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PART ONE: How Weight is Classified
What ever happened to the phrase “baby fat” and
the notion that some kids might be chubby at certain times
of their lives but then outgrow it? Is this an old-fashioned
myth or a normal process we’ve forgotten about (or don’t
want to admit to)? On the other hand, what is going on in
America with kids and their weight? If you look around at
an elementary, middle or high school now, you’ll see
lots of kids who look chubby—indeed more than we used
to see. But even more startling is the number of kids who
are very heavy, so much so that they can’t move as well
as the other kids. We’re now hearing about health problems
in children and teens—for example, type 2 diabetes—that
we used to find only in adults. Clearly, this is not good.
How has this happened? To find the answers, we’ll look
at this issue in a three-part web article.
First we need to understand what normal growth looks like,
because, the truth is, some kids are meant to be fatter than
others, both in general, but also specifically at certain
times in their development. Therefore, high weight is
normal for some kids, especially at certain times. In other
kids, quickly climbing weight (above what their genetic heritage
determines they should weigh) may be a reflection of a feeding
or eating problem. So, we need to know how to look at patterns
of growth and weight. We’ll actually cover that in Part
3. But first, in Part 1, we’ll look at weight classifications
for children and where they came from. Then Part 2 discusses
how weight has changed in American children (we know it’s
up, but we’ll look at some numbers and what it all means).
In Part 2 we’ll also introduce the notion that weight
standards can be problematic. Part 3 will talk about the why
of the children’s weight issue--perhaps the most interesting
part! But we need to begin with definitions.
How is overweight defined? What about underweight?
Using government standards from the Centers for Disease Control
(CDC), an agency of the Department of Health and Human Services,
adults can be classified as overweight or obese depending
on their weight versus their height. In children, however,
currently, there is no official definition of obesity. Children
are classified as overweight if they’re at or above the
95th % for body mass index (BMI) for their age and gender
on the CDC BMI growth charts from 2000. Children between the
85th and 95th % officially fall in a newer category is called
“at risk for overweight.” Children who fall under
the 5th % for BMI are classified as underweight. So what does
this mean? A few definitions...
What is BMI?
Body Mass Index is really just a way to measure weight versus
height. You get BMI by dividing weight in kilograms by height
in meters squared. If you typically do your measurements in
pounds and inches, you can use the alternate equation on the
right side in the box.
Don’t
like math? There are published tables for adults and online
BMI calculators which give you the BMI for various heights
and weights. The handy thing about BMI is that it takes height
into consideration, unlike just looking at a person’s
weight at a given age. The idea is that then we can assess
relative fatness of people of different heights. In children,
BMI is higher in babies, dips down somewhat before age 6,
and then climbs again as children mature, with a significant
amount of fat accumulated in the couple years before puberty
(more on pre-puberty weight in a later article). Even if children
did not accumulate a higher percentage of body fat as they
aged, BMI would increase through childhood because children
get broader and wider as they get taller.
How are BMI standards decided for American children?
The range of expected BMIs for children of all ages has been
based on height and weight measurements of very large numbers
of kids from national assessments done in this country since
the 1960s—called the NHANES (National Health and Nutrition
Examination Survey). In these surveys (still going on!), thousands
of adults and children from all over the country have been
interviewed and measured. The NHANES assessments were designed
to gather information about living arrangements, food intake,
income status, and health issues. Everything from food and
supplement consumption to drinking water sources for children
and adults has been analyzed. NHANES staff members (numbering
in the thousands) have also taken all sorts of health measurements,
including height and weight. For each survey period, all the
data collected take several years to analyze and immense reports
are published for each study period.
In 1977,
the National Center for Health Stastistics (NCHS), a department
within the CDC, created graphs from the large pool of measurements
from NHANES I (as well as a couple of other smaller surveys).
These graphs, called weight curves, show the typical range
of weights for infants and children all the way through adolescence.
Separate curves for weight-for-age, stature-for-age (length
or height), and weight-for-stature were made for infants and
children of both genders. You may have seen a nurse or doctor
plotting your child’s measurements on just such a graph.
The New and Revised Growth Curves Were Unveiled in 2000...
Because BMI calculations take height into account, the CDC
decided to create BMI-for-age curves, as well as revise the
1977 weight-for-age curves. Data from NHANES I, II & III
(with one exception, explained in Part 2) were thrown into
the calculator and BMI curves were established for children
2 years old and older. The data pool was also updated to include
measurements more ethnically and socioeconomically representative
of the U.S. population, especially for babies. (The early
infant growth curves were based on data mostly from white,
middle class, formula-fed infants.) These new BMI growth curves
have taken the place of the old weight-for-stature curves.
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How
do you read the weight-for-age growth curves?
Let’s use 4-year old girls as an example. Looking at
the 2000 weight-for-age
curves, all the measurements collected reveal that the
vast majority of these four year olds are likely to weigh
between 28 and 48 pounds. About half the girls (50%) will
weigh under 34 pounds, half over. So 34 pounds is called the
50th percentile for weight for 4-year old girls. The 75th
% for 4-year old girls is 38 pounds. That means that if there
are 100 4-year old girls in a room, you could expect that
about 75 girls would weigh at or under the 75th percentile
weight (38 pounds) and 25 would weigh more than that. (The
truth currently is a bit different, but more on that soon.)
The line following the same weight percent over different
ages is referred to as that percentile—for example, if
you join the dots for the weights at the 75th percent from
ages 2 to 19 years old, you get a line called the 75th percentile.
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What
about the new BMI-for-age curves?
Using the new BMI
curves, we see that the 75% BMI for four-year old girls
is a number—not a height or a weight—which is 16.2.
The idea with the BMI curves is that we can more easily compare
the weights of children of different heights. So, in general,
tall four-year old girls who weigh more than short four-year
old girls wouldn’t be considered fatter, if they each
have the same BMI. Because BMI increases as children age,
the BMIs at each percentile also increase as the child ages.
For example, at the 75th%, the BMI for girls at age 2 is 17.4,
at age 4 it’s 16.2 [that’s because of the preschool
dip down in BMI], it’s back up to 16.4 at age 6, 17.3
at age 8, 18.6 at age 10, and so forth. Since the BMI curves
were just released in 2000, doctors’ offices and WIC
clinics (Women, Infants and Children, a government supplemental
food program) may be using both BMI and weight-for-age charts
for kids, and perhaps even the old weight-for-stature curves.
Now that we know a bit about where those weight standards
came from, we’ll discuss in the next column (Part 2)
weight changes in American children over the last 30 years—with
some words of caution about using population standards for
individual kids.
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