1. On the basis of their chemical constitution the nutritive substances are classified as:
(i) Proteins, (ii) Carbohydrates, (iii) Fats, (iv) Mineral salts, (v) Water and (vi) Vitamins. They may again be classified as nitrogenous and non-nitrogenous substances subject to the presence or absence of nitrogen element in their constitution. Protein is the only nitrogenous substance because it contains nitrogen element in its constitution, while rest of the substances are known as non-nitrogenous because they do not have nitrogen element
in their constitution. Similarly they may also be classified as organic and inorganic substances depending upon the presence or absence of carbon-element in their constitution. Thus, proteins, carbohydrates and fats are referred to as organic substances because they have carbon element in their constitution while mineral-salts and water are referred to as inorganic substances as they don’t have any carbon element in their constitution.
2. On the basis of their functions in the body the nutritive substances fall under the following heads:
1. Body builders:
Proteins, mineral salts and water are referred to as body-builders because they are the only substances which enter into the composition of the body.
2. Energy producers:
Carbohydrates and fats on their oxidation yield certain amount of energy which is consumed by the animal in order to perform the various vital activities of the body. These substances, therefore, are referred to as “energy producing substances” or energy producers. Energy is always valued in terms of calorie.
Carbohydrates are said to be the best energy producers because they are completely oxidized easily. Fats are said to be the second best energy producer, while proteins are oxidised only in the deficiency of carbohydrates and fats.
Mineral salts and vitamins are capable enough to regulate the different functions of the body, therefore, they are referred as regulating substances or regulators.