He knows people who have lost weight and kept it off with diets, including a colleague in his department. “He lost 30 or 40 pounds in the 1970s and kept it off all these years,” Dr. Sacks said. But about 15 to 20 percent of patients respond to measures short of surgery, with response defined as a change in their body weight that is maintained without constant hunger or cravings. While people seldom end up thin, their permanent weight loss is a proof of principle, showing that treatments can lower the weight the brain forces a person to maintain. She has tried one diet after another, losing weight and gaining it back.