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Body Mass Index for Children

May 3, 2006

Body Mass Index for Children

In the mid-19th century, a Belgian mathematician famous for his statistical prowess developed a technique for measuring the amount of fat in the human body. The “Quatelet Index” created by Lambert Adolphe Jacques Quatelet became what we know as “body mass index,” a statistical correlation of the relationship between the height and weight of an individual arrived at by dividing body weight in kilograms by height in meters squared.

Used throughout the world in the centuries after Quatelet arrived at his formula, body mass index (BMI) as an indicator of health risk was not commonly practiced by clinicians in the United States until the late 1900s. It has now been widely publicized in this country--a computer search engine given the descriptor “body mass index” comes up with 145,000 entries, offering not only calculators that instantly convert height and weight in pounds and inches into the Quatelet index but also a host of commercial diet and weight control programs that promise to correct any unwelcome conditions the BMI reveals.

Recent converts to the BMI are schools in states with some type of student BMI reporting requirements currently in place (including, as of 2005, Arkansas, California, Florida, Illinois, Missouri, Pennsylvania, Tennessee, and West Virginia). An unknown number of school districts appear to have decided on their own to measure BMI as part of traditional height-and-weight assessments. All of this new interest means that a substantial percentage of children are now having body mass index percentiles added to their school health records.

Given that so much BMI assessment is taking place in schools, it is especially important to emphasize that “BMI is used differently for children than it is with adults,” according to the Centers for Disease Control and Prevention (CDC) in the United States Department of Health and Human Services. “Children’s body fatness changes over the years as they grow. Also, girls and boys differ in their body fatness as they mature,” the CDC points out.

This is why BMI for children is referred to as BMI-for-age and is plotted on gender-specific growth charts for measuring body fat in children, with each chart containing a series of curved lines indicating specific “percentiles.” Those percentiles simply mean that if a child is in a given percentile—say the 60th percentile, for example—60 percent of children of the same age and gender have a lower BMI.

Body mass index decreases during the preschool years, then increases into adulthood, and the percentile curves show this pattern of growth. Useful for measuring body mass index from ages 2 to 20, BMI-for-age provides a reference for adolescents that can be used beyond puberty, the CDC points out.

The CDC advises healthcare professionals to use the following established percentile cutoff points to identify underweight and overweight in children:

  • Underweight—BMI-for-age less than 5th percentile
  • Normal—BMI-for-age 5th percentile to less than 85th percentile
  • At risk of overweight—BMI-for-age 85th percentile to less than 95th percentile
  • Overweight—BMI-for-age 95th percentile or more.

Those age-related BMI charts were developed by the National Center for Health Statistics (NCHS) in response to increasing use of body mass index as a way of assessing children’s health. BMI gained momentum in 1994, when an expert committee recommended that body mass index be used routinely to screen for overweight in children 11 to 21 years of age. A second recommendation, in 1997, by a committee looking into assessment and treatment of childhood obesity, fueled further interest by concluding that BMI could be used to screen for overweight in children 2 years of age and older (2 years was chosen as the first age at which stature could reliably be measured).

The NCHS notes that pediatric growth charts in one form or another have been used by pediatricians, nurses, and parents to track the development of infants, children, and adolescents in the United States since 1977, when the first charts were developed by the NCHS and were adopted by the World Health Organization for international use.

It was assumed in 1977 that the growth charts might need to be revised periodically, to reflect changes in the growth patterns of Americans as reflected in data from the National Health and Nutrition Examination Survey (NHANES), which has periodically collected height and weight and other health information on the American population since the early 1960s. When the charts were revised, in 2000, the new body-mass-index-for-age charts for boys and girls ages 2 to 20 years were added, and the NCHS explained they were to be used in place of the 1977 weight-for-stature charts.

Some Caveats

The increasing popularity of body mass as an indicator of health in children is leading to some cautions. The Centers for Disease Control and Prevention points out that growth charts, including the BMI Index-for-Age charts, “are not intended to be used as the sole diagnostic instrument.” “Instead,” the CDC says, “growth charts are tools that contribute to forming an overall clinical impression for the child being measured.”

In a study published in the journal Pediatrics in March 2006, researchers asked whether changes in the body mass index percentile really reflect changes in the body composition of children and concluded that although high correlations have been reported between BMI and both total body fat and percentage of body fat during childhood, BMI is not a precise indicator of the underlying proportion of fat and lean tissue. “The extent to which BMI percentile changes may or may not reflect corresponding changes in body fatness (or leanness) in children is not known,” the researchers noted.

Although it dealt only with U.S. adults, another study has found that the “lipid accumulation product” performs better than body mass index in predicting cardiovascular risk. The researchers noted that “obesity is commonly understood to imply excess fat,” but what may be more significant is how fatty tissues are distributed in various parts of the body—at the waistline, for instance. In other words, at least in adults, having a “pot belly,” regardless of body size, is believed to predispose to increased prevalence of obesity-related disease, while fat predominantly deposited around hips and buttocks does not seem to carry the same risk.

Questions have been raised as to whether schools are the appropriate site for body mass index screenings, and it remains unclear to what extent the BMI results are being used by parents or school health personnel to counsel or refer for treatment students found to be overweight. Also not known is whether emphasis on body mass is creating concerns among adolescents leading to eating disorders such as bulimia or anorexia.

A wide body of research since the mid-1990s does show that body mass index, for both children and adults, is now an accepted measure of the causes of and possible treatments for a broad range of disorders, including not only diseases such as diabetes and heart trouble, but also seemingly less directly related conditions such as asthma. In a 2006 report on obesity in children, the journal Future of Children notes that taking measures such as height, weight, and body mass annually, and converting them to an age-and-gender-specific BMI percentile for each child “makes it possible to monitor individual children over time” and provides an opportunity for early intervention in obesity prevention.

BMIs for Surveillance

Body mass index can be used to identify students who are overweight or at risk of overweight—“to monitor individual children over time”—but such individual identification raises questions, including whether parents are to be notified and whether appropriate follow-up treatment and counseling is available in the community. That is leading some school systems to use BMIs for another purpose—to determine the extent of overweight as a problem in their students. Using sampling techniques, school systems are able to administer BMIs to representative samples of the school population, generating information that can be used to develop pilot programs or as a guide to preventive education. This surveillance function may be a useful approach in districts that find it difficult to meet the cost and time constraints of individual identification and treatment.

The CDC has issued a number of tools for calculating BMI-for-age, including:

  • A web calculator and tables of calculated BMI values for selected heights and weights of children 2 to 20 years of age;
  • A BMI web calculator for English and metric systems;
  • A CDC table--Calculated Body Mass Index Values for Selected Heights and Weights for Ages 2 to 20.
A 44-page booklet, the BMI Table for Children and Adolescents, is available in print. Most of these documents are available in PDF (Acrobat Reader required) at the CDC website http://www.cdc.gov/nccdphp/dnpa/growthcharts/bmi_tools.htm.

A 44-page booklet, the BMI Table for Children and Adolescents, is available in print. Most of these documents are available in PDF (Acrobat Reader required) at the CDC website .

A 44-page booklet, the BMI Table for Children and Adolescents, is available in print. Most of these documents are available in PDF (Acrobat Reader required) at the CDC website .

A 44-page booklet, the BMI Table for Children and Adolescents, is available in print. Most of these documents are available in PDF (Acrobat Reader required) at the CDC website .