Why are some diets unhealthy?

Diets: Fad diets are marketing ploys to promote a concept that has more to do with making money than with a well-balanced diet that promotes long-term compliance conducive with a healthy lifestyle. Many of these diets are also low calorie diets. A low calorie diet is a diet that provides extremely low calories in order to produce significant weight loss. They can be unbalanced and cause problems.