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 upto 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 and drie, 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.