This doesn't seem to be a common symptom and I'm wondering if there is another cause besides fast metabolism? I am not underweight but I would just like to be less skinny. However, it is so hard for me to gain weight and keep it on. I can gain a couple pounds but usually within a week I lose it. I started taking Creatine recently to help, as well as Maca which can cause weight gain. I also eat a lot of peanut butter and high calorie meals. For being mostly sedentary, it does not seem like it should be this hard to gain weight. I will start drinking a lot of whole milk as well, but it does seem to make me feel a little sluggish. I have been doing a little better where I am able to do light exercising to try to build up the muscles in my legs. But it almost seems like I am losing muscle weight because I then feel weaker after a while? And I have muscle twitching after I haven't done anything. I also take B12, Vit. C and D, B-Complex, COQ10, Probiotics, Curcumin, and Iron. I don't have hyperthyroid symptoms. Does anyone else have this symptom? Or what else could it be?