Every few months, a new study purports to prove that a calorie is a calorie is a calorie, and that the only way to lose weight is to burn more than you take in.

But veteran dieters know something that some researchers apparently don't: Certain foods seem to fuel the appetite like pouring gasoline on a fire. Some people find that once they start eating bread, cookies, chocolate, potato chips -- or leftover Easter candy -- they lose all sense of fullness and find it difficult to stop.

That's the concept behind "The Skinny," a new book by Louis J. Aronne, longtime director of the Comprehensive Weight Loss Program at NewYork Presbyterian Hospital/Weill Cornell Medical Center. He makes the best case yet why what you eat and when you eat it can make a big difference in appetite, satiety and how much willpower it takes to cut down. "It's true that a calorie is a calorie," Dr. Aronne says. "But what that doesn't take into account is how some calories affect what people eat later on."

After 23 years of treating patients -- some of it espousing liquid diets -- Dr. Aronne has concluded that refined carbohydrates and foods with high sugar and fat content promote what he calls "fullness resistance." They interfere with the complex hormonal messages the body usually sends to the brain to signal that it's time to stop eating. People feel hungrier instead.

This happens in part because refined carbohydrates raise blood-sugar levels, setting up an insulin surge that drives blood sugar down again, causing rebound hunger. That insulin spike also interferes with leptin, the hormone secreted by fat cells that should tell the body to stop eating. Obese people have loads of leptin, but it either doesn't get to the brain, or the brain becomes resistant to it. "This is not a failure of willpower, it's a physical mechanism," Dr. Aronne writes. The body also becomes resistant to insulin, setting the stage for diabetes.