Hello,
Thanks for posting your query.
Being lean with a good height is not a reason for back pain. It is more likely to be due to calcium or vitamin D3 deficiency or due to bad posture. Taking calcium supplements and improvised posture will help reducing the pain.
To gain weight, a good balanced diet with fruits, vegetables, proteins, milk and yoghurt, cereals etc is required to be taken daily. The other thing she can do is exercise regularly like swimming, cycling or jogging. She can also join a gym as appetite increases after exercising. Exercise is of paramount importance because it releases endorphins from the body.
Weight gain requires eating calorie-rich but also nutrient-rich foods -- not just high-calorie foods with lots of fat, sugar, or empty calories. Try to eat six times a day, with each meal (or at least three of them) containing protein, starch, vegetables, and fat.

Foods highest in calories and nutrients are foods with fats, especially plant fats. Plant fats such as nuts, peanuts, seeds, peanut butter, almond butter, avocados, hummus, and oils are great sources of healthy fats loaded with nutrients and calories. Eat nuts by the handful or sprinkle them on top of soups, salads, cereal, desserts, and casseroles for added protein, fiber, healthy fat, and calories.

I hope this answers your query.
In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.
Please accept my answer in case you do not have further queries.
Wishing you good health.
Regards.
Dr. Praveen Tayal.

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