I am having the same issue! Before I discovered my intolerance for wheat/gluten I was hovering around 170lbs. After getting sick and losing 15-20lbs within the span of 1.5-2 months (prior switching to gluten free), I have been incapable of putting weight back on. I have increased my protein intake in an attempt to put on weight along with helping to build muscle and I've also stopped watching my sugar/carb/calorie intake. I've been purposely letting myself eat worse in hopes that it will help put on any kind of weight! Nothing seems to be working!! I hope you have more luck with it than I and would be interested to hear any other tips / success stories!