Constituents of food:

1. Carbohydrates:

They are the chief source of energy in our diet. They are chemical compound containing carbon, hydrogen and oxygen. They provide instant energy to our body.

The chief sources of carbohydrates are: rice, wheat, maize, barley, potato, sugarcane, beetroot, banana, etc. Carbohydrates are of three types-

i) Sugars

ii) Starch


iii) Cellulose

Sugars are called simple carbohydrates. They provide instant energy. Sugars are present in milk and fruits like grapes, banana, sugarcane and beetroot.

Starch is found in potato, rice wheat, maize, etc. Starch is the main carbohydrate in out diet, since it is present in cereals, which form the major part of our diet.

Carbohydrates are burnt in our body to release energy, which is used up, by the body to carry out the life processes. Carbohydrates are thus broken down to produce carbon dioxide, water and energy. This process is known as oxidation and can be represented in the form of a chemical reaction:


Oxidation of foodstuffs takes place in our body during the respiration process.

Respiration takes place in our body at the normal body temperature, i.e., 37 0 C. The energy is released during oxidation is stored in the body inside special molecules just as you store money in your money-box. As and when energy iis needed by the body, these special molecules break down to release energy just as you take out money from the money-box whenever you need some.

The energy released by a foodstuff on oxidation can be measured in calories. A calorie is defined as the amount of heat required to raise the temperature of one gram of water by 10 C. A calorie, however, is very small to measure the energy requirements of our body. Therefore, energy changes are measured in kilocalories.

1 kcal = 1000 calories.

2. Proteins:

Proteins are body-building food. They are essential for the growth and repair of the body tissues. Proteins are made up of amino acids. Proteins are formed by different combinations of twenty amino acids. Each amino acid contains carbon, nitrogen, hydrogen and oxygen. Some proteins contain elements like sulphur, phosphorus and iron as well.


Proteins can be classified into two groups depending on their source:

i) Animal proteins are obtained from animal products like milk, cheese, egg, fish or meat.

ii) Vegetable proteins are obtained from plants like pulses, soyabeans, nuts like cashew nuts, groundnuts, grains like barley, etc.


Animal proteins are considered to be better than vegetable proteins as they are more easily digested and absorbed by the body. Daily requirement of proteins for adults is about 1 gram per kg body weight. Children may require 2 to 3.5 grams per kg body weight because of their continuous growth.

Proteins act as body-building materials. Muscles, skin, hair and nails are made up of proteins. Wool and fur of sheep, lamb and bear and silk produced by silkworm are chemically proteins. Haemoglobin, the red pigment of red blood cells also contains a protein ‘haeme’.

Enzymes are chemical substances that take part in several chemical reactions. Enzymes are chemically proteins. For example, salivary amylase is an enzyme produced by our salivary glands that breaks down starch into sugar.

Proteins also help to repair damaged body tissues. Proteins can also be utilized to provide energy during starvation. One gram of protein when burnt yields about 4 calories.

3. Fats:

Fats like carbohydrates are energy-giving foods but are greatly concentrated sources of energy. One gram of fat when burnt gives 9 calories of energy. Fats are made up of carbon, hydrogen and oxygen. However, compared to carbohydrates, fats contain lesser amount of oxygen, and hence produce larger amount of energy when oxidized.


Fats can be classifies as animal fats and vegetable fats depending on their source. Butter, ghee, milk, fish, meat, etc., are sources of animal fat while nuts and vegetable oils like groundnut oil, sunflower oil, mustard oil and sesame oil are sources of vegetable fat.

A major part of the food you eat is used to derive energy for day-to-day activities. A small part of the remaining food is converted into fat and stored in the body. Fats thus constitute an energy bank in the body which provides energy whenever the need arises. Fat is mainly stored under the skin and protects internal body organs from jerks and shocks. Fats help in the absorption of vitamins A, D, E and K as these vitamins are soluble in fats. Fats also make food tastier. This is why people crave for fried food like pakoras and samosas. Fats take a longer time for digestion, hence we do not feel hungry for a long time after eating fried food.

Many animals like bear, walrus, whale and camel sore fat in their bodies for future use. The animals of the arctic region like walrus and seals survive the harsh arctic winters because they possess a thick layer of fat called blubber under their skin. The polar bear has a thick fur that traps air to keep the bear warm. In addition to this it has a thick layer of fat to provide more warmth. During the winter months, the polar bear hibernates. Since the bear does not eat anything during hibernation (winter sleep), it eats voraciously before winter and stores the energy as layers of fat under its skin. The stored fat is used up during hibernation and the bear appears much leaner when it wakes up in the spring.

4. Vitamins:

They are protective foods which are necessary for the well-being of the body. Vitamins are necessary for normal growth and good health of an individual and shortage of one or more vitamins in the body results in deficiency diseases in the individual. Vitamins do not provide energy like the carbohydrates and fats. They are therefore required in very small quantities and are essential for proper utilization of carbohydrates, fats, proteins and minerals by the body. Since the presence of vitamins in the diet prevents the occurrence of many deficiency diseases, they are known as protective food. The various vitamins are named as vitamins A, B, C, D, E, K, etc. most of the vitamins cannot be produced by our body. They must, therefore, be supplied by our diet. Since no single food item contains all the vitamins, we must eat a variety of foods to obtain all the vitamins our body requires.


Vitamins are required to keep our teeth, gums, eyes, blood, bones, skin, etc., healthy. Deficiency of vitamin A in the body causes poor vision and night-blindness which is a state wherein a person cannot see in the dark. Deficiency of vitamin A can be cured by giving spinach, carrots, butter and yellow-coloured fruits like papaya and mangoes to the patient.

5. Minerals:

In addition to carbohydrates, fats, proteins and vitamins, our body also requires minerals like iron, calcium, phosphorus, iodine, sodium, zinc, copper, etc., for its growth and proper functioning.

Both vitamins and minerals are required in small quantities by our body. We get many minerals from plants which in turn absorb them from the soil. Salts of calcium are needed for the formation of strong bones and teeth. Iron is required to form haemoglobin which transports oxygen to the tissues. Deficiency of minerals in our diet leads to deficiency diseases.

6. Water:

Water forms about 70% of our body weight and is an important constituent of all body cells. Water is required for all the biological processes in our body. Although water does not provide energy, it is a very important nutrient. It performs the following functions in the body:

1. Water transports food, wastes, gases and other chemicals (like hormones) throughout the body.

2. Water helps in digestion by dissolving the nutrients which can then be absorbed or digested by the body.

3. Water carries waste out of the body as sweat and urine.

4. Water helps to regulate the body temperature.

The amount of water needed by a person depends on one’s age, type or work and climate. Athletes and persons doing more of physical work must consume plenty of water as they lose a large amount of water as sweat. For the same reason more water is required by our body in summer than in winter.

7. Roughage:

As you already know, the cell walls of all plant cells are made up of cellulose. Cellulose cannot be digested by our digestive system. Although cellulose does not have any nutritive value for us, it is needed in our diet for proper functioning of the digestive system. Cellulose forms the fibre content of our diet and is referred to as roughage. Roughage provides the alimentary canal muscles with bulk against which they contract easily. This allows for more efficient movement of food in the alimentary canal, especially in the large intestine. Roughage helps in the regular movement of bowels. People who do not include roughage in their diet suffer from constipation.

Salad, fruits, vegetables and cereals constitute the main sources of roughage in our diet. Bhutt (corn) and dalia are also rich sources of roughage.