BMI has been getting a lot of attention lately, particularly in schools. In fact, several states started using “BMI report cards” to alert parents of their child’s BMI and further informing them if their child is underweight, overweight or obese. These have proven controversial and some states have already stopped sending them. Those in favor of BMI report cards feel that knowing the possible health risks that are linked with obesity will motivate parents and others to help a teen make changes in their eating and exercise habits. This guide was created to help you understand how BMI is measured, the limitations of BMI measurement, and when to be concerned about a teen’s BMI.
What is Body Mass Index (BMI)?
BMI, short for Body Mass Index, is a term used to estimate a person’s body fat. Although having some body fat is healthy and normal, having too much or too little body fat isn’t. Having a high BMI has been linked to certain health problems such as high blood pressure and Type 2 diabetes. Similarly, having a low BMI is not healthy.
How is BMI measured?
To figure out a teen’s BMI, an accurate weight and height is needed. BMI is then measured using a mathematical formula. It is the ratio of a person’s weight in pounds times 703 to their height in inches squared.
How is BMI interpreted?
Once a teen’s BMI has been calculated, it can be interpreted using growth charts designed by the Center for Disease Control (CDC). These charts (one for boys and one for girls, aged 2-20) are used to convert a teen’s BMI into a percentile. This percentile tells us how a teen’s weight and height compares to other teens of the same age and gender.
The best way to interpret a teen’s BMI is to track it over time on his growth chart. Teens should stay at about the same BMI percentile during their teen years. If a teen’s BMI percentile goes up or down quickly, he should talk with her health care provider to talk about the possible reasons for the change.
|less than 3rd percentile||Underweight|
|between 5th and 85th percentile||Normal weight|
|between 85th and 95th percentile||Overweight|
|greater than 95th percentile||Obese|
Example: A 13 year old male has a BMI of 18.5. Using the boys’ 2-20 years BMI growth chart, his BMI plots on the 50th percentile, which classifies him as normal weight.
What is BMI used for?
BMI is used to identify teens who may be at higher risk for weight-related health problems. A teen who classifies as underweight, overweight, or obese should be seen by her health care provider for further evaluation. A health care provider can screen for health problems and then help develop a personalized weight management plan if it is needed. A referral to a dietitian may be suggested.
What are BMI report cards?
Schools commonly measure the heights and weights of their students and calculate their BMI. Some of these schools also send this information home to parents with a report, thus the term “report card”. Students do not receive a grade as with traditional report cards. The “BMI report card” is simply a letter with information that is meant to help parents understand if their child is possibly at risk for weight-related health conditions such as eating disorders, high blood pressure, high cholesterol, and Type 2 diabetes.
What are the limitations of BMI?
BMI is just an estimate of body fat. Most teens that have a BMI in the overweight or obese category also have a high amount of body fat, but there are some that don’t. This may be true for very muscular teens since they may have a higher weight because of their muscle mass but still have a healthy amount of body fat. Additionally, BMI should be cautiously interpreted in teens that are growing or going through puberty. During these times, weight, height, and body fat change at different times and rates, which can affect BMI.
If a teen classifies as overweight or obese, he needs to be examined by his health care provider who can evaluate potential health risks and offer personalized resources.
Nihiser AJ, et al. Body mass index measurement in schools. Journal of School Health. 2007; 77(10): 651-671.