editorNadia replied over 2 years ago:
As a first step, try taking our body health quiz (http://www.youbeauty.com/quizzes/body-health) to make sure you're actually underweight. If you are (or even if you just want to put on a few pounds), it's important to gain weight without sacrificing your health. Snacking on potato chips all day might add pounds, but it won't be good for your body. Instead, snack on nuts and avocados since they're calorie-dense but good for you. You can also just increase your portions of the usual foods we recommend--fruits and vegetables, whole grains, lean meats and legumes. If you feel like you're eating a lot without gaining weight, there might be something else going on. Some medical problems (like a thyroid disorder) can make weight gain really difficult, so mention to your doctor that you're struggling to gain weight and see if he/she has some insight.