It’s almost impossible to stay on 500-600 calories a day without medication or structure or both. 500-600 calories a day is the fastest weight loss, but it’s almost no food. Eating Jols or very low calorie, very low carbohydrate, sugar-free candy and gum can help. Caffeinated fluid (coffee, tea, Diet Coke, Diet Pepsi, Diet Mt. Dew, Diet Jolt, Diet Red Bull, etc.) can help. 5% vegies (anything you like, except peas, potatoes or corn) can help.