Measurement of Energy Value of Foods | Biochemistry

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.