Our bodies require a variety of nutrients and vitamins to stay healthy and be in good shape. So by eating a balanced diet, you ensure that your body is getting all the nutrients it needs in the right amounts! I definitely recommend a balanced diet and everything in moderation!
A great way to think about diets is to ‘eat the rainbow’ so try to eat fruit and veg of every different colour you can find. Add in a good helping of protein (meat or what ever takes your fancy) and some dairy or soy products and you should be in fairly good health. Variety and not eating to much of any one thing are key.
I agree with all the others. My wife studies genes that are involved in controlling body weight and she keeps telling me that you can do all the fancy science you like: but for most people if you want to keep your weight down its pretty simple: eat a balanced, healthy diet and do some exercise.