In 2008, the U.S. Centers for Disease Control and Prevention reported that 34 percent of Americans were overweight, another 34 percent were obese and approximately 6 percent were extremely obese. To identify these categories, the CDC uses a parameter called "Body Mass Index," or BMI. First developed in 1835 by Adolphe Quetelet, BMI has become a near-universal tool for evaluating body proportions and, more importantly, the health risks that arise from being overweight.
BMI Has Not Always Been Utilized
Even though the Quetelet index, or BMI, has been discussed among mathematicians for nearly two centuries, it was not commonly employed by health professionals until the 1980s. According to a 2000 review in "American Journal of Clinical Nutrition," physicians previously used weight-for-height charts to monitor their patients' nutritional status. BMI was adopted by doctors and researchers when the limitations of weight-for-height charts became more apparent in an increasingly obese population.
Calculation of BMI
Your BMI is calculated by dividing your weight, measured in kilograms, by the square of your height, measured in meters. Many online sites offer tools which calculate BMI from data measured in pounds and inches, as well. The BMI is a mathematical formula derived from principles of regression analysis, which is why the concept makes so little sense to most people. In a population of relatively uniform individuals, BMI more accurately portrays a person's body "fatness" than simple weight-for-height tables which permitted a fairly wide range of body types to fit within the clinical range of "normal." Some of these tables were devised using data from healthy people who were purchasing life insurance, highlighting their inadequacies for use among the chronically ill or even a predominantly overweight population. (Reference 3, Table 1)
Shortcomings of BMI
Like many weight-to-height indexes, BMI has some distinct shortcomings. It does not account for the observation that a given volume of muscle weighs more than the same volume of fat, so elite athletes' BMIs often reflect a degree of "fatness" that is not really present. A weight-based index cannot account for such differences in body composition. In addition, BMI does not account for differences in frame size. For these reasons, some experts believe a waist-to-hip ratio is a better predictor of obesity-related disease, as outlined in the June 2006 "American Family Physician."
Considerations
If you are not an athlete with a low body fat percentage and a fair amount of muscle mass, BMI is a reasonably good indicator of your body type and your risk for obesity-related conditions, such as heart disease, osteoarthritis and type 2 diabetes mellitus. Currently, a BMI above 24.9 kg/m2 indicates that you are overweight. However, as demonstrated in the January 2010 "Medical Hypotheses," mathematicians and nutritionists are already considering changes in the way your doctor analyzes those numbers the next time you climb on a scale. A more accurate determination of your health status might rest in a "meters-cubed" BMI -- requiring a new set of standards -- or a combination of BMI plus waist-to-hip ratio.
References
- US Centers for Disease Control and Prevention: Prevalence of Overweight, Obesity, and Extreme Obesity Among Adults: United States, Trends 1976--1980 Through 2007--2008
- "Medical Hypotheses"; Human Allometry: Adult Bodies Are More Nearly Geometrically Similar than Regression Analysis Has Suggested; R.F. Burton; January 2010
- "American Journal of Clinical Nutrition"; Criteria for Definition of Overweight in Transition: Background and Recommendations for the United States; R.J. Kuczmarski, K.M. Flegal; November 2000
- "American Family Physician"; Waist-to-Hip Ratio Better Predictor of Disease than BMI; A.D. Walling; June 2006
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