While most Americans are concerned about losing excess pounds, some have the opposite problem - how to add pounds onto an extra-thin body.

Genetics can be an important factor in the inability to gain weight. Other causes may be inadequate nutrition or excessive athletic activity. In some cases, persistent weight loss can be a sign of a mood or eating disorder, a digestive disorder such as malabsorption, endocrine problems such as thyroid disease, or of cancer.

It is important to see a physician...