TABLE 6.

Relative strengths and weaknesses of zinc biomarkers¹

Biomarker	Usefulness for the purpose	Advantages	Disadvantages	Analytical considerations
Dietary assessment	For dietary assessment of zinc intake, the 3 major instruments are FFQs, 24-h food-intake recalls, food diaries or weighed records, or weighed duplicate portions.	The dietary assessment provides an assessment of zinc sources. Dietary assessment methods do not accurately quantify “usual” zinc intake, but dietary data can be used to identify the most important food sources of zinc. If dietary phytate is estimated, the bioavailability of zinc can also be determined. This information is useful to determine risk of zinc deficiency in a population or to design zinc intervention strategies.	It is time consuming to collect data, because it is difficult to identify all zinc sources.	All FFQs and 24-h recalls need to assess specific brands or types of cereal or bread products to determine if the product is zinc-fortified. Questions with regard to the specific cut of meat need to be assessed because zinc content varies with the type of muscle.
	FFQs assess the frequency and portion sizes of zinc-containing foods and/or food groups consumed over a predefined time frame, usually 1 y or several months. The FFQ method captures zinc-rich sources that are irregularly consumed and accounts, to some extent, for day-to-day variation in the overall consumption patterns.	National food balance sheets can be used to estimate risk of zinc inadequacy in a population.	Food-composition databases may not contain information on the zinc content of all foods. The amount of zinc added as a fortificant may vary among similar foods (i.e., breakfast cereals). Dietary phytate, which influences zinc absorption, should also be assessed, but it is rarely included in food-composition tables.	Portion sizes need to be carefully quantified to estimate zinc intakes from meat, fish, poultry, cereals, grains, and seeds.
	Twenty-four-hour recalls assess intakes over the past 24 h. To capture day-to-day variation in dietary zinc intake, 24-h recalls must be repeated preferably on nonconsecutive days, the number of repeats depending on the within-person variation in zinc intakes of the study group.
Plasma or serum zinc concentration	Plasma zinc may be used to predict a functional response to an intervention, i.e., growth in children or an immune response.	The decline in plasma zinc with severe zinc depletion reflects a change in total body zinc.	A meta-analysis of high-quality studies of the relation between zinc intake and plasma zinc showed a high degree of heterogeneity in all population groups.	Analysis of plasma zinc concentrations requires special care to avoid contamination. Use stainless steel needles and trace element–free tubes and syringes and avoid hemolysis of blood.
	Plasma zinc responds consistently to zinc supplementation. It also decreases with very low zinc intakes (<2 mg/d).	Plasma zinc responds quickly (within 5–10 d) to zinc supplementation in all population groups. This response occurs irrespective of initial plasma zinc concentrations.	Plasma zinc does not respond to short-term exposure for fortified zinc foods; some response may occur when children are given fortified food sources for longer periods.	An AAS, an ICP-OES, or an ICP-MS are required for the analysis.
	Plasma zinc may be used to predict a functional response to an intervention, i.e., growth in children or an immune response.	Plasma zinc reference limits have been established for children, men and women, and pregnant women.	It is not known if functional changes occur without changes in plasma zinc.
		Different cutoffs are available for fasting, morning, or afternoon blood draws to adjust for diurnal variation.	Biological factors, other than zinc intake, influence plasma zinc concentrations. Examples include infection, food intake, time of day, sex, age, pregnancy, oral contraceptive use, severe stress, position of subject during blood drawing, and length of time subject‘s arm is occluded with a tourniquet.
Stunting	A growth response to a zinc supplement reflects a pre-existing zinc deficiency. However, it does not rule out other factors that may be limiting growth. There is no pharmacologic effect of zinc on growth. Thus, an increase in growth with supplemental zinc reflects zinc deficiency.	Standardized measurements of length or height are noninvasive and require simple equipment (i.e., a wooden or acrylic length board or stadiometer, preferably fitted with a digital counter). Ideally, the clothing should be minimal for length or height measurements so posture can be seen clearly. Shoes and socks should not be worn. WHO growth standards are available for evaluating the rates of growth in children up to 5 y of age. A WHO growth reference is also available for school-aged children and adolescents, which is closely aligned with the WHO Child Growth Standards at 5 y. The WHO provides computerized programs for calculating the degree of deviation from the age-specific reference median for a male or female child of the same age, i.e., the z score.	There are no definitive cutoffs for predicting an elevated risk of zinc deficiency within a population. A prevalence of 20% of low length- or height-for-age (defined as < −2 z scores) for children <5 y has been used as a reasonable cutoff. Low height cannot be used to evaluate the prevalence of zinc insufficiency among individuals who are no longer growing. Accurate measurements of length or standing height require calibrated equipment and strict adherence to standard procedures for making the measurements. In longitudinal studies involving sequential measurements on the same individual, 1 person should conduct all of the measurements to eliminate between-examiner errors.	Several well-trained anthropometrists are often rotated in large cross-sectional surveys to reduce measurement bias. To accurately measure growth velocity in infants, the measurements should be made every 2 wk in infants between 2 and 6 wk of age, monthly for ages 2–12 mo, and bimonthly in the second year. Time of measurement should be recorded because the spine gradually compresses during the day.

¹AAS, atomic absorption spectrometer; ICP-MS, inductively coupled plasma mass spectrometry; ICP-OES, inductively coupled plasma optical emission spectrometer.