Human Nutrition 3(3+0)

Lesson 5 : Energy-Units, Methods of Measuring Energy Value of Food

Loss of Energy in Metabolism due to Incomplete Oxidation

There is no loss in metabolism in the case of carbohydrates and fats. But in the case of proteins, a part of the energy is lost as urea due to incomplete oxidation. This loss has been estimated as 1.2 cal. per gram of protein oxidized. The physiological energy values of carbohydrates, fats and proteins are4, 9 and 4 after making allowances for losses of food energy in digestion and metabolism.

Relation between Oxygen required and Calorific Value
It is of importance to know the heat produced when 1 litre of oxygen is used for the oxidation of carbohydrates, fats or proteins. This can be calculated from the data obtained by the oxy-calorimeter and bomb calorimeter as explained below:

1 litre of o2 oxidises 1.25g. Carbohydrate and produces 5 Kcal heat
1 litre of o2 oxidises 0.49g. Fat and produces 4.5 Kcal heat
1 litre of o2 oxidises 0.83g. Protein and produces 4.6 Kcal heat.

One liter of oxygen oxidizing carbohydrates, fat or protein produces nearly the same amount of heat i.e., 4.5 to 5 Kcal: This is an important conclusion as this is the basis for indirect determination of energy requirements.