Body mass index (BMI) is a measurement of weight relative to height that is used as an indicator of obesity levels in Americans. The measurement is based on a mathematical formula that was initially developed in 1832. It is not completely accurate for all people but is intended as a general diagnostic tool for body composition.
History
In 1832, a scientist named Adolphe Quetelet developed the equation that would later become known as BMI. His aim was to create a measurement that helped define what the average man looked like. He was not attempting to create a diagnostic tool for obesity. The phrase "body mass index" was not introduced until 1972 in a study of obesity by physiology professor Ancel Keys. Keys discovered a correlation between a person's measured body fat percentage and Quetelet's formula. The National Institutes of Health latched onto the measurement in 1985 and began using it as an official indicator of the health risks that go along with being overweight or obese. With a few minor adjustments, it has been used in that capacity ever since.
Formula
The original formula is quite simple, but it is based on metric measurements. The translation to English forms of measurement makes the formula a bit more complicated. The original formula reads: Weight (in kg) / [height (in meters)]^2
So, for a man who weighs 76 kg and stands 2 meters tall, the formula would be 72/2^2 which equals a BMI of 19.
Instead of using the specific conversions from kg to lbs. and meters to inches, the U.S. Centers for Disease Control and Prevention has simplified the conversion by using the following formula: Weight (in lbs.) / [height (in inches)^2 x 703. So, for example, a person who weighs 180 lbs. and stands six feet (72 inches) tall would have a BMI of 24.4.
BMI Ranges
The potential BMI measurements based on height and weight have been divvied up into numerical ranges that tell you whether you are underweight, normal, overweight or obese based on a healthy level of body fat percentage for your height. A BMI below 18.5 is considered underweight; 18.5 to 24.9 is considered normal; 25 to 29.9 is considered overweight; and anything above 29.9 is considered obese.
Adults and Children
The same formula is used to determine the BMI of both adults and children, but the numbers are interpreted differently for the two age groups. For adults, the interpretation of the numbers does not change based on age or gender. This is not the case for how the numbers for children and teens are interpreted. Rather than being placed on a set scale people under the age of 20 are compared to other people their same age and sex. Their BMI is therefore most often presented as a percentile of people within the same demographic. For example, a 13-year-old boy who is five feet tall and weighs 125 lbs. has a BMI of 24.4. This BMI is higher than approximately 93 percent of boys his same age and weight, so he is considered to be overweight.
Limitations
The National Heart Lung and Blood Institute acknowledges that BMI as measurement of obesity has its limitations. BMI cannot account for someone with an athletic build who has more muscle than fat. A high BMI may not necessarily indicate obesity in these people. And it cannot account for the muscle loss that the elderly experience. A low BMI may not accurately represent such a person's body fat.
Member Comments