POPKIN REPLIES: Many studies have shown that soup does make us feel full, whereas caloric and noncaloric beverages do not. We are unsure if this appetite satisfaction is because of the salt or fat in soup, the way it is consumed, our perception of it as a filling food, or other reasons. Similarly, many studies have demonstrated that beverages containing sugar, high-fructose corn syrup or alcohol are handled differently by the body than when sugar or high-fructose corn syrup is incorporated in solid foods; as a result, the overall caloric intake from solid food does not adjust to account for the calories in these beverages. The mechanism responsible for that weaker compensatory response to fluids is unknown. I posited the hypothesis that humans may lack a physiological basis for processing carbohydrate or alcoholic calories in beverages because only breast milk and water were available for the majority of our evolutionary history. Another possibility is that carbohydrate- and alcohol-containing beverages may produce an incomplete satiation sequence that prevents us from becoming satiated on them. George Bray of Louisiana State University, my co-author for a study that will amplify these theories, suggests that one possible mechanism is the way the gastrointestinal tract responds to the form in which it is exposed to nutrients.