1) write down (in a short form) everything you already know about specific illnesses such as Type 2 diabetes, osteoporosis, anorexia nervosa, bulimia nervosa, and so forth. This is a brainstorming exercise to help you revise and activate your knowledge!
2) Then, write about what you want and need to learn about diet-related illnesses. This will help you manage your learning and fill in the gaps.