Although nutrition experts still debate the precise limits of what constitutes a healthy weight, there's a good working definition based on the ratio of weight to height.
This ratio, called the body mass index (or BMI for short), takes into account the fact that taller people have more tissue than shorter people, and so tend to weigh more.
Dozens of studies that have included more than a million adults have shown that a body mass index above 25 increases the chances of dying early, mainly from heart disease or cancer, and that a body mass index above 30 dramatically increases the chances.
Based on this consistent evidence, a healthy weight is one that equates with a body mass index less than 25. By convention, overweight is defined as a body mass index of 25 to 29.9, and obesity is defined as a body mass index of 30 or higher. Nothing magical happens when you cross from 24.9 to 25 or from 29.9 to 30. These are just convenient reference points. Instead, the chances of developing a weight-related health problems increases across the range of weights. Muscle and bone are more dense than fat, so an athlete or muscular person may have a high body mass index, but not be fat.
It's this very thing that makes weight gain during adulthood such an important determinant of weight-related health—few adults add muscle and bone after their early twenties, so nearly all that added weight is fat.
