Gaining weight can change the structure of the heart and its ability to pump blood. But losing weight can reverse that process, which is potentially lethal. Overweight is defined as a body mass index (BMI) of 25 or more and obesity as a BMI of 30 or more. "People who gain weight, even just 5 percent, are more likely to have thickening on the left side of the heart, which is a well-established indicator of heart failure," said researcher…