Body Mass Index


Body Mass Index for Kids

While we Americans are supersizing our meals, we’re also supersizing ourselves. More than 60 percent of adults in the United States are overweight or obese, creating many health problems, including an increased risk of heart disease, stroke, diabetes, high blood pressure, arthritis, varicose veins, and even some types of cancer.

Obesity among children is also on the rise, and with increased weight comes risk for diseases like diabetes and heart disease, even for kids. A BMI chart shows a measurement called body mass index.

BMI is used as a screening tool to identify possible weight problems for children. The CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight and obesity in children beginning at 2 years old.

This does not measure exact body fat percentage. BMI charts help children, parents and medical professionals quickly see whether a child is normal, underweight, overweight or obese for his age, weight and height. Percentiles compare the weight limits of children of the same age and provide benchmarks for determining what is healthy. However, BMI is not a diagnostic tool. For example, a child may have a high BMI for age and sex, but to determine if excess fat is a problem, a health care provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

To use BMI charts, you must first calculate BMI. Although BMI does not directly tell you an exact body-fat percentage that you get from using skinfold calipers, underwater weighing or electrical impedance methods, BMI does correspond to body-fat percentage, according to the Centers for Disease Control and Prevention. You can then compare your child's BMI on a BMI-for-age chart to discover your child's BMI percentile. To calculate BMI, use a BMI calculator. Select your child's gender first. Then, enter in the birth date, height measurements in feet and inches, weight measurement in pounds and the date of measurements. Then press the "calculate" button. A BMI calculator such as the one at KidsHealth automatically graphs the BMI results on a percentile chart for you. (See references & links) Lines on the graph mark the 5th, 85th and 95th percentiles so you can see where the BMI fits.

A child has a healthy BMI if his percentile is between the large range of the 5thto 85th percentiles, according to the CDC. Children with lower BMIs, less than the 5th percentile, are considered underweight. Overweight children fall above the 85th percentile. Any child who has a BMI equal to or greater than the 95th percentile is obese.
Gender and Age
Doctors use different charts for male and female children. This is because boys and girls grow at different rates and their body-fat amounts differ as they mature. If calculating the BMI yourself, be sure that you select the correct gender so the online calculator shows you the appropriate chart.

BMI versus Weight
BMI-for-age charts compare body mass index, not simply weight for age. Although it might be easier to simply compare your child's weight to a weight range for their age, this would not give you as accurate information because a healthy weight changes not only as height increases, but also monthly as a child ages, according to the CDC. BMI takes height into account and the calculations take the year and month of a child's birthday into account, giving a more accurate representation of a child's weight range as healthy, underweight, or overweight.

Limitations of BMI
BMI is an easy tool for estimating the amount of fat in your body. However, it is not a direct measure of body fatness, since your body weight is comprised of fat as well as muscle. Because of muscle mass and growth variations in children, the BMI chart my not be fully accurate for all children. The main limitation is that the BMI will yield inaccurate values for people that have large muscle mass. Despite these limitations, the use of BMI does yield some useful information for evaluating patterns and trends in groups of people. Other tests for weight can be performed on your child to ascertain fat levels with greater accuracy, but it is important to remember that weight is only one factor related to risk for disease. Many other variables may affect your child's health, so even if he/she is at a healthy BMI percentile, he/she should still see a pediatrician regularly.

References & Links
Center for Disease Control & Prevention: About BMI for Children and Teens
Kids Health: Body Mass Index (BMI) Charts