"BMI is bullshit!" cries the ever-informed fat activist.

Fat activists always like to point out the one flaw of the BMI system: the fact that it does not differentiate between muscle and fat. Yet they don’t understand that this flaw does not make the system flawed for them—it makes BMI flawed for people of large muscle mass.

Two men go to the doctor, same age, same height, same weight. Let’s say the weight is at 250 pounds, which might be overweight for whatever height by the BMI. One man is a professional athlete. The other man’s muscle mass accounts for little of his weight. The doctor, because he is not an idiot, would not suggest weight loss for the professional athlete. The doctor, because he is not an idiot, can differentiate between if a person is overweight by unhealthy copious amounts of fat or overweight by perfectly fine muscle mass.