The BMI equation has become the medical community's commonly accepted way to measure body fat, although "for years, critics of the body mass index have griped that it fails to distinguish between lean and fatty mass," according to "Beyond BMI," an article in the July 20, 2009, issue of "Slate" magazine.
The equation has become accepted because the method that leads to the equation is "inexpensive and easy-to-perform," according to the Centers for Disease Control and Prevention (CDC).
An Explanation
The BMI equation (W/H squared) is an effort to tell you how much you should weigh based on your height. It is much simpler than the weight-for-height tables because it treats all adults the same regardless of whether they are women or men or whether they have wide or thin frames.
The equation is used by doctors in countries all over the world, so it is based on the metric system. It requires division. The top number is your weight in kilograms. The bottom number is your height in meters squared or times itself.
The Equation's Results
The result of the equation shows you whether you are at a "normal" or "healthy" weight that minimizes your risk of heart attacks, strokes, high blood pressure and other obesity-related diseases. Your BMI should be between 18.5 and 24.9 when you divide your height squared into your weight. If your weight divided by your height results in a BMI below 18.5, you are underweight. If W/H squared results in a BMI between 25 and 29.9, you are overweight. If your BMI is 30 or above, you are obese.
Converting from Metric System
You need to convert your weight from pounds to kilograms and your height from inches to meters if you don't know your metric measurements. One pound equals 2.2 kilograms. If you weigh 182 pounds, you divide 2.2 into 182 (182/2.2) and you weigh 82.72 kg., according to "Essentials for Health and Wellness." One meter equals 39.4 inches. If you are 5 feet 10 inches tall or 70 inches, you divide 39.4 into 70 (70/39.4) and your height is 1.79 m. Your BMI is 82.72/1.79 times 1.79 or 25.8.
Reliability Questioned
The BMI equation's reliability is questionable, according to the CDC. One problem is that your weight measures fat and muscle, but heavily muscled people don't have a higher risk of obesity-related diseases. In addition, a woman has more fat than a man with the same BMI, and an older adult has more fat than a younger adult with the same BMI. The BMI equation also measures children's fat, but the classifications such as underweight and overweight depend on gender and age.
History
The BMI equation was created by Adolphe Quetelet in 1832 because he was trying to ascertain the standard human build, and he discovered that weight varied more in proportion to height squared than height. The equation wasn't called the BMI equation until researcher Ancel Keys concluded in 1972 that it accurately measured body fat, reported the "Beyond BMI" article in "Slate." The National Institutes of Health decided the BMI equation was more accurate than the weight-for-height tables in 1985.
References
- CDC: About BMI for Adults
- "Essentials for Health and Wellness"; Gordon Edlin, Eric Golanty, Kelli McCormack Brown; 2000
- CDC: About BMI for Children and Teens
- Slate: "Beyond BMI"
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