I'm 21 & not gaining weight. every possible investigation done & negative. family is healthy but i'm only 45 kgs height 172 cm. tried everything...:(?

Why underweight?: There are lots of reasons for being underweight, ; these should be ruled out by medical evaluation. Assuming your work-up identifies no maldigestion, malabsorption, inflammatory disease, chronic infection, metabolic abnormalities, ca, or non-gi systemic disease then weight gain is a simple matter of taking in more calories than you burn. Supplement with fortified milk shakes/ commercial products.