According to the Centers for Disease Control and Prevention (CDC), the Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI acts as an inexpensive and reliable indicator of screening for different weight categories that can lead to health problems. BMI does not measure body fat directly, but does correlate well to more complicated and expensive measures of body fat taken from methods like underwater weighing and dual energy x-ray absorption.
Although BMI is used as a screening tool to identify weight problems, it is not a diagnostic tool in itself, and even if a person has a high BMI number, a healthcare provider would need to perform further assessments to determine if excess weight is actually a health risk. Other than that, BMI is one of the best methods for population assessment of overweight and obesity in use today because the calculation requires only height and weight and it is inexpensive and easy to use. The result is that BMI is a great way for people to compare their own weight status to that of the general population at a glance.
Some of the other methods used to measure body fat include skin folding thickness measurements underwater weighing, bio-electrical impedance, dual-energy x-rays, and isotope dilution. Obviously, all of these alternative fat measurement methods are expensive and require highly trained personnel. They are all also quite different and difficult to standardize, a fact that complicates comparisons across different studies and time periods.
BMI is calculated for adults and children in the same manner using calculations based on a mathematical formula that divides a person’s weight in pounds by their height in inches squared and multiplying the result by a conversion factor of 703. In the metric system, the formula is weight in kilograms divided by height in meters squared. However, just because children and adult use the same BMI formulas does not mean that the results are interpreted the same way. BMI is interpreted for adults 20 years old and older using standard weight status categories that are the same for all ages and for both men and women. For children and teens the BMI interpretation is both age- and sex-specific.
BMI numbers have been shown to be a reliable indicator of body fat, although the correlations do vary by sex, race, and age. Results have shown that at the same BMI number, women tend to have more body fat than men and older adults with the same BMI tend to have more body fat than younger adults. The numbers can be skewed by various other factors though, as athletes can have a high BMI number due to their muscularity instead of more body fat. As far as looking at a person’s risk of developing obesity-related diseases, the National Heart, Lung, and Blood Institute guidelines recommend looking at the individual's waist circumference as well as other factors for diseases and conditions associated with obesity like high blood pressure or physical inactivity.
Based on the relationship between body weight and disease and death, obese individuals are at increased risk for many diseases and health conditions, including hypertension, high LDL cholesterol, low HDL cholesterol, Type 2 diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep and respiratory problems as well as endometrial, breast, and colon cancers. However, it is important to remember that a person’s weight is only one factor related to increased risk for disease and that BMI is not a direct measure of body fatness.