All the 2 percent want to do is gain weight

Fewer than 2% of adults in the United States are underweight, according to 2007 to 2010 data collected by the National Center for Health Statistics. To be considered underweight, individuals must have a body mass index of less than 18.5. A woman who is five-foot-six, for example, would weigh 114 pounds or less.

For some, difficulty gaining weight can be a frustrating problem and must be approached in a healthy way, experts say.