One of the tests for BMI is a floatation test (this is why the height/weight BMI charts are nonsense).
Possibly terminology is different in the USA, but in Australia the floatation test is used to calculate Body Composition, and particularly Body Fat Percentage (%BF), rather than
Body Mass Index (BMI). BMI is just a number relating height to body mass, and it needs to be interpreted intelligently with an understanding of its
considerable limitations as a diagnostic tool:
BMI was originally intended for statistical studies, not clinical diagnosis, and its main virtue is ease of calculation. No
competent medical professional should use it as the sole criterion for judging whether an individual is over or under weight. Simple height vs. weight tables are particularly potentially misleading when applied to athletes, children and adolescents (school nurses please note), elderly people, and people who are unusually short or tall because the height/weight ratio in BMI doesn't correctly reflect the way real human bodies "scale" (My hometown boy Yao Ming, pictured above, has a BMI of around 27. Overweight? Not so much, I think).
The criticism of BMI I read in forums like this one focuses
entirely on the perception that BMI over-diagnoses overweight condition in fit muscular people, but BMI also under-diagnoses obesity in people with levels of body-fat strongly correlated with bad health outcomes. As the authors of the paper I link above put it:
"The implications of mislabeling patients are not trivial. By using BMI as a marker of obesity, we misclassify ≥ 50% of patients with excess body fat as being normal or just overweight and we miss the opportunity to intervene and reduce health risk in such individuals. Conversely, BMI may lead to misclassification of persons with normal levels of fat as being overweight, a fact that could cause unnecessary distress and prompt to unnecessary and costly interventions. In addition, such mislabeling has a deleterious effect on public trust for healthcare providers, particularly from fit patients with evident preserved muscle mass."Having said all that, BMI is not without some value because of its simplicity. If you are a fairly sedentary urban adult, as opposed to an athlete or person engaged in heavy manual labour, and your BMI suggests that you are overweight (25+), you
possibly are, and you should consider talking to your doctor about it. If your BMI is over 30 you
probably are, and you definitely should.
I obviously spend
way too much time around doctors...