One possible version:

Jane,

     I' m very glad to receive your letter. But at the same time, I' m sorry to hear that you are not very well

these days. In your letter you wants to know how to stay healthy. Here is my advice.

     Firstly, you should keep a balanced diet. You should avoid eating food high in fat and too much sweet

food. You' d better eat more fresh vegetables and fruits. Secondly, taking exercise every day helps building(up) a strong body. Why not take more exercise in your spare time? As we all know, regular exercise is

an important part of keeping us healthy. Finally, make sure you have enough sleep and you mustn' t stay

up too late at night.

     By eating properly and exercising regularly, you can keep your body at a proper weight and stay

healthy. I truly hope my advice is helpful and you' ll get well soon.

Best wishes!                  

                                                                                                         Yours sincerely.

                                                                                                                 Li  Hua