I use 500-600 calorie diets, because they create the fastest, medically monitored, safe weight loss possible. The problem is, you may not be able to stay on 500-600 calories a day, without prescription medication to help curb your hunger and sweet and starch cravings. If you are having trouble staying on 500-600 calories a day (it’s almost no food, unless you are eating mostly egg whites and low carb vegetables), bump your calories up to 600-800 or even 1000 a day. If you go much beyond 1500 calories a day, you won’t lose weight fast enough, to stay motivated enough to stay on your diet.