Explanation The food that we eat can be grouped into five classes called nutrients. These are carbohydrates, fats, proteins, mineral salts and vitamins. Carbohydrates: Carbohydrates form our main energy source. They are oxidised in cells to release energy. Carbohydrates are substances containing the elements of carbon, hydrogen and oxygen, with hydrogen and oxygen in the ratio of 2:1. Starch and sucrose belong to this class and are made up of simple sugars like glucose. Based on their simple sugar content, carbohydrates are divided into monosaccharides or simple sugars, disaccharides or double sugars and polysaccharides or complex sugars. Monosaccharides: Monosaccharides are simple sugars that can be absorbed directly in the body. The common examples of monosaccharides are glucose, fructose and galactose. Glucose is the most common form of sugar found in organisms. It is called as grape sugar. Fructose is common in plants and is called as fruit sugar. Galactose does not occur freely in nature but is combined with glucose to form lactose a disaccharide sugar found in milk. Simple sugars have the chemical formula C6 H12 O6. Disaccharides: Disaccharides are made up of two simple sugars. The chemical formula is C12 H22 O11. They cannot be absorbed by the body and are digested to form two simple sugars that can be absorbed. The common disaccharides are sucrose, maltose and lactose. Sucrose is cane sugar. The commercial sugar that is used at home is sucrose. Sucrose is obtained from cane sugar and sugar beet. Sucrose is a double sugar, made up of two simple sugars - glucose and fructose. Maltose is malt sugar and is made up of two glucose molecules. Lactose is milk sugar and is made up of glucose and galactose. Polysaccharides: Polysaccharides are complex sugars. The common polysaccharides are starch, cellulose and glycogen. Starch is an insoluble carbohydrate consisting of glucose units arranged in two forms, amylose and amylopectin. Plants store food in the form of starch. The chemical formula of starch is (C6 H10 O5) n. The rich sources of starch are potatoes, rice, maize, wheat and barley. Cellulose is unbranched polysaccharide composed of beta glucose units. The cell wall of plants is made up of cellulose. It has high tensile strength and is fully permeable. Animals store excess carbohydrates in the form of glycogen. Fats and oils: Food rich in fats are butter, cream, lard, fish liver oil, animal fat and vegetable oil. Fats and oils are made up of carbon, hydrogen and oxygen. Fats are made up of fatty acids and glycerol. Fats and oils produce more energy than carbohydrates. Fats are important 'storage' food and serve as insulators. Besides providing energy to the body, fats are solvents for some vitamins. Protein: Food rich in proteins are lean meat, fish, egg, milk, cheese, beans, peas and nuts. Proteins are made up of carbon, hydrogen, oxygen and nitrogen. Amino acids are the building blocks of proteins. Some proteins contain sulphur and phosphorus. Proteins are required for the growth and repair of body cells. Protein deficiency in children causes Kwashiorkor and Marasmus disease. Kwashiorkor : Kwashiorkor is a malnutrition disease that occurs due to the severe deficiency of proteins in the diet of children, though the diet may contain adequate amounts of carbohydrates. The child becomes weak, irritable and listless. The abdomen of the child swells, accompanied by swelling of the feet. The skin becomes dark and scaly. The child suffers from anaemia and the liver is damaged. The child may die before the age of 5 years. Kwashiorkor is a Ghanian term, which means the sickness the baby gets when the new baby comes. This happens as the child does not get the mother's milk and he is fed on a starchy diet of fruits and vegetables. Marasmus: Marasmus is a malnutrition disease that occurs due to the deficiency of proteins and carbohydrates in the diet of small children. Children suffering from Marasmus lose body weight rapidly. There is wasting of muscles. The child becomes so thin that loose folds of skin are visible all over the body. The ribs of he child look very prominent. Oedema and skin pigmentation are however absent in children afflicted with Marasmus. Mineral salts: Mineral salts are required in the diet in small quantities as they regulate various metabolic activities in our body. Calcium is required for healthy teeth, muscle contraction, clotting of blood and strengthening of bones. Milk, eggs and green vegetables are a rich source of calcium. Phosphorus is required for strengthening of bones and for the production of energy rich compounds ATP. Milk, fish, pulses and green vegetables are rich