122 Obesity
health consequences. An individual is considered obese when
body fat content equals or exceeds 30% to 35% in women or
20% to 25% in men (Lohman, 2002). However, this decep-
tively simple de“nition obscures the complexities involved
in the measurement and classi“cation of body composition.
Direct measurement of body fat can be accomplished through
a variety of methods, including hydrostatic (underwater)
weighing, skinfold measurement, bioelectrical impedance,
dual energy x-ray absorptiometry (DEXA), and computer-
ized tomography (CT). Direct measurement is typically
either expensive (as is the case with DEXA and CT) or
inconvenient (as is the case with hydrostatic weighing and
skinfold measures). Consequently, for practical purposes,
overweight and obesity often have been de“ned in terms of
the relation of body weight to height.
“Ideal” Weight
Actuarial data from insurance companies have provided
tables of •idealŽ weights for mortality rates (Metropolitan
Life Insurance Company, 1983). For many years, 20% or
more over ideal weight for height was commonly used as the
de“nition of obesity (National Institutes of Health [NIH]
Consensus Development Panel on the Health Implications of
Obesity, 1985). In recent years, however, the limitations of
this approach have become increasingly apparent. For exam-
ple, insurance company data are not representative of the
U.S. population, particularly for women and minorities
(Foreyt, 1987). In addition, alternative weight-to-height
indices have shown greater correspondence to direct mea-
sures of body fat and to the negative health consequences of
obesity (L. Sjöstrom, Narbro, & Sjöstrom, 1995).
Body Mass Index
Body Mass Index (BMI), also known as Quetelet•s Index, is
an alternative weight-to-height ratio that has gained general
acceptance as the preferred method for gauging overweight.
BMI is calculated by dividing weight in kilograms by the
square of height in meters (kg/m^2 ). BMI can also be calcu-
lated without metric conversions by use of the following
formula: pounds/inches^2 704.5. BMI is not encumbered by
the problems inherent in de“ning •ideal weight,Žand it corre-
sponds more closely to direct measures of body fat than
alternative weight-to-height ratios (Keys, Fidanza, Karvonen,
Kimura, & Taylor, 1972; L. Sjöstrom et al., 1995).
While BMI provides an •acceptable approximation of
total body fat for the majority of patientsŽ (National Heart,
Lung, and Blood Institute [NHLBI], 1998 p. xix), it does not
discriminate between weight associated with fat versus
weight associated with muscle. For example, an athlete may
have a high BMI as a result of the higher body weight associ-
ated with greater levels of muscle mass rather than excess fat.
In addition, because one can be overfat,even in the context of
a healthy BMI, other measures such as waist measurement
should be used concurrently for a comprehensive assessment
of a person•s •risk due to weightŽ status.
Table 6.1 presents body weights (in pounds) by height (in
inches) that correspond to BMI values of 18.5, 25, 30, 35, and- These selected values correspond to the various cut points
used by the World Health Organization (WHO) system to cat-
egorize overweight and obesity.
The WHO Classification SystemThe WHO (1998) has developed a graded classi“cation sys-
tem for categorizing overweight and obesity in adults accord-
ing to BMI. In the WHO system, overweight is de“ned as a
BMI 25, and obesity is de“ned as a BMI 30. The WHO
system, which has also been accepted by NIH (NHLBI,
1998), employs six categories based on the known risk of co-
morbid conditions associated with different BMI levels (see
Table 6.2). For example, the risk of comorbid conditions is
considered •averageŽ in the normal weight category and
•very severeŽ in the obese class III category. Thus, the WHO
classi“cation system facilitates the identi“cation of individu-
als and groups at increased risk of morbidity and mortality,
and it allows for meaningful comparisons of weight status
within and between populations.TABLE 6.1 Body Mass Index
Body Mass Index
18.5 25 30 35 40
Height Body Weight
58 89 119 143 167 191
59 92 124 149 174 198
60 95 128 153 179 204
61 99 132 158 185 211
62 100 136 164 191 218
63 104 141 169 197 225
64 108 145 174 204 232
65 111 150 180 210 240
66 115 155 186 216 247
67 118 159 191 223 255
68 122 164 197 230 262
69 125 169 203 236 270
70 130 174 207 243 278
71 133 179 215 250 286
72 136 184 221 258 294
73 139 189 227 265 302
74 144 195 234 273 312