How to gain weight as i am 32years old and my weight is only 51.

See endocrinologist: You could have several problems. I assume that you eat normally. Question too brief to answer correctly. But, you need to see a specialist that can help you not only put on weight, but lean weight. Physical exercise such as weight lifting, drinking protein drinks, eating foods high in carbohydrates may help. Dairy products, white breads can also pack on weight.