ur offcorse underweight ideally u shud be 65 kg. well it depends on so many things..like genetic make up n all..as long as ur healthy less weight is not a problem. there r certain ways which cn increase ur body mass apart from vitamins odr medicine claiming wt gain is not approved. so main theme is to increase appetite and hv high energy food which ur body cn assimilate before dat to knw ur body status i need to ask u sm tests to rule out odr nutritional deficiency causing this.