Things you might include in your letter:Names of fast food:sandwiches/hamburgers/pizza/ fries/hot dogs/coca cola/ice-creamReasons for not having too much fast food:have little nutrition;have less fibers/vitamins;not provide well-balanced nutrition;full of fat;get ill/sick/unhealthy;have side/bad effect on the health.Some advicenot eat takeaways/ready-made meals/junk food so frequently;eat/have fresh fruit/vegetables as often as one can;eating/having some light food is good for sb.’s heart/liver;drink green/black tea/fresh fruit juice as often as possible;Well-balanced diet keeps somebody fit/healthy.Useful expressions you may use to show your concern for your friends:I’m writing to express my worry about you.As soon as I know/learn about your work and life, I miss you more.I’m sorry to learn that you are too busy to cook.Please don’t push yourself too hard.Transitional words or phrases you may use to join the sentences togetherbut, if, because, so, as often/frequently/much/many as possible/you can