Introduction to Human Nutrition

28 Introduction to Human Nutrition

body composition in the elderly and changes in body
composition with age. For the same reason, the BMI
as an indicator of body fatness is only suitable when
corrections for age and gender are made; for example,
a BMI of 25 kg/m^2 at the age of 20 years refl ects a
much lower percentage of body fat than at the age of
70 years. Body fat in females is always higher than
body fat in males with the same BMI and age. Recent
studies have shown that the relationship between
BMI and body fat percentage also differs among
ethnic groups. Figure 2.6 gives a good insight on how
misleading a low BMI can be. The biodata also show
the differences in body build, which is at least part of
the reason for the paradox. Prediction equations
based on impedance are dependent on body build,

among other factors, and there are no universally

valid prediction equations based on impedance.

Choice of method to assess body composition in a

population would also take into consideration within-

and between-observer variability. Thus, impedance

may be better than skinfold thickness measurements

in a study of 1000 people, where variability between

technicians is important. One may argue that, for

population studies, a slight overestimation or under-

estimation of body fat is not important. However, if,

for example, obesity is defi ned as a certain amount of

body fat that is achieved at a certain distinct level of

BMI, even minor differences in body fat percentage

or in BMI will result in large differences in the preva-

lence of obesity.

Age (years) 21 22

Height (cm) 175.6 158.0

Weight (kg) 77.2 43.9

Relative sitting height 0.53 0.55

Wrist (cm) 5.6 4.4

Knee (cm) 9.1 8.4

Arm span (cm) 177.0 157.5

Waist (cm) 82.0 62.2

Hip (cm) 104.0 87.0

BMI 25.0 17.6

BF (%) 31.1 29.6

Dutch Asian

Figure 2.6 The difference in the relationship between BMI and body fat percentage across populations is best demonstrated in this fi gure
and the given biodata. Note that the two young women are the same age and their percentage body fat as determined by a chemical four-
compartment model (bias free!) is the same. The Asian woman has relatively shorter legs and a more slender body build (determined as height/(sum
of knee and wrist diameter). Relative leg length and “slenderness” are main determining factors in the BMI/percentage body fat relationship in
addition to physical activity level.