28.18: Multiple biomarkers (24c.18)

The efficient regulation of zinc homeo­stasis com­plicates the assessment of zinc defi­ciency and excess, and currently there is no single, sensitive, specific biomarker of zinc status. At the popu­lation level, three biomarkers have been recom­men­ded by WHO / UNICEF / IAEA /IZiNCG (de Benoist et al., 2007) and endorsed by the BOND Zinc Expert Panel (King et al., 2015) to assess zinc status and/or evaluate nutrition interventions to com­bat zinc defi­ciency. They are itemized in Section 24c.1.7. Details on the measure­ment and interpretation of these biomarkers are available from IZiNCG (Technical Briefs No.2 and No.3) and a table listing the relative strengths and weaknesses of the biomarkers is summarized in (King et al., 2015). For each biomarker, an indicator has been developed to estimate risk of zinc defi­ciency at the popu­lation level. These are: (a) the prevalence of usual zinc intakes below the estimated average require­ment (EAR), (b) the percentage of the popu­lation with low serum zinc concen­trations below the appropriate reference limit, and (c) the percentage of children less than 5y with length- or height-for-age less than −2SD below the age-specific median of the reference popu­lation (WHO, 2006). For each indicator, a “trigger level” for the prevalence considered indicative of elevated risk and of public health concern is given, at which level an intervention to improve popu­lation zinc status is warranted. Ideally, all three indicators should be included in a survey to strengthen the conclusion regarding the zinc status of the popu­lation and the need for an intervention, although in practice, this is not always possible. de Benoist et al., 2007 suggest selecting indicators according to the level of available resources (Box 24c.7).

Box 24c.7. Resource availability guidelines

• Limited resources available. Review existing evidence: Stunting rates (height– or length–for–age Z-score < −2SD) of children under 5y; Dietary patterns and likelihood of providing adequate bioavailable zinc in the food supply; Results from clinical, com­munity-based studies of functional response to zinc supple­ments; Results from com­munity-based studies on dietary zinc intakes or serum zinc concen­tration
• Some resources available Augment existing information with data collected in a prospective survey. This would normally be linked with other health/nutrition surveys with broader goals: In at least a sub­sample of the survey popu­lation, or samples focused among sub­popu­lations that are likely beneficiaries of nutrition or health interventions. Include serum zinc concen­tration and/or dietary zinc intake adequacy as biomarkers; where food-based interventions may be considered, at least 1 day of 24‑hour recall per individual in the sample should be com­pleted
• Adequate resources available Where resources permit: Serum zinc concen­tration should be deter­mined in a representative sample of all relevant sub­popu­lation groups; 24‑hour dietary recalls, or other locally validated quantitative dietary assessment methods, should be repeated on nonconsecutive days on at least a sub­sample to estimate the distribution of usual zinc intakes

For individuals, unless zinc defi­ciency is severe, moderate or mild zinc defi­ciency can be present in the absence of abnormal zinc biomarkers, including serum zinc, despite the presence of distur­bances such as growth retardation, loss of appetite, and impaired immune function. Hence, for assessing the zinc status of individuals, the recom­men­ded approach is based on a medical history, a qualitative assessment of the diet, and a clinical assessment, with plasma or serum zinc providing only supple­mentary data (King et al., 2015), as noted earlier.