Belgian statistician Adolphe Quetelet developed the body mass index, or BMI, in the early 19th century when he observed that an adult's body weight typically increases in proportion to height. In the 1970s, after researchers found that excess body weight could lead to problems such as heart disease, type 2 diabetes and early mortality, BMI became an important assessment tool in the United States. Today, BMI is a common health screening method for adults as well as children.
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Although BMI isn’t a direct measure of body fat, it can be an accurate reflection of whether or not a person is carrying too much fat. The basic principle behind BMI is that taller people have more tissue -- more bone, muscle and fat -- than short people, and therefore tend to weigh more. The formula for BMI is universal, meaning BMI is calculated the same way for all people, regardless of gender, age, ethnicity or even body composition. You can find your BMI by dividing your weight in pounds by your height in inches, squared, and then multiplying that result by a metric system conversion factor of 703: BMI = weight in pounds/(height in inches)2 x 703.
BMI scores are interpreted within a specific set of parameters, or category ranges, that are universally applied to all adults. According to these boundaries, any adult -- male or female -- who has a BMI between 18.5 and 24.9 is considered normal in weight. A BMI below 18.5 is classified as underweight; overweight is defined by a BMI of 25 to 29.9, while obesity is defined as having a BMI of 30 or above.
For example, an adult who is 5 foot 9 inches tall and weighs 145 pounds has a normal BMI of 21.4, regardless of gender. If that same adult were to gain 25 pounds, he -- or she -- would have an overweight BMI of 25.1.
Because BMI uses a person’s total body weight and doesn’t distinguish lean mass from fatty tissue, two people with the same BMI can have very different body compositions. This is especially true when comparing males and females -- women typically have a higher percentage of body fat than men, and men tend to have a higher proportion of muscle mass. Essential body fat, or the amount of fat you need to function normally, accounts for 10 to 13 percent of a woman’s total body fat. According to the American Council on Exercise, just 2 to 5 percent of a man’s fat is essential. So even though the Centers for Disease Control and Prevention estimates that the average BMIs for men and women in the United States are 26.6 and 26.5, respectively, it doesn’t mean American men and women are equally fat.
Merits and Shortcomings
While BMI can provide an accurate snapshot of weight trends across a population, it can also be inaccurate for certain individuals. Bodybuilders, for example, are often classified as overweight or obese because muscle is denser than fat and they tend to have more muscle than other people. Because gender, age and ethnicity affect a person’s body composition and fat percentage, BMI can seem ambiguous -- at the same BMI, older adults tend to have more fat than younger adults; black people tend to have more fat than white people; and a woman is likely to have more body fat than a man. That doesn’t mean, however, that a tall woman with low body fat couldn’t have the same BMI as a shorter, heavier man.
Perhaps one of the most important benefits of BMI is how accurately it correlates with certain health risks. The Harvard T.H. Chan School of Public Health notes that, as your BMI rises above 21, your risk of developing heart disease, type 2 diabetes and other chronic illnesses increases progressively.