Your Ideal Weight Isn't What You Think It Is
BMI provides an estimation of fat content in a person's body, and is probably accurate for most people. Results may vary, however, because BMI doesn't take into account the differences between lean weight and fat weight. For example, athletes may develop significantly higher muscle mass. Muscle contributes more to overall weight than fat. As a result their BMI may be higher than average despite a lower fat content.
Conversely, elderly people who have suffered some deterioration of muscle mass may have lower BMI with higher fat content.
Again, using our Fat Albert/Jay Cutler example, we find that their BMI is an identical 39.1 (obese). Remember that we're using Jay's real statistics here, and Jay is certainly not obese. That's the danger of relying only on BMI as an indicator of fitness - without differentiating between lean body mass and fat body mass, the result is skewed.
Still, for the average American without Jay Cutler-size muscles, the BMI can be a fairly accurate measurement of health.