What can i do if i'm underweight?

How do you FEEL?: If you're losing weight and don't know why, and don't feel well, your physician needs to work you up for any of a variety of causes including tumor and malabsorption / maldigestion. "Healthy weight" charts are subscience; most elite track-and-field athletes are "underweight". If you want to gain muscle mass, resistance training and eating sensibly when you're hungry is your key.