Diseases or disorders occur mainly due to the deficiency of one or more nutrients in the diet over a long period of time. For example, wheat is rich in carbohydrates, but poor in nutrients like proteins and fats. Too much intake of wheat products results in a deficiency of proteins and fats, which reduces growth.  Lack of proteins also results in stunted growth, skin diseases, swelling of the face and discolouration of the hair, and even causes diarrhoea. So a balanced diet is required to avoid deficiency diseases.

Lack of iron in the diet leads to a deficiency disease called anaemia. The symptoms of anaemia are headache, dark patches under the eyes, weakness and feeling tried. A balanced diet should, therefore, include iron-rich foods such as apples, fish and raisins to avoid anaemia.
Lack of iodine in the diet leads to a deficiency disease called goitre. In adults, goitre results in swollen glands at the throat. In children, iodine deficiency causes stunted mental and physical development. Iodine deficiency can be avoided by using iodised salt in the diet.

Lack of calcium in the diet makes the bones weak and brittle, and also causes tooth decay. Milk, eggs and other dairy products contain calcium and give strength to the bones and teeth.

Lack of vitamin A in diet causes loss of vision. Symptoms include night blindness, i.e. poor vision in dim light. Eating food rich in vitamin A, such as carrot, mangoes, butter and egg yolk, helps avoid loss of vision.
Lack of vitamin B1 in the diet causes a deficiency disease called beriberi. The symptoms are loss of weight and weak muscles. Food rich in vitamin B1, such as beans, meat, eggs and corn, helps avoid beriberi.
Lack of vitamin C in the diet causes a deficiency disease called scurvy. The symptoms of scurvy are bleeding gums. Moreover, wounds take longer to heal. Oranges, tomatoes, lemon, guavas and amla are rich in vitamin C, so eating these helps avoid scurvy.
Lack of vitamin D in the diet causes a deficiency disease called rickets. The symptoms are soft and bent bones. The diet should include fish, eggs, milk and butter to avoid rickets. In addition, exposure to sunlight produces vitamin D in the body.