Two years ago, a young organ transplant doctor told me a harrowing story. Recently he had stood by and watched helplessly as a 15-year-old African-American girl died from an enlarged heart. A transplant might have saved her, but high blood pressure, diabetes, and a body mass of more than 400 pounds made surgery impossible. The memory haunted him as he continued to treat more and more children experiencing the deadly effects of chronic obesity. Mostly poor black kids, they marched through his office suffering from high cholesterol, high blood pressure, enlarged hearts, and adult-onset diabetes that promised to fill their...