Some people who have celiac disease or gluten sensitivity are underweight because their gluten issues prevent them from absorbing nutrients. So you may actually need to gain weight to be at the peak of health.
When you go gluten-free, your gut usually heals quickly, and you begin to absorb nutrients and calories. Your weight usually normalizes quickly simply as a result of being gluten-free.
Sometimes when people go gluten-free, they inadvertently cut calories by leaving out things like bread and butter. If you’re already underweight, you may need to supplement a healthy diet with high-calorie foods like nuts, avocados, tuna, and salmon.