(introduction...)

The anthropometric assessment of undernutrition among adolescents presents several problems that are not relevant to the assessment of young children. This section will outline some of these complications, including:

Changes in body proportions with age

Pubertal development

Inter-ethnic differences in genetic growth potential

Changes in body proportions with age

Thinness, as measured by weight-for-height and BMI, changes with age in healthy, well-nourished children, adolescents, and adults. In populations in industrialised countries, BMI reaches a nadir at about 6 years of age, then rises steadily until middle-age ^35-37. (see figure 1) The Cormic Index (sitting height divided by standing height) measures the ratio of leg-length to trunk length, and is sometimes called the sitting height to standing height ratio. The Cormic Index declines throughout childhood because leg length increases faster than trunk length during prepubertal growth ³⁸. However, the adolescent growth spurt is made up disproportionately of growth in the trunk, leading to a rise in Cormic Index in later adolescence ^38-40. At least one study shows a sharp change in the index with onset of adolescent growth ⁴⁰. Other anthropometric measurements also change with age in adolescence. MUAC rises progressively throughout adolescence at a rate greater than that in early childhood ⁴¹.

Figure 1 Weight-for-height (W/S), BMI (W/S2), and Rohrer Index (W/S3) as a function of age during childhood and adolescence, constructed from medians of NCHS reference population (from Cole, 1991 ³⁷)

Populations of many developing countries do not know their own ages. In young children age can be approximated by asking a mother about significant events on a local calendar which coincided with the child's birth. Such a technique may be much more difficult when asking about local events which occurred 10-19 years before. In any case, using such techniques to ascertain age takes substantial time, both in preparation of a local calendar and in posing extra questions during data collection.

Pubertal development

Superimposed on the more gradual age-related changes, more rapid changes in anthropometric measurements occur during sexual development. For example, during the adolescent growth spurt, the highest rate of weight gain follows the highest rate of height gain ⁴². This leads to an acceleration in the rise of BMI shortly after reaching the peak height velocity, and this rise is more related to pubertal development than chronologic age ^{42, 43}. Moreover, sexual development occurs at different ages in different populations ⁴⁴. As a result, differences between study populations and reference populations in sexual maturation can confound comparisons. Therefore, the comparison of a pre-pubertal child in a study population to post-pubertal individuals of the same age in a reference population would be invalid because the normal BMI for these two persons would be quite different. Chronic undernutrition can also delay sexual maturity and the adolescent growth spurt ^{4, 5, 44}. This delay can exaggerate differences in age of sexual maturation and the growth spurt between undernourished survey populations and well-nourished reference populations ⁴⁵.

Associations are also found between sexual development and skinfold thicknesses and MUAC. The onset of puberty changes the rate of subcutaneous fat deposition and fat distribution, as measured by skinfold thickness,^46-49 and these changes occur independently of chronologic age ⁵⁰. Similarly, MUAC changes with onset of puberty ^{47, 49}.

Adjustment for differences between the survey and reference populations, as recommended by WHO, requires the collection of data on the age at which certain landmarks of sexual maturation occur in the survey population. Each subject must be assessedfor pubertal changes; however, in practice, judging Tanner breast or genital stages during field surveys is very difficult. Health workers require extensive training and, in most field situations, exposing breasts and genitals of adolescents is not culturally acceptable. The single maturational landmark which might easily be collected in the field is age of menarche. In all populations where adolescents know their ages, age at menarche can be recalled by girls and women, and has been measured successfully in many surveys ^{2, 4, 43, 49, 51-53}. However, the situation is very different for boys, for whom no such easily obtained landmark exists. Self-assessment of genital and breast stage, using photographs or line drawings, has been validated in some countries, but has not been evaluated in developing countries or in a variety of cultures ^54-56. (See figure 2 for an example of drawings used in self-assessment.)

Figure 2 Examples of line drawings of various Tanner stages of sexual development

Interethnic differences in body size and shape

Although well-nourished, healthy children younger than 5 years of age are of similar size and shape worldwide,⁵⁷ school-age children and adolescents may not be similar. Older adolescents who have completed their growth spurt and have essentially become adults may reflect adult differences in body shape. Adults display differences in Cormic Index, which is one measure of body shape ⁵⁸. Figure 3 demonstrates the substantial effect which differences in Cormic Index have on BMI. As a result, it may be inappropriate to compare older adolescents, who may manifest ethnic differences in anthropometric indices, to a single universal reference population. (See accompanying report on assessment of adult nutritional status for further details ⁵⁹.)

Figure 3 BMI as a function of Cormic Index in a hypothetical 70 kg, 175 cm man. (from Norgan, 1994 ⁵⁸)