The illustrations here were created from scans of six people, who were all 5 feet 9 inches tall and 172 pounds. This means that though their bodies look very different, they all have exactly the same body mass index, or B.M.I. At 25.4, technically each of them could be considered overweight. (By the most common definition people with a B.M.I. over 25 are overweight and those with a B.M.I. over 30 are considered obese.)
How is it possible that the same B.M.I. can look so different? The simple explanation is that muscle and bone are denser than fat and some people carry more or less weight in their torso or legs. As B.M.I. factors in only height and weight, it sidesteps these differences.
This is a crucial shortcoming as some research now suggests that body fat percentage and body composition are more important indicators of health than weight alone. That puts the emphasis on exercises that don’t just burn calories, but also build muscle. This also means that, to get a summer “beach body,” a strict weight loss goal might not be the right path.