I use very low calorie, almost exclusively protein diets – that are almost impossible stay on without prescription medication. Prescription medication can be exceedingly helpful on very low calorie programs, to help you control your hunger and sweet and starch cravings. 500-600 calories a day is almost no food. You can eat a lot more 20 calorie per ounce shellfish, than 80 calorie per ounce prime rib on a 500-600 calorie diet, so choose your protein carefully. Protein drinks should contain at least 15 grams of protein (my favorite contains 20 grams of protein), and no more than 100 calories per serving. Protein bars / wafers / cookies should contain at least 15 grams of protein, and no more than 150 calories per serving.