Whole grain pastas, sauces with brown lean beef and fish are also positive choices, Primack says.

Gaining a new perspective

As society in general has become increasingly overweight, underweight individuals may feel they stand out.

Others should be conscious of how their remarks that someone looks "too skinny" could be hurtful, Eang says.

"Some people have said, 'Oh, you look fine,' ... and there are other people who say, 'You're skinny.' It's the same way, that if a person were fat, (commenting on) it would be offensive."

Bacon says it is more important to practice healthy habits than to worry about weight and societal ideas about beauty and attractiveness.

"I don't think the answer is to gain weight to meet social standards or meet some kind of arbitrary idea of health," she says. "We have so many social and health expectations that aren't predicated on what's true for the individual."

If underweight individuals are living healthy lifestyles but still find themselves on the lower end of the weight spectrum, it may be helpful to change how they view their bodies, she says.

"The best avenue is to start to appreciate the functionality of our bodies," she says. "It is just amazing I have legs that can carry me from one place to another ... that we have mouths that allow many of us to be able to speak. There is so much we can just appreciate and marvel at in our bodies."

Are you part of the "2 percent"? Do you feel as if you are underweight and want to gain a few pounds? Is weight even the best way to determine if an individual is healthy? Share your thoughts and experiences in the comment section below.