28.4: Linear growth (24c.4)

The limited sensitivity and specificity of linear growth and other similar “bioindicators” means linear growth must be measured alongside other biomarkers of zinc (i.e., plasma or serum zinc) and other growth-limiting nutrients to establish the role of zinc in poor growth. There is no pharmacological effect of zinc on growth in zinc-replete individuals and linear growth is considered the best functional bioindicator associated with the risk of zinc defi­ciency in popu­lations (King et al., 2015). Other functional bioindicators known to be responsive to zinc supple­mentation (e.g., diarrheal episodes) are difficult to defi­ne in a standardized manner. Justification for the selection of linear growth as the zinc functional bioindicator is summarized below in Box 24c.5. (Fischer -Walker and Black, 2005).

Box 24c.5 Justification for linear growth as the zinc functional bioindicator

• Low height- or length-for-age is often responsive to zinc supple­ments.
• Standardized methods exist to measure height or weight that are widely used.
• Reference data (i.e.,WHO growth reference data) (WHO , 2006) are available for interpretation.
• Linear growth is likely to be the primary response to an increased intake of absorbable zinc whereas weight gain is likely to occur as a result of increased linear growth.

In a study by Wessells and Brown (2012), the prevalence of inadequate zinc intakes in 138 low- and middle-incom­e countries was estimated from food balance sheet data. In these countries the prevalence of stunting in children less than 5y was positively correlated with the estimated prevalence of inadequate zinc intake (r=0.48. P=0.0001) as shown in Figure 24c.7.

Figure 24c.7 Relationship between the estimated prevalence of inadequate zinc intake and the prevalence of childhood stunting. Stunting data (low height-for-age) are for children less than 5y in138 low- and middle-income countries. The solid line represents the line of identity (intercept=0, slope=1). The dashed line represents the best-fit regression line. Dotted lines demarcate countries with a high risk of inadequate zinc intake and where the prevalence of stunting is > 20%. From Wessells & Brown (2012).

The figure also defines an at-risk group of 32 countries where the estimated prevalence of inadequate zinc intake is > 25%, and the prevalence of stunting is > 20%.Nevertheless, both inadequate intakes of zinc and stunting only provide suggestive evidence of zinc defi­ciency, and the multi-factorial causes of childhood stunting may be responsible, at least in part, for the marked variability around the “best-fit” regression (dashed) line.

24c.4.1 Interpretive criteria

The percentage of children < 5y with height- or length-for‑age less than −2SDs below the age-specific median of the WHO Multicentre Growth Reference Study (MGRS) popu­lation is recom­men­ded for assessing the zinc status of popu­lations (WHO, 2006). Risk of zinc defi­ciency is considered to be of public health concern when the prevalence of low height‑ or length-for‑age Z-scores among children aged less than 5y is ≥ 20% (de Benoist et al., 2007. The prevalence of low height- or length-for-age Z-scores (HAZ) for children 0–5y can be calculated from the WHO Child Growth Standard (WHO , 2006) and the com­puter program WHO (AnthroPlus). Note that in a healthy popu­lation of children the mean Z-score will be about 0.0 and the SD of the Z-score about 1.0; 2.5% of all the childen will have an HAZ-score < –2.

24c.4.2 Measurement of height or length

For infants and children ≤ 85cm (i.e. ≤ 2y), recumbent length is the recom­men­ded measure, preferably with the use of an infantometer with a range of 30–110cm, equipped with a digital counter reader. Recumbent length should be recorded to the nearest millimeter, or even more precisely (i.e., 0.1mm) when possible. Wooden or acrylic length measuring boards can be used, but they are rarely fitted with digital counters so are less reliable. Note that recumbent length for a child of about 2y is about 5mm greater than standing height for the same child (Haschke and van’t Hof, 2000).

Children > 85cm and adults should be measured in the standing position, preferably using a free-standing stadiometer (range 65–206cm), again equipped with a digital counter reader capable of measuring stature to 0.1mm. Platform scales with movable measuring rods should not be used as they are less accurate. Clothing should be minimal when height measure­ments are taken so that posture can be clearly seen. Shoes and socks should not be worn. The timing of the measure­ment should be recorded; diurnal variations in height occur due to com­pression of the spine as the day progresses (Buckler, 1978). Consequently, in popu­lation studies, standing height should always be measured at the same time of day, preferably in the afternoon.

When measuring recumbent length or standing height, attempts should be made to minimize measure­ment errors. In longitudinal studies involving sequential measure­ments on the same group of individuals, one person should conduct all of the measure­ments throughout the study to eliminate between-examiner errors. This is especially critical when growth velocity is estimated; growth increments are generally small and are associated with two error terms, one for each measure­ment occasion. Recom­mendations of the minimal intervals necessary to provide reliable data on growth increments during infancy and early childhood are available (de Onis et al., 2004). In the WHO MGRS, the minimal interval recom­men­ded for reliable data on length measure­ments was every two weeks for infants from 2–6 weeks of age, monthly for ages 2–12 months, and bimonthly in the second year. During adolescence, increments measured over 6 months are the minimum interval recom­men­ded (WHO, 1997). For shorter intervals, the com­bined errors may be too large in relation to the expected mean increment.

In large regional surveys, several well-trained anthro­pometrists are often needed to rotate among the participants to reduce the effect of measure­ment bias. Regular standardization sessions to assess both within‑ and between-examiner reliability should be conducted throughout the data collection period to maintain the quality of the measure­ments and to identify and correct systematic errors in the measure­ments; details of the procedures used in the WHO Multicenter Growth Reference Study (MGRS) are given in de Onis et al. (2004). An anthro­pometric training video prepared for the WHO MGRS is also available on request from WHO.

The WHO MGRS recom­mends that the maximum allowable differ­ence in length for acceptable precision between measure­ments by two anthro­pometrists is 7.0mm. (de Onis et al., 2004). Details of the measure­ment techniques and standardization protocols for both recumbent length and stature are given. Statistical methods exist for removing anthro­pometric measure­ment errors from cross-sectional anthro­pometric data; details are given in Ulijaszek and Lourie (1994).