sources of phosphorus. Iron is required for the formation of haemoglobin. Deficiency of iron causes anaemia. Green leafy vegetables and liver are rich sources of iron. Iodine is required for the proper functioning of the thyroid gland. Deficiency of iodine causes goitre. Sea foods and iodised salts are rich sources of iodine. Potassium is required for the proper transmission of nerve impulses. Sodium is required for the proper transmission of nerve impulses and the proper functioning of the kidneys. Chlorine is required for the proper functioning of the gastric juice and for tissue fluids. Household salt is a rich source of sodium, potassium and chlorine. Vitamins: Vitamins are organic substances required by the human body in minute quantities for normal growth and maintenance of health. Vitamins are not energy giving food but are essential to keep our body healthy and free from diseases. Vitamins are generally classified into two groups, fat soluble vitamins and water soluble vitamins. Fat soluble vitamins include vitamins A, D, E and K. Water soluble Vitamin A (Retinol) is fat soluble, mainly found in carrots, yellow fruits, leafy green vegetables, butter, liver, egg yolk and fish liver oil. It is required for normal growth and keeps the skin healthy. It forms a component of visual purple in the retinal cells of the eye. Deficiency of vitamin A causes night blindness, xerophthalmia and dermatosis. Vitamin B complex is a group of water soluble vitamins, which generally function as coenzymes in metabolic reactions occurring in the cells. The complex consists of vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B5 (pantothenic acid), vitamin B6 (pyridoxine), biotin, nicotinic acid, folic acid and vitamin B 12 (cobalamin). Whole grains, yeast, liver, eggs and lean meat are rich in Vitamin B1. Its deficiency causes beri-beri. Yeast, lean meat, liver, milk, eggs, groundnut and whole grains are rich in nicotinamide. Its deficiency causes pellagra. Liver and leafy green vegetables are rich in B 11 or folic acid. Deficiency of folic acid causes anaemia. Vitamin B12 or cobalamin is required for the normal functioning of the red blood cells. Lack of it in the diet causes pernicious anaemia. Liver, meat, milk and cereals are rich sources of vitamin B12. Vitamin C is water soluble and mostly found in fresh citrus fruits like limes, lemons, oranges, and grapefruit. Its deficiency causes scurvy. Vitamin D is fat soluble and is the only vitamin that the human body can synthesise. It is produced by the skin in the presence of sunlight. Fish liver oils, milk and eggs are rich in vitamin D. Its deficiency causes rickets. Vitamin K is a fat soluble vitamin. Leafy green vegetables, cabbage and spinach are rich in vitamin K. It is essential for normal clotting of blood. Its deficiency leads to haemorrhage. Water: Water is very important for our body. About two-third of the body weight is water. Water serves several functions in the body including acting as a solvent for thousands of substances, both organic and inorganic; helping in the production of digestive juices, transportation of digested food and oxygen throughout the body, helping in the excretion of soluble wastes and in maintaining the body temperature. Water is lost from the body through sweat, urine and as water-vapour in breath. Therefore water must be constantly replaced. It is recommended that an adult individual drinks at least six glasses of water each day. It is essential to drink sufficient water for proper functioning of the body. Roughage: Roughage is the undigestible portion of the diet. It largely consists of the cellulose of the plants. The roughage enables the food to move easily through the intestines and also helps to avoid constipation. Balanced Diet: A balanced diet has all the principal constituents of food in correct proportion and quantity. A balanced diet includes all the nutrients, water and dietary fibres (roughage) needed to maintain good health. Balanced diet of individuals vary according to the age, gender, body weight, occupation and the temperature of their external environment. It is obvious that manual workers require more calories than sedentary workers; adults need more calories than small children and usually men need more calories than women. Individuals living in warm climates need less calories than individuals living in cold climates. For a diet to be balanced, the essential dietary nutrients should be present in the required and correct proportion. It is recommended that for a healthy adult, the calorific food must be evenly distributed among carbohydrates, fats and proteins, on average, in a ratio by weight of 5:1:1.