The most important thing that you have to remember about gaining or losing weight, is that everything you eat or don't eat will have an effect on whether or not you will reach your goals. We are what we eat, because if we don't eat well then our bodies will not have the fuel needed to convert into energy or muscle.
You have to feed your body with the right amount of and types of food for it to be able to function properly, at the same time keeping you healthy and mentally fit. fad diets don't generally work to either lose weight or gain weight in the long term. You may lose a few kilos but unless you modify your etaing patterns in the long term you will still either put the wieght back on or lose it again.
Regardless of what anyone says about eating a healthy and well balanced diet, you will never get all the proper vitamins and nutrients you need just from the food you eat. The main problem is that we all eat snacks and meals that fill us up but don't have the daily doses of vitamins, minerals and other bits and pieces.
The moral of the tale is that we need to include supplements in our diet, such as vitamins, calcium etc. The first rule of health eating is just that eat well and take vitamin supplements. I have put together two guides for diets for either losing or gaining weight, so have a look, but remember that they are guides only and are not emant to be the definitive guide of diets.
Training to lose fat requires different methods than training to gain muscle. Losing fat involves eating a low calorie diet, while building muscle requires a higher calorie diet.