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What Every Parent Should Know About BMI (WebMD Medical Reference)

Posted Apr 12 2012 11:30am

Bmi-chart As children grow and their bodies change, it's not always easy for parents to tell if a child falls within a healthy weight range. Body mass index, or BMI for short, is a standard measurement of body fat. Your child's BMI can help you determine if he is at risk for health problems based on his weight. Measuring the waist or neck circumference is another way to measure body fat in kids. Your health care provider may also do this.

The CDC and the American Academy of Pediatrics recommend BMI screenings for all kids age 2 and older. Here's what you need to know about checking on your child's BMI and what to do with the info once you have it.

BMI estimates how much body fat you have. Calculating a child's BMI number starts out just like calculating an adult's BMI. It's based on height and weight. But for kids, height and weight alone aren't as accurate as they are for adults. Why? Because kids' body fat percentages change as they grow. Kids' BMIs vary based on their age and gender.

That's why when health care professionals talk about a child's BMI, you won't usually hear a plain BMI number, like 25, but rather a BMI percentile, like 75th. These BMI percentiles show how a child's BMI compares to other children of the same age and gender. To calculate the BMI percentile -- which is also called "BMI for age" -- a health care provider or an online tool like WebMD’s FIT Kids BMI Calculator takes a kid's BMI (along with age and gender) and looks it up on a pediatric growth curve. This gives the child's BMI percentile.

BMI percentiles are grouped into weight categories:

  • Underweight: below the 5th percentile
  • Healthy Weight: 5th percentile up to the 85th percentile
  • Overweight: 85th percentile up to the 95th percentile
  • Obese: 95th percentile or higher

So, for example, a 6-year-old boy with a 75th percentile BMI has a higher BMI than 75 out of 100 6-year-old boys. And though you may think that means he is heavy, he is considered a healthy weight.

Many parents assume that if their child had a high BMI, their pediatrician would tell them. But that's not necessarily the case. Sometimes pediatricians may not bring up weight issues with parents. So if you're interested in your child's BMI percentile, it's best to ask directly.

Some school districts have started to measure all children's BMIs in school. The school then sends home a BMI report card to alert parents to any weight issues. Although some parents don't like the idea of schools sending report cards with their child's BMI, experts say that the point is not to embarrass anyone. It's to let parents know about a health problem with serious consequences. 

Studies from the U.K. show that children's BMI report cards can work. One study found that after getting a BMI report, about 50% of the parents with overweight children made some healthy changes to their lifestyle.

How Accurate is BMI for Kids?

Experts generally consider BMI for kids to be a good measure of body fat, at least among heavier children. But there are some cases in which BMI might be misleading. Athletic kids, in particular, may fall into the overweight category when they are actually muscular.

Your child's BMI is important, but it is only a piece of the picture. If a BMI percentile indicates that your child is not within the healthy range, she needs a complete weight and lifestyle evaluation with a pediatrician.

The pediatrician will likely follow up with an exam to see how far along your child is in her development and perhaps tests for weight-related health conditions, and by asking questions about her diet and exercise, whether weight is an issue for her, and your family history. This information will allow the health care provider to determine the best way to respond to an underweight, overweight, or obese BMI percentile.

Experts recommend that kids of all ages and all weight categories follow these healthy guidelines to help keep weight in check. It's easy to remember them as 5-2-1-0 every day.

  • 5: Everyone in your family needs five servings of vegetables and fruits. Keep serving them even if kids don't eat them. Familiarity increases the likelihood that they'll eventually try a food. Give a fruit or vegetable with every snack or meal.
  • 2: Limit TV-watching to no more than two hours a day. Family members who use other "screens" -- video games or computers, for instance -- get less TV time.  Experts also recommend not having TVs in bedrooms.
  • 1: Get one hour of physical activity. Add up the minutes each family member is moving -- it should be 60 minutes or more. Start small and keep adding if necessary. The goal is to have all those minutes be at least moderate activity, sweating after about 10 minutes.
  • 0: That's how many sugar-sweetened beverages you should have a day. Juice drinks such as lemonade and fruit punch, sodas, tea, and coffee can all have added sugar. Stick to water and reduced-fat milk instead.

Here are some other expert-recommended eating guidelines.

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