My best advice regarding supplements is to stay away. Eat regular whole foods (not processed crap), and eat as much as you want. You are apparently naturally thin – learn to love it. I have never much cared for muscle mass either. I’m 5’9” and 150lbs. I do yoga daily (power vinyasa style). Vinyasa is a style that puts a lot of emphasis of building strength in addition to flexibility. With yoga, you are not using weights, so the mass is not gained, but you get long lean muscles. Also, the definition and tone is amazing.