Quetelet's Index

Interpretation for Adults

BMI is often used to classify overweight and obesity, but can be interpreted for the entire weight spectrum. Typically, a person is considered underweight if he or she has a BMI of 18.5 or less and normal weight between 18.5 and 24.9. A BMI of 25.0 to 29.9 is classified as overweight, and 30 or above is considered obese. The obese category can be further subdivided into Class I obesity (30.0 to 34.9), Class II obesity (35.0 to 39.9), and Class III or extreme obesity (40 or above). Recent estimates suggest that two thirds of the U.S. adult population are classified as overweight according to these categories, and nearly one third are obese.

Interpretation for Children and Adolescents

Although the calculation for BMI is the same for adults and children, the interpretation differs. For adults (aged 20 years and older), BMI is interpreted the same for both men and women using the categories listed above. However, for children and adolescents (aged 2 through 19 years), the interpretation is based on age- and sex-specific percentiles, which reflects the fact that adiposity changes with age and differs among boys and girls. Therefore, it is not appropriate to use the BMI categories for adults to interpret BMI and determine the weight category for children and adolescents. BMI can be used to identify children and adolescents who are either currently overweight or at risk of becoming overweight, based on BMI-for-age growth charts provided by the Centers for Disease Control and Prevention. After obtaining the BMI using the method described above, this number is plotted on the BMI-for-age growth charts for the appropriate sex to yield a percentile ranking, which indicates how the child's BMI compares to children of the same age and sex. Although children are usually not classified as obese, the current recommendation is that BMI values that meet or exceed the 95th percentile of the BMI growth charts for their age and sex should be categorized as overweight, and those who are in the 85th percentile to the 95th percentile are classified as at risk of overweight. These growth curves are J-shaped and were constructed to identify BMI scores that show a trajectory toward overweight (BMI ≥25) or obesity (BMI ≥30) in adulthood. [Page 1177]Childhood overweight is of particular concern because research suggests that it frequently tracks into adulthood, with the vast majority of obese adolescents going on to become obese adults.

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