From the point of view of the science of diet, the food we eat should contain five constituents, namely, carbohydrates, proteins, fats, mineral salts, and vitamins. The first three constituents in this list are supposed to yield energy that is required for the life activity of an individual. One gram of carbohydrate or protein gives 4.1 calories of energy, while one gram of fat is found to give 9.2 caloriesof energy. We get large amount of carbohydrates from cereals, potatoes, sugar, etc. The proteins, in addition to supplying energy, perform another important function, that of building the muscles of the body. They are of two types, namely, plant proteins, which are contained in large quantities in pulses, and dried nuts, and animal proteins, which may be received from animal sources, like eggs, meat, fish, poultry, milk, and so on. The animal proteins are more valuable, and hence vegetarians must include sufficient quantity of milk in their diet, so as to fulfil the requirements of animal proteins. The fats can also be had from two sources, namely, plants and animals. Both these are found to have equal energy value. But the animal fats, which are contained in butter, meat, egg, fish and poultry, are more valuable in as much as they are rich in vitamins A and D.Many minerals, like Calcium, Magnesium, Phosphorus, Sodium, Iodine, Sulphur, etc., are found in the human body. They play an important role in controlling the biochemical activity going on in the body.Calcium and Phosphorous are largely required for the proper development of bones and teeth. Many disorders are caused due to deficiencies of various minerals, and hence, it is necessary to have them in sufficient amounts in one's diet.The vitamins, although needed in very minute amounts, are often found to be lacking in diets. They are obtained from fruits, vegetables, milk, etc., and are very intimately concerned with the maintenance of health and vigour of the body. Along with these nutrients, water is also needed in large amounts, because nearly 65% of the weight of our body is made by water. The body fluids, like saliva, the blood, the digestive juices, and endocrine secretions contain large amounts of water. Water helps in maintaining the body temperature within the normal range.The mouth is the receiving organ for food. The food is cut into pieces with the teeth, and is ground into finer forms, and is moistened with saliva, so that it can be easily passed down the throat. The pharynx is a part next to the mouth, where seven paths come and meet: two from the nose, one from the mouth, two from the ears, one going down to the lungs, and one to the stomach. The food masticated and moistened in the mouth passes through the pharynx, down this last path, made by the gullet or oesophagus, which leads ultimately to the stomach. The food remains in the stomach for about two hours, and gets mixed with the Hydrochloric Acid and digestive juices secreted by the stomach wall. It then passes to the duodenum which has the shape of an inverted horse-shoe. Three digestive juices get mixed with it here, namely, the pancreatic juice (secreted by the glands called pancreas), the bile (produced in the liver), and the juice of the duodenum itself. As a result of the action of various digestive juices, the constituents of food, especially the fats, proteins and carbohydrates are broken down to simpler substances which can be assimilated in the body. The small intestines which are over twenty feet long, are responsible for absorption of the digested constituents of food. The remaining part goes to the large intestines (nearly five feet long), and is ultimately eliminated through the anus after absorbing water from it in the large intestines. The assimilated constituents of food are largely stored in the liver, and are supplied to the tissues, muscles, and all the parts of the body, through the agency of blood.