8.1: Mea­sure­ments, indices, and indicators

Anthro­pometric mea­sure­ments are of two types. One group of mea­sure­ments assesses body size, the other group appraises body composition. The most widely used mea­sure­ments of body size are stature (length or height), weight, and head circum­ference; see Chapter 10 for more details. The anthro­pometric mea­sure­ments of body composition are based on the classical “two component model” in which the body is divided into two major compartments, fat mass and the fat free mass. Skinfold thickness mea­sure­ments are used to estimate of the size of the sub­cu­taneous fat depot, which, in turn provides an estimate of total body fat: over one third of total body fat is estimated to be sub­cu­taneous fat. The distri­bution of body fat is also important, with the mea­sure­ment of waist circum­ference used increasingly as a proxy for the amount of intra-abdominal visceral fat. Waist circum­ference is recom­mended for use in population studies (WHO, 2011), as well as in clinical practice for the evaluation and management of patients with over­weight or obesity (Ross et al., 2020).

The fat-free mass consists of the skeletal muscle, non-skeletal muscle, soft lean tissue, and the skeleton. A major component of the fat-free mass is body muscle. As this is composed of protein, assessment of muscle mass can provide an indirect assessment of the protein reserves of the body. Measure­ments of thigh circum­ference and mid-upper-arm circum­ference (MUAC) can be used to assess skeletal muscle mass (Müller et al., 2016).Measure­ment of MUAC is especially useful for young children < 5y in emergency settings such as famines and refugee crises. In such settings, children often have a small amount of sub­cutan­eous fat, so changes in MUAC tend to parallel changes in muscle mass; see Chapter 11 for more details.

Anthro­pometric indices are usually calculated from two or more raw mea­sure­ments, and are essential for the inter­pretation and grouping of mea­sure­ments collected in nutri­tional assessment. For example, the mea­sure­ment of a child's body weight is meaningless unless it is related to the age or height of a child. In young children the three most commonly used growth indices are weight-for-age, height-for-age, and weight-for-height. The first two indices reflect body weight or height relative to chronological age, whereas weight-for-height assesses body weight relative to height.

Body mass index (BMI) is also widely used in children and adults to assess under­weight, over­weight, and obesity, and is calculated as (weight kg) / (height m)². When height cannot be measured, as may occur in bed-bound or frail individuals, published equations based on a range of body mea­sure­ments such as knee height, lower leg length, arm span, and ulna length can be used to provide an approximate estimate of height. Examples of equations for estimating height from these body mea­sure­ments in adults are given in Madden et al. (2016). However, their usefulness for hospitalised patients may be questionable, if the equations have been derived from young and healthy populations (Reidlinger et al., 2014).

Examples of body composition indices include a combination of triceps skinfold and mid-upper-arm circum­ference, which together can be used to estimate mid-upper-arm fat area and mid-upper-arm muscle circum­ference or area, surrogates for total body fat content, and muscle mass, respectively. Other mea­sure­ment combinations include the waist-hip ratio (i.e., the waist circum­ference divided by the hip circum­ference), an additional index of the distri­bution of body fat which can be measured more precisely than skinfolds. Moreover, mea­sure­ments of waist-hip ratio as a surrogate for abdominal obesity, appear to be a stronger inde­pen­dent risk factor for risk of myocardial infarction, stroke and premature death than BMI, especially among men (Larsson et al., 1984; Lapidus et al., 1984).

In an effort to obtain more reliable estimates of per­cent­age body fat and fat-fat-free mass based on anthro­pometric mea­sure­ments in healthy adults, the sum of skinfold thickness mea­sure­ments from multiple anatomical sites is also used in conjunction with population-specific or generalized regression equations to predict body density, and in turn, the per­cent­age of body fat using one of three empirical equations. Once the per­cent­age of body fat is calculated, total body fat content and the fat-free mass can be derived (see Chapter 11 for more details). Again, many of the prediction equations were developed on young, healthy, lean Caucasian population groups and, hence, are less appro­priate for malnourished, obese, or elderly subjects or for other racial groups.

Anthropometric indices are often evaluated by comparison with the distri­bution of appro­priate anthropometric reference data using standard deviation scores (Z‑scores) or per­cen­tiles. (see Section 9.4.3). From this, the number and proportion of individuals (as %) with anthropometric indices below or above a predetermined reference limit or cutoff are often calculated. A commonly used reference limit for the three main growth indices is a Z‑score of −2 (i.e., below the WHO reference median) (Section 9.4.2). When used in this way, the index and its associated reference limit or cutoff become an “indicator”; these are discussed below.

Anthro­pometric indicators are constructed from anthro­pometric indices, with the term “indicator” relating to their use in nutri­tional assessment, often for public health, or socio-medical decision-making at the population level. Indicators are also used in clinical settings to identify individuals at risk of malnutrition. To be valid, a substantial proportion of the variability of an anthro­pometric indicator should be associated with differences in nutri­tional status. WHO (1995) provide a detailed classification of recom­mended anthro­pometric indicators based on their uses for both targeting and assessing response to inter­ventions, identi­fying deter­minants of mal­nu­trition, or predicting mal­nu­trition in populations of infants and children.

Anthro­pometric indicators should be chosen carefully in relation to both their proposed use and their attributes. Indicators vary in their validity, sensitivity, specificity, and predictive value; these characteristics are discussed briefly in Section 9.4.3. For example, although the indicator weight-for-age < −2 Z‑score is still widely used in health centers in many low-income countries for screening young children at risk of malnutrition, it is inappro­priate. Children who are stunted but of normal weight, or alternatively, tall and thin may be incorrectly diagnosed as “healthy”. Instead, in these countries, the indicator length/height-for-age < −2 Z‑score should be used (Ruel et al., 1995).

Further, several factors will affect the magnitude of the expected response of an anthropometric indicator. These may include the degree of defi­ciency, age, sex, and physiological state of the target group. Some examples of frequently used anthro­pometric indicators and their corresponding application are shown in Table 9.1.

Table 9.1 Anthropometric indicators and their corresponding applications.

Anthro­pometric indicator	Application
Proportion of children (of defined age and sex) with WHZ < −2	Prevalence of wasting
Proportion of children (of defined age and sex) with HAZ < −2	Prevalence of stunting
Proportion of children (of defined age and sex) with WAZ < −2	Prevalence of under­weight
Proportion of children 0–5y (of defined age and sex) with BMIZ > +2 or BMIZ > +3	Prevalence of over­weight or obesity
Proportion of adult women or men with waist- hip ratios > 0.85 (F) and > 0.90 (M)	Prevalence of abdominal obesity and thus risk of metabolic syndrome
Proportion of children 6–60mos with MUAC < 115mm	Prevalence of severe acute mal­nu­trition (SAM)
Proportion of children with SAM who have MUAC > 125mm and no edema for at least 2wk after receiving treatment for SAM	Prevalence of children ready for dis- charge following treatment for SAM