O

OBESITY

American culture values thinness. From supermodels
to laptop computers, the American public equates
thinness with beauty; yet, the prevalence of obesity in
the United States is higher than at any time in history.
Estimates of overweight and obesity among children
and adolescents in the United States have doubled
since the 1970s, to almost 25 percent in 2001. Once
obesity develops in childhood or adolescence, there
is a risk that it will persist into adult life. The risk ap-
pears to be greatest for children who are obese in the
prepubertal years (between ages nine and thirteen),
with more than 50 percent of such children remain-
ing obese as adults, and for children with one or two
obese parents.

William Dietz, of the Centers for Disease Control
and Prevention (CDC), described three critical peri-
ods in the development of obesity, corresponding to
periods of adipose tissue (the connective tissue where
fat is stored) proliferation: gestation and early infan-
cy, ages five to seven years, and adolescence. Body fat
increases over the first twelve to eighteen months of
life. Loss of ‘‘baby fat’’ over the subsequent eighteen
months leads to a decrease in fatness, which lasts until
the age of five to seven years, when the adiposity re-
bound occurs and fatness begins to increase again.
Children who experience the adiposity rebound earli-
er, before five years and six months of age, are more
prone to later obesity.

Definition

Obesity is a condition of excessive fatness. Fatness

is often expressed as a percentage of body weight.

Prepubertal boys and girls typically have about 15

percent of their body weight as fat, while the average

adult male is 20 percent fat, and the average adult fe-

male is 30 percent fat. Body fatness is measured using

a variety of techniques, such as body densitometry,

electrical impedance, and skin-fold thickness. Total

body weight, although it includes muscle, bone, and

internal organs, in addition to body fat, can be used

as an index of fatness, especially when expressed in

relation to body height. A ratio of weight and height,

called the body mass index (BMI), has been adopted

for use in the assessment of children, adolescents, and

adults. BMI is calculated by dividing the weight, in

kilograms, by the square of the height, in meters (kg/

m2). If pounds and inches are used, then the quotient

(pounds divided by inches squared) multiplied by

704.5. For adult men and women, BMI greater than

25 signifies overweight, and BMI greater than 30 in-

dicates obesity. During childhood, BMI varies by gen-

der and normally increases with age. Obesity is

determined with a graph or reference table that gives

the eighty-fifth percentile for age (as a criteria for

overweight) and the ninety-fifth percentile for age (as

a criteria for obese) for boys and girls. In 2000 the

CDC published revised percentile standards for BMI

(see Table).

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