Body Mass Index

Body Mass Index (or BMI) is a quick way of converting one's height and weight into one metric.  The problem is that the BMI metric is largely without value.  It is not any better of a metric of health than someone's height or weight alone.  According to the Mayo Clinic:

FINDINGS: We found 40 studies with 250,152 patients that had a mean follow-up of 3.8 years. Patients with a low body-mass index (BMI) (ie, <20) had an increased relative risk (RR) for total mortality (RR=1.37 [95% CI 1.32-1.43), and cardiovascular mortality (1.45 [1.16-1.81]), overweight (BMI 25-29.9) had the lowest risk for total mortality (0.87 [0.81-0.94]) and cardiovascular mortality (0.88 [0.75-1.02]) compared with those for people with a normal BMI. Obese patients (BMI 30-35) had no increased risk for total mortality (0.93 [0.85-1.03]) or cardiovascular mortality (0.97 [0.82-1.15]). Patients with severe obesity (> or =35) did not have increased total mortality (1.10 [0.87-1.41]) but they had the highest risk for cardiovascular mortality (1.88 [1.05-3.34]).

Their data shows that BMI is not a reliable metric for predicting health outcomes.  From a medical or scientific standpoint the metric BMI is just not valuable.  Another study published in the American Journal of Public Health found:

Traditional BMI categories do not conform well to the complexities of the BMI–mortality relationship. In concurrence with conclusions from previous literature, I found that the current definitions of obesity and overweight are imprecise predictors of mortality risk.

If a gut level reaction to assessing health by height and weight alone is not enough, take a look at the Wikipedia section on it.

Some argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health.  University of Chicagopolitical science professor Eric Oliver says BMI is a convenient but inaccurate measure of weight, forced onto the populace, and should be revised.

The medical establishment has generally acknowledged some shortcomings of BMI. Because the BMI is dependent only upon weight and height, it makes simplistic assumptions about distribution of muscle and bone mass, and thus may overestimate adiposity on those with more lean body mass (e.g. athletes) while underestimating adiposity on those with less lean body mass (e.g. the elderly).

The problem with making the connection between BMI and overall healthfulness is that the metric is then used inappropriately, for example by health insurance companies.

In the United States, where medical underwriting of private health insurance plans is widespread, most private health insurance providers will use a particular high BMI as a cut-off point in order to raise insurance rates for or deny insurance to higher-risk patients, thereby ostensibly reducing the cost of insurance coverage to all other subscribers in a 'normal' BMI range. The cutoff point is determined differently for every health insurance provider and different providers will have vastly different ranges of acceptability.

Another article published in The Dartmouth, Dartmouth College's student newspaper, proposes that insurance and pharmaceutical companies actually have a financial incentive for embracing and perpetuating the use of the flawed BMI metric.

According to Oliver, the "obesity mafia" consists of government health agencies such as the National Institute of Health, the Center for Disease Control and the Food and Drug Administration. These agencies are linked in symbiotic relationships with drug companies and academic researchers, with the trio feeding off one another to perpetuate the myth of widespread obesity in America -- all for financial gain.