Carbohydrates are the chemical compounds made up of carbon, hydrogen and oxgyen. They release energy on biological oxidation with the help of cellular enzymes. They are the cheapest source of energy.

Complete biological oxidation of one gram of carbohydrate yields about 18 kilo joules of energy. One kilo calorie of heat is required to raise the temperature of 1 litre of water through 1°C.

1 kilocalorie = 4.18 kJ

1 kilojoule = 1/4.18 × 1000 calories

Carbohydrates in the diet provide about 60-80% of total energy required by the body.

Types of Carbohydrates

The three types of carbohydrates that we consume in food are:

  1. Sugars
  2. Starch
  3. Cellulose

Common Sources of Carbohydrates


Cereals (wheat, rice, maize), millets (bajra, jowar, barley), roots and tubers (sweet potato, tapioca, potato)


Cane sugar, beet root, fruits (banana, mango, sapota or chiku), milk, honey, and cereals


Cell walls of fruits, vegetables, and cereals

During digestion both starch and sugars are absorbed as glucose. The surplus glucose is changed into glycogen which is stored in the liver for subsequent use.

Cellulose is a fibrous substance which is not digested by human body. However, it serves as roughage and facilitates bowel (stool) movement.

A normal person needs about 400-500 grams of carbohydrates daily in the diet. A growing child, a lactating mother and a person doing hard physical work need more carbohydrates than an average person because of their greater energy requirements.

Functions of Carbohydrates

  • Lactose sugar promotes growth of intestinal bacteria that facilitate the absorption of calcium.
  • Excess carbohydrates are converted into glycogen and fat and serve as reserve sources of energy.
  • Cellulose helps in bowel movement.
  • Glucose is the only source of energy for the central nervous system.