At first she thought regular vigorous exercise would do it, but her weight began climbing. Eventually, she discovered a lifestyle program that lets her keep her weight between 155 and 180 pounds — her weight fluctuates because each time she relaxes her guard the pounds return. Dr. Kerns, 42 — now an obesity medicine specialist at the Veterans Affairs Medical Center in Washington — reached a peak weight of 300 pounds. She knows all too well how hard it is to lose weight and keep it off. Diets would work, but then the weight would come back.