Diet and lifestyle affect how much of your overall weight is stored as body fat. If you or your doctor need a quick way to estimate your degree of body fat or identify whether you need to lose weight, calculating your body mass index, or BMI, can be a useful tool. The Centers for Disease Control and Prevention also uses BMI analysis to assess the general weight status of populations. As an individual, you should look at BMI as a screening, rather than as a diagnostic, tool.
Identification
Excess body fat can put you at risk for developing serious health conditions such as high blood pressure, heart disease and type-2 diabetes. Although using a weight scale can provide your total body weight, this method weighs bone and muscle as well as body fat and can be somewhat misleading. BMI separates body fat weight from bone and muscle weight and provides a good estimate of whether your weight is putting your health at risk, according to the National Heart, Lung and Blood Institute.
Calculation and Interpretation
The formula for calculating BMI is the current body weight times 703 divided by height in inches and divided by height in inches. For example, if your current body weight and height equals 150 lbs. and 62 inches, 150 times 703 divided by 62 divided by 62 equals a BMI of 27.4. After obtaining this number, you can compare it against standard weight categories that the CDC provides. A BMI below 18.5 places you in the underweight category, a BMI of between 18.5 and 24.9 identifies you as normal, a BMI of 25.0 to 29.9 indicates that you're overweight and a BMI over 30.0 indicates that you're obese.
Analysis
Researchers also use BMI calculations to analyze the general effect of diet within large groups and populations. A 2003 study published in the "International Journal of Obesity" provides an example of how researchers use BMI in diet analysis. The study analyzed the BMI of 37,875 adults between the ages of 20 to 97 against four basic diets, including meat eaters, fish eaters, vegetarians and vegans, to determine the effect each has on body fat accumulation. Results showed meat-based diets result in the highest BMI, with men averaging about 24.41 and women averaging 23.52, mainly due to the high calorie, protein, total fat, saturated fat and monounsaturated fat content in the diet. In contrast, vegans had the lowest BMI, averaging 22.49 for men and 21.98 for women.
Considerations
A reason the CDC suggests using BMI as a screening tool instead of a diagnostic tool for individuals is that it has certain limitations. BMI doesn't consider gender, age or the effect of large amounts of muscle on total weight. In addition, BMI doesn't take into account factors other than weight, such as smoking habits or lack of exercise, which can increase your risk of developing weight-related medical conditions.
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