Underweight, like over, is a relative terms, being based on the ideal weight for a given height, built and sex. A person can be regarded as moderately underweight if he or she weighs 10 per cent below the ideal body weight and markedly so if 20 per cent below the ideal.

Appropriate body weight is among the most important physical attributes and has a deep influence upon the health and personality of an individual. For a healthy body, weight slightly above the average is favorable up to the age of 30 years, as it serves as a good defense measure against certain diseases, especially tuberculosis. Between 30 and 40 years of age, the endeavor should be to maintain the weight at the average level as during this period, many future diseases have their beginning. After the age of 40, it will be advisable to keep the weight slightly below the average, so as to lighten the burden on the heart, kidney and other vital organs.

There are two types of thin people. One type is wiry and energetic, who eat heartily but never put on weight. Presumably, they burn up energy due to constant activity. Such persons need not worry as chances are that they do not have any disease as such. The other type of thin persons lack energy dire, and are unable to take normal meals and find that rich food usually makes them sick. Their body lacks fat cells thus providing no storage place for added fat and the calories they consume are probably wasted.

Symptoms
Underweight due to an inadequate caloric in take is a serious condition, especially in the young. They often feel easily fatigued, have poor physical stamina and lowered resistance to infection. Diseases like tuberculosis, respiratory disorders, pneumonia, circulatory diseases like heart disorders, cerebral hemorrhage, nepthritis, typhoid fever and cancer are quite common among them. The occurrence of the complications of pregnancy in your women may result from malnutrition due to an inadequate energy intake.

Causes
Thinness may be due to inadequate nutrition or excessive bodily activity or both. Emotional factors or bad habits such as skipped meals, small meals, habitual fasting and inadequate exercise are some of the other causes of thinness. Other factors include inadequate digestion and absorption of food due to a wrong dietary pattern for a particular metabolism; metabolic disturbances such as an overactive thyroid and hereditary tendencies. Disorders such as chronic dyspepsia, chronic diarrhea presence of parasites like tapeworm in the alimentary canal, liver disorders, diabetes mellitus, insomnia, constipation, and sexual disorders can also lead to thinness.

Treatment
Diet play an important role in building up health for gaining weight. Nutrients which help keep the nerves relaxes are of utmost importance as nervousness causes all the muscles to become tense and the energy which goes into the tenseness wastefully uses up a great deal of food.

Although all vitamins and minerals are required for a sound health the most important ones are vitamin D and B6, calcium and magnesium. The richest sources of vitamin D are milk, cod liver oil and the rays of the sun. Calcium is also supplied by milk and yogurt. Magnesium can be obtained from green leafy vegetables such as spinach, parsley, turnip, radish and beet tops. These vegetables should preferably be taken in salad former should be lightly cooked.

Lack of appetite can result from an inadequate supply of vitamin B, which leads to low production of hydrochloric acid by the stomach. Hydrochloric acid is essential for the digestion of food and absorption of vitamins and minerals into the blood. It is, therefore, necessary that the daily diet should be rich in vitamin B for normal appetite. Proper digestion and absorption of foods and regular elimination. Foods rich in vitamin B are all whole grain cereals, blackstrap molasses, nuts, soyabean, eggs and butter. Vegetable oil is of special value to those wishing to gain weight as it is rich in vitamin E and essential fatty acids .

Underweight persons should eat frequent small meals as they tend to feel full quickly. Meals may be divided into six small ones instead of three big ones. These may consist of three smaller meals and three substantial snacks between them. The weight-building quality of a food is measured by the number of calories it contains. To gain weight, the diet should include more calories than are used in daily activities so as to allow the excess to be stored as body fat. The allowance of 500 calories in excess of the daily average needs is estimated to provide for a weight gain of one pound weekly.

All refined foods such as products containing white flour and sugar should be avoided, as they destroy health. Excessive intake of refined carbohydrates and fats may help the individual to put on weight but this will be detrimental to general health. The diet should be tilted towards alkaline-forming foods such as fruits and vegetables. Alkaline foods should comprise 80 per cent of the diet. The other 20 per cent should consist of acid forming foods such as cereals, and lentils. Beverages containing caffeine like soft drinks, coffee and tea should be curtailed. Smoking should be given up. Water should not be taken with meals but half an hour before or one hour after meals.

Milk Cure
An exclusive milk diet for rapid gain of weight has been advocated by some nature cure practitioners. In the beginning of this mode of treatment, the patient should fast for three days on warm water and like juice so as to cleanse the system. Thereafter, he should have a glass of milk every two hours from 8 a.m. to 8 p.m. the first day, a glass every hour and half the next day, and a glass every hour the third day. Then the quantity of milk should be gradually increased so as to take a glass every half an hour from 8 a.m. to 8 p.m. If such a quantity can be tolerated fairly comfortably. The milk should be fresh and unboiled , but may be slightly warmed, if desired. It should be sipped very slowly through a straw. The milk should be unpasteurised, if possible.

Figs are an excellent food remedy for increasing weight in case of thinness. The high percentage of rapidly assimilable sugar make them a strengthening and fattening food.