BMI's predictive usefulness is based on people looking like when it was developed (it's an index, so you have to choose values for "normal", "overweight", etc).

BMI's predictive usefulness is based on people looking like when it was developed (it's an index, so you have to choose values for "normal", "overweight", etc). Back then, people in the "normal" band had more muscle and less fat than they do today. From what I understand, body-fat % is a much better metric for individual health outcomes, and BMI is a rough proxy to that. Taking two populations of the same BMI, one from say 40 years ago and one now, the one now will be fatter and less healthy. The core problem is that BMI is a proxy for what you're actually trying to measure: body composition. Height and weight gets you there on average, if you make some assumptions about the average composition. If average composition gets worse, BMI becomes more inaccurate.