Quark is a type of fresh cheese, close to yogurt, that is allowed during the Dukan diet due to it’s low fat content. However quark can be very hard to find in the United States and is much more common in other countries. You may be able to special order quark in the United States but make sure that it is low in fat. Some substitutes are much higher in fat than the original. Many people use fat free or 2% ricotta as a substitute for quark in recipes.