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Weighty Wisdom by: Elizabeth
Jackson, M.S., R.D.
Looking Beyond the Numbers — Part One
/ Part Two
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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.
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.
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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PART TWO: Weights are Up Across the Board, but a Child Isn't
a Population
In the last Weighty Wisdom column (2, Part 1), we looked at
the origins of weight classifications. In this column, Part
2, we’ll talk about what’s happened on a population-wide
basis but give a few warnings about interpretation of weight
classes. Part 3, coming at the end of August 2003, will tackle
untangling normal from abnormal weight by looking at patterns
of growth.
Why are we so worried about kids’ weights? How have
weights changed in American children over the last 30 years?
In the beginning, the percentage of kids at a certain percentile
basically matched the percentile itself. This means about 5%
of American kids or so were at the 95th percentile and above
for weight at any age. As we discussed in the last column, weight
classifications have come from large national government surveys,
called the National Health and Nutrition Examination Surveys
(NHANES), in which thousands of children have been measured
and weighed. The NHANES surveys tell us that 6% or fewer of
children and teens aged 6-19 years old were defined as overweight
up until the mid 1970s. Prevalence of overweight (defined at
or above the 95th %) in younger kids, aged 6-11, nearly doubled
from 4% of kids to 7% between NHANES I and NHANES II (the 1970s).
The big increase in overweight for children of all ages came
in 1980s. During the period beween the NHANES II and the NHANES
III, the prevalence of overweight jumped from 7% and 5% for
6-11 and 12-19 year olds, respectively, to 11% for both groups.
Then prevalence jumped again to 15% during the 1990s. This means
that now, in a room of 100 American kids of the same age, about
15 (instead of 5) will have a weight that is at the 95% or greater.
This is true across ethnic groups.
If kids across the board have gotten heavier, then wouldn’t
the new weight curves shift so the percentiles would really
reflect the percent of kids at those weights (meaning, only
5% of kids would be at or above the 95th %)?
In fact, because of the upward trend, weight data from NHANES
III for kids over age 6 was not included in the 2000 weight
curve revisions in order to keep weight for height standards
comparable to earlier times. So kids are definitely getting
heavier. But before we talk about why this is happening and
what we can do about it, we need to know some basic things about
weight and BMI in kids.
1) Overweight and underweight or simply high weight and low
weight? It is a bit problematic to take a range of values
for a population and say that the numbers at each end (the top
and bottom 5%) are automatically abnormal. The top and bottom
weights percentiles in a population certainly may have a higher
likelihood of reflecting growth or medical problems, but in
a group of individuals, some members always have to have the
top and bottom numbers or degrees for characteristics—for
example oldest and youngest, darkest and lightest skin or eye
color, straightest and curliest hair, etc. There are many, many
children for whom being at the top or bottom of the BMI scale
has no negative health consequences. But we do need to know
how to recognize actual growth problems (coming in Part 2 of
this article!).
2) Remember the BMI is a calculation based on weight and
height and is not a direct measurement of body fat. Medically,
there is more worry about being overfat versus being overweight.
BMI is generally a good estimate of fatness in people, but individuals
who are very large boned or muscular (including very fit athletes)
can have BMIs that fall in the overweight or even obese range.
In other words, BMI is a measure of weight versus height, but
higher weight does not always mean higher fat. To actually measure
body fat directly, the only method you might see used in a clinic
would be skinfold measurements. Fat under the skin is measured
(by a trained clinician!) with a special instrument called a
caliper at several sites on the body. This has been done in
the NHANES surveys. There are other very high tech and even
more accurate ways to measure body fat, but these methods are
used primarily in research studies and include scanning/imaging
devices and body density measuring.
3) Single BMI measurements can be misleading because kids often
get fatter right before a growth spurt (again, we’ll
discuss patterns of growth in Part 2).
4) Perhaps most importantly of all, no matter what is going
on in our culture that may be making kids heavier, we can’t
forget that weight is still very strongly determined by genetics,
meaning the natural size of the parents. A very famous study
from the 1970s, conducted by Garn & Clark, showed that weights
of children strongly resembled weight combinations of parents.
The leanest kids came from two lean parents and fattest kids
came from two obese parents, with other combinations in-between.
We might think this is simply because fatter people eat less
well or are less active and so have bigger kids because of “bad
habits” they pass down. Yet a study from the 1980s by Albert
Stunkard looked at the weights of Danish adopted children and
found their weights correlated largely to their biological parents’
weights rather than the adoptive parents’ weights. In 1990,
Stunkard published another study based on Swedish adoptees with
the same results. In other words, we’re not all meant to
be thin. We certainly understand that short parents tend to
produce short kids and tall parents, tall kids—but as a
culture we haven’t been comfortable thinking that weight
is also largely determined by genetics. At a time when we’re
very legitimately worried about the increased numbers of very
heavy kids, we can’t forget that there are always going
to be some kids who are naturally fatter than their naturally
thinner friends.
Also, kids from different ethnic groups naturally tend to be
built a bit differently. As mentioned, the CDC revised growth
charts of 2000 were based on measurements of groups of children
that included ethnic minority groups in proportion to the total
U.S. population. That would mean, for example, that since about
17% of kids in the U.S. are Hispanic/Latino, about 17 percent
of measurements used when combining data to make the growth
curves would have come from Hispanic/Latino children. About
61% of American kids are identified as white, so we can say
that the majority of the measurements used for the growth charts
were those of white children. But for exactly this reason, some
researchers feel the BMI growth charts may not be the most accurate
standards for non-white children. To give some interesting examples,
Hispanic children naturally tend to be shorter and heavier than
Caucasian children, and this even shows up in babies. Children
of Chinese ethnicity in Singapore are built smaller than Chinese
children in Beijing (same % of body fat but lower BMI). Children
from the Chaldean ethnic group (of Middle Eastern heritage)
in the Detroit area have denser bones and more lean body mass
(e.g., muscle) in general, as well as more body fat, and so
end up on average with higher BMIs than Caucasian kids. Up to
a certain point, children with higher BMIs may reflect better
nourished kids: for example, immigrant Mayan Indian children
growing up in Florida and California are taller and heavier
and have more muscle and fat than Mayan village children back
in Guatemala, though the immigrant children are still generally
shorter than the white, African-American and Mexican-American
children in their cities. We have to be very careful when comparing
children to standards.
So: if we presume that children do naturally come in different
sizes based on genetics, and if this has been true forever,
how do we explain why so many American kids are heavier now?
And heavier to such a great degree? This would suggest that
while some children are simply meant to be larger, others are
indeed experiencing de-regulated weight patterns—they are
not growing as nature intended. What’s going wrong and
what are we to do about it to keep kids healthy? And, in the
process, how do we address these issues without overreacting
to normal patterns of growth and fatness in children? In Part
2 of Looking Beyond the Numbers we’ll discuss patterns
of growth—many measurements taken over time. We’ll
see how patterns of growth are the key to to determining when
weight truly reflects a problem and how the best standard to
compare a child to is him- or herself.
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Looking Beyond the Numbers - Part Three: Patterns of Growth
and Weight: Untangling What’s Normal from What’s
a Problem coming soon.
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