Hi, Sure will help you There are many reason of being underweight. 1. Endocrine disturbances 2. Genetic 3. Vitamins and minerals deficiency 4. Psychological problem 5.IBS 6. Eating disorder 7. Liver dysfunction And many more Due to these reason something you cannot  gain weight. Its better to follow  a balance weight gain diet . Which not only increase your weight but also cure your deficiencies  mainly of vitamins and minerals.   So my opinion is to consult any near by dietician or if not possible contact me  directly  so that we can find the cause of weight reduction , gathering all your data and according ly we will suggest you diet and supplements which will soon help you to achieve ideal weight.