# Explaining the quirks of childhood BMI

## The body mass index is a flawed measure of your kids' health, but its odd math is proof of our obesity crisis

Topics: Obesity, Food,

A friend of mine said she has a dumb obesity-related question for me: How can 15 percent of kids be above the 95th percentile?

Let me explain.

My friend’s girls — like most kids nowadays — are monitored for obesity as part of their health screening at school. My friend looked at the screening results — in which BMI is expressed as percentile for age and gender (more on that below) — gave some thought to the definitions of overweight and obesity, and realized that the math just doesn’t make sense. We’re hearing time and time again that obesity has reached epidemic proportions, and that a third of our kids are overweight, half of these obese. Yet, if overweight in kids is defined as having a BMI above the 85th percentile how can a third of U.S. kids be classified as overweight? If obesity in kids is defined as having a BMI above the 95th percentile how can 15 percent of the kids be obese? There can only be 5 percent above the 95th percentile, right?

It’s not a dumb question at all, so I’d like to devote this post to some really basic concepts in the diagnosis of childhood obesity, and solve this question for those of you wondering if — as in the imaginary Lake Wobegon of Prairie Home Companion fame — all kids can be above average.

BMI for kids: What is it and why bother with BMI percentiles

Body Mass Index (BMI) is a number calculated from a child’s weight and height (weight in kilograms divided by height in meters squared). BMI is a useful and easy screening tool for body fatness for most kids — it doesn’t measure body fat directly but studies have shown that BMI correlates quite well with body fat in most people.

In adults BMI on its own can assess obesity — we consider an adult (man or woman) with a BMI above 25 to be overweight, and above 30 to be obese — yet in kids a BMI on its own tells us very little. For example: A BMI of 21 — a healthy weight for an adult — is indicative of obesity in a 6-year-old boy, would categorize a 10-year-old boy as overweight, but would put a 16-year-old well within the range of a healthy weight.

Why isn’t kids’ BMI on its own informative? Kids’ body shape and composition change with age. The amount of body fat, muscle and bone transform dramatically with age, and the amount of body fat differs quite greatly between boys and girls. That’s why pediatricians use BMI-for-age charts, in which they plot your kids’ BMI comparing it to kids of the same age and gender.

The BMI percentile allows medical professionals to categorize kids’ weight: A BMI below the 5th percentile indicates underweight. BMI percentiles between the 5th and 85th percentile are considered healthy weight. BMI percentiles between the 85th and 95th percentile are overweight and BMI above the 95th percentile indicates obesity.

Who decides what’s a normal weight?

To establish a percentile chart you’d need a reference population. As we all know, weight (unlike other body characteristics such as eye color, blood type or even height) changes remarkably as a population changes its diet and activity level. Indeed, our kids’ weights as a collective have shifted dramatically in the past 30 years.

Therefore, in establishing the BMI-for-age charts the reference population is the kids of the past — mostly of the ’60s and ’70s, before the obesity epidemic started. There are several charts used around the world, each developed referencing a different population, but they all carry a historic picture of past generations, when childhood obesity was much less prevalent. When you get your kids’ BMI-for-age he’s not compared to the kids of today, but rather to what experts feel is a “normal” population.

And that explains how 15 percent of kids can plot above the 95th percentile — they’re plotted compared to BMIs of kids before the obesity epidemic began in full force.

Is a statistical norm a good indicator of health?

Why do we insist on seeing the weight of the past as the norm, and not accept our new dimensions as the new “normal”?

Obesity isn’t merely a statistical description of the extreme upper end of a bell curve. Obesity is a health issue, and we have plenty of evidence showing that kids with excessive body weight have higher blood pressure, higher cholesterol, are at risk of early onset of many diseases — heart disease, type 2 diabetes, some types of cancer, obstructive sleep apnea, to name a few — and overall have a shorter life span.

Unfortunately, when it comes to weight and BMI, too many kids are above average. Looking at population surveys over time, we see that kids’ weights, as a whole, have shifted up, and that kids are generally more than 10 pounds heavier than kids 30 years ago .

We all want a healthy future for our kids — that’s why we need to get childhood obesity under control.

## The week in 10 pics

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• Lisa Montgomery embraces her nephew Thursday after a tornado tore apart her home in Cleburne, Texas. The twister killed six people and destroyed entire swaths of the North Texas town.
Credit: AP/LM Otero

• Jack McMahon, the defense attorney for abortion doctor Kermit Gosnell, speaks outside the Criminal Justice Center in Philadelphia Tuesday. His client was convicted of killing three babies in his clinic, and will serve multiple life sentences.
Credit: AP/Matt Rourke

• A photo taken Monday captures Vice President Joe Biden's response to a Milwaukee second-grader's innovative proposal to end America's epidemic of gun violence. This guy!
Credit: AP/Jenny Aicher

• Sen. Rand Paul, R-Ky., flanked by a grouper-eyed Michele Bachmann, addresses the IRS' admission that it targeted Tea Party groups in advance of the 2012 election. In an op-ed for CNN Thursday, the Kentucky senator slammed the president for his faux outrage.
Credit: AP/Molly Riley

• Ousted IRS chief Steven Miller is sworn in on Capitol Hill Friday. Miller testified before the House Ways and Means Committee on the extra scrutiny the agency gave conservative groups applying for tax-exempt status.
Credit: AP/J. Scott Applewhite

• Attorney General Eric Holder pauses as he testifies on Capitol Hill before the House Judiciary Committee Wednesday. Holder is under fire, among other things, for the Justice Department's gathering of phone records at the Associated Press.
Credit: AP/Carolyn Kaster

• O.J. Simpson sits during an evidentiary hearing at Clark County District Court in Las Vegas, Nev., Thursday. Simpson, who is currently serving a nine-to-33-year sentence in state prison for armed robbery and kidnapping, is using a writ of habeas corpus to seek a new trial.
Credit: AP/Las Vegas Review-Journal/Jeff Scheid

• Major Tom to ground control: On Sunday astronaut Chris Hadfield recorded the first music video from space, a cover of David Bowie's "Space Oddity."

• When it rains it pours. President Barack Obama speaks during a news conference Thursday with Turkish Prime Minister Recep Tayyip Erdogan, inexplicably inspiring an #umbrellagate Twitter meme.
Credit: AP/Jacquelyn Martin

• A smoke plume rises high above a road block at the intersection of County A and Ross Road east of Solon Springs, Wis., Tuesday. No injuries were reported, but the the wildfire caused evacuations across northwestern Wisconsin.
Credit: AP/The Duluth News-Tribune/Clint Austin