Hi, Whatever you weight, it is not caused or affected by masturbation. Masturbation does not harm your body or cause any health problems. It's possible that you have an inherited lean constitution. One or both of your parents or grand parents might be lean (but this is not a rule). The other reasons could be nutritional deficiency, thyroid, sleeplessness, excessive worry or mental preoccupation, anemia and so on. See a general physician, who will examine you and suggest food, medicines and exercises to help you put on a healthy weight. Remember, being lean is not a disgrace. You have lots of advantages over your heavier counterparts.

Please consult an andrologist for opinion.