Requirement of Man:
The energy requirement of man can be determined either directly by measuring his output in a calorimeter or indirectly by measuring his oxygen consumption.
A. Direct Method:
1. This is the most accurate method but is time-consuming, laborious, and demanding very expensive apparatus.
2. The subject is placed in an insulated chamber. His heat production is measured directly by recording the total amount of heat transferred to a weighed quantity of water circulating through the calorimeter.
3. The oxygen intake, the CO2 output, and the nitrogen excretion in the urine and feces are also measured during the whole period of observation.
B. Indirect Method:
1. This method consists of measuring gas exchange and determining the respiratory quotient.
2. This method is simple and applicable to field studies and clinical analysis.
Respiratory Quotients (RQ) of foodstuffs:
It is the ratio of the volume of CO2 eliminated to the volume of oxygen utilized in the oxidation.
1. Carbohydrates:
The complete oxidation of glucose is as follows:
2. Fats:
The oxidation of tristearin is as follows:
Fats have a lower R.Q. value because the oxygen content of their molecule in relation to carbon content is very low.
3. Proteins:
Since the chemical structure of proteins is variable their oxidation cannot be so readily expressed. The R.Q. of proteins by indirect method has been calculated to be about 0.8.
4. Mixed Diets:
The R.Q. of mixed diets is about 0.85. The R.Q. is lowered if the carbohydrate metabolism is impaired.
Total Heat Production:
To obtain the heat production, the grams of each foodstuff oxidized are multiplied by the caloric value of that food. The sum of these caloric values is equal to the total heat production of the diet. Therefore,
Significance of R.Q.:
1. R.Q. helps in the determination of metabolic rate.
2. It is the guide for assessing the type of food burning or the nature of synthesis taking place in the whole body or in any particular organ.
3. The determination of R.Q. aids in the diagnosis of acidosis, alkalosis and diabetes mellitus, etc.
4. Non-protein R.Q. is used for calculating the total energy output and the proportions of various foodstuffs being burnt.