That advice comes by way of a study from the nutrition department at Pennsylvania State University. Researchers already knew that watered-down foods with a "low caloric density" could fill you up just as well as foods that had more calories in every spoonful.
But what if the food and water were consumed separately? To test the notion, one group of people ate a bowl of soup, and another group ate a casserole made with the same ingredients as the soup, then drank a glass of water.
The results: people who ate the soup were full after 1,209 calories. The people who had the casserole with the water on the side kept going until they had reached 1,657 calories.