The problem with BMI is that is not a strong indicator of over all health. Let me put it this way. People with poor health (heart attack, diabetes, etc) tend to have high BMI's but people with high BMI's don't necessarily have poor health. Think of the guy that's 5' 8'' but lifts weights. He will have a very high BMI but is probably in good shape. To accomplish what your suggesting you would use lipid profiles (levels of triclycerides, glucose, etc.), etc.