Being underweight is not a common problem in the United States, affecting only about 2 percent of adults, compared with two-thirds who are overweight or obese.

But people who are too thin can be vulnerable to disease because they may have weakened immune systems; they are also at higher risk of osteoporosis.

There are varying reasons why a person may be underweight. Some may have fast metabolism and burn calories off quickly. Others may be recovering from an eating disorder like anorexia or bulimia or from the side effects of disease such as cancer or AIDS.

People looking to shrink their waistlines often have a wealth of information to reach their goals from diet books to weight-loss programs to support groups. But for those wanting to fatten up, it is often a lonely struggle.

"We are so preoccupied with the idea that people might gain too much weight that we almost don't want to admit that any weight gain is normal," said Joanne Ikeda, the co-director of the Center for Weight and Health at the University of California, Berkeley.

While there isn't a one-size-fits-all plan to weight gain, the key is to take in more calories than you burn. Consuming an extra 500 calories a day may add a pound a week.

People suffering from eating disorders or the side effects of disease must first break through mental and physical barriers before they can start counting pounds. For the naturally underweight looking to beef up, health experts advise eating five or six times a day, spacing the meals to avoid feeling stuffed and keeping in mind that not all foods are created equal.

Choose nutrient-rich foods like bread, cereal and pasta and dairy-based products like milk, cheese and yogurt.