Body Mass Index (BMI) is the most common method to measure adult obesity. However, BMI is now becoming a popular tool, which is combined with BMI-for-age percentiles, used to measure childhood obesity. This measure is used to assess weight relative to height which estimates how much body fat a person has and is much easier to measure. BMI does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA). BMI can be considered an alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.
BMI is defined as weight in kilograms divided by height in meters squared. BMI also can be calculated in pounds and inches. (see measuring childhood obesity) for further details.
To calculate a child's body mass index, follow these steps:
► Divide weight in kg by height in meters squared, or wt/ht2.
► For pounds and inches, divide weight by height squared and,
► Multiply the result by the conversion factor 703.
BMI is the standard for defining obesity in adults, but its use for obesity in children is not accepted universally. BMI is not perfect. Children can also have a high BMI if they have a large frame or a lot of muscle, not excess fat. And a child with a small frame may have a normal BMI but too much body fat. Therefore, for children and teens, BMI is age- and sex-specific percentile and is often referred to as BMI-for-age percentile (see childhood obesity charts)
BMI is not a perfect measure of body fat and there are situations where BMI may be misleading. For example, a muscular person may have a high BMI without being overweight (because extra muscle adds to a person's body weight but not fatness).
In addition, BMI may be difficult to interpret during puberty when kids are experiencing periods of rapid growth. It's important to remember that BMI is usually a good indicator but is not a direct measurement of body fat. BMI is particularly helpful for identifying kids and adolescents who are at risk for becoming significantly overweight as they get older. In older kids and teens, there is a strong correlation between BMI and the amount of body fat. So those with high BMI readings and, probably, high levels of fat are most likely to have weight problems when they are older. By identifying these at-risk kids, doctors can monitor their body fat carefully and try to prevent child obesity through changes in eating and exercise habits.
Use of BMI percentile to screen obesity in children
For children and adolescents (aged 2–19 years), BMI is calculated and the BMI number is plotted on the BMI-for-age growth charts (see childhood obesity growth charts) to obtain a percentile ranking. Separate charts for boys and girls are used to account for differences in growth rates and amounts of body fat as the two genders mature. However, it should be kept in mind that this method, among other methods, should be used as a tool, and only a physician can best determine and diagnose (see diagnosing childhood obesity) weight status in your child.
BMI-for-age percentiles is a favored method to measure the size and growth patterns of individual children in the United States. The percentile indicates the relative position of the child’s BMI number among children of the same sex and age. The growth charts show the weight status categories used with children and teens (underweight, healthy weight, overweight, and obese).
BMI-for-age percentiles and the corresponding weight status categories are shown in the table bellow:
BMI percentiles show how child’s measurements compare with others the same gender and age. For example, a BMI at the 95th percentile means that, 95% of kids of the same gender and age have a lower BMI.This definition is based on the 2000 CDC Growth Charts for the United States.
|Percentile Range||Weight Status Category||BMI below the 5th percentile||underweight||BMI at the 5th and less than the 85th percentile||normal weight||BMI at the 85th and below 95th percentiles||overweight||BMI at or above 95th percentile||obese|
The American Obesity Association also defines those children above the 95th percentile as "obese", which corresponds to a BMI of 30 (considered obese in adults).
Here is an example of how some sample BMI numbers would be interpreted for a 10-year-old boy.
While BMI is an essential indicator of healthy growth and development, if you think your child may be gaining or losing weight too fast, concern soon to your doctor.
|Tips for parents
What can you do as a parent or guardian or caregiver to help preventing childhood obesity? We have some ideas in our Childhood Obesity Prevention section.
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|Childhood Obesity Definition
Body mass index (BMI)is a measure used defining child obesity. BMI can be calculated using either English or metric units. BMI is a measure of weight in relation to height that is used to determine weight status.[Read more]
|Measuring Childhood Obesity
For children and adolescents (aged 2–19 years), BMI is calculated and the BMI number is plotted on the BMI-for-age growth charts to obtain a percentile ranking. Separate charts for boys and girls are used.[Read More]
|Childhood Obesity Charts
These charts give general ranges of healthy weights and overweight for a child's height. Many health care providers define obesity in children as weighing 20% or more over the healthy range.[Read More]
|Childhood Obesity Growth Charts
Growth charts show doctors how kids are growing compared with other kids of the same gender and age. It also helps to see the way kids' height and weight gain over time, and whether they're developing proportionately.[Read More]
|Diagnosing Childhood Obesity
Doctors and other health care professionals are the best people to find out whether a child or teen's weight is healthy, and they can help in ruling-out rare medical conditions as the effects of childhood obesity.[Read more]