The letter should include a introductory statement addressing the friend. It should say why/how the topic of the letter was thought of and why it could/does concern the addressee. The letter could include the writer's state of health at the moment, eating habits, favorite foods and and an opinion on what are healthy foods and what are not. It could include a separate paragraph where healthy eating habits is linked to other healthy habits like exercise or sleep. The letter could end with advice for the addressee and if the friend receiving the letter is close, the closing statement could be informal; otherwise, the closing statement should include a formal goodbye.