At the age of sixty-six, Bryanna Heartley had what most would consider an ideal life. She was the matriarch of a growing family, each of her children successful in his or her own way. She had a happy marriage. She had close friends, financial security, overseas holidays, and a psychology career that she truly loved.

However, there was a problem: Bryanna was obese. She suffered from bouts of asthma, problems with reflux, high cholesterol, irritable bowel syndrome, and the early signs of hypertension and renal failure. If she wanted to live much longer, she needed to make a change. With...