28.2: Dietary Zinc Intake (24c.2)
- Page ID
- 117292
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Assessment of dietary zinc intakes is the best method for estimating zinc exposure in individuals and populations. To determine dietary zinc intakes, a quantitative dietary method such as a weighed or estimated food record or a validated 24-h recall must be used. Details of the procedures to estimate dietary zinc intakes using a modified 24-hr recall specifically for use in lower income countries are available in a Dietary Technical Monograph (#8) (Gibson and Ferguson, 2008).
Information on dietary zinc intakes can be used for several purposes. At the population level, data can be used to: estimate the prevalence of inadequate zinc intakes, classify subpopulations at elevated risk, design and monitor zinc intervention programs, and identify dietary patterns that contribute to inadequate zinc intakes. At the individual level, those at risk of inadequate intakes can be identified for dietary counseling. Tools have been developed to facilitate the collection of dietary zinc intakes and analyze the data to assess dietary adequacy (IZiNCG Technical Briefs) No.3 and No.7.
Inadequate intakes of dietary zinc can arise from low intakes of zinc per se, poor bioavailability, or a combination of these dietary factors, as noted earlier. In low income countries, where unrefined cereals and legumes containing high levels of phytate are often the major source of energy and zinc, inadequate intakes of bioavailable forms of zinc are the most likely cause of zinc deficiency. Other diets based on starchy roots or tubers have a low total zinc and phytate content. Values for the phytate content of raw and processed plant-based staples are available in the FAO / INFOODS / IZiNCG Global Food Composition Database (Infoods Phytate Database). When collecting dietary intake data for zinc, intakes of dietary phytate should also be determined, whenever possible.
Data on both zinc and phytate intakes permit the calculation of phytate-to-zinc molar ratios from which an estimate of the bioavailability of zinc can be obtained, as noted in Section 24c.1.5
24c.2.1 Measurement of zinc intake for populations
The recommended dietary indicator to identify a population or population subgroup at elevated risk of zinc deficiency is the prevalence of usual zinc intakes below the estimated average requirement (King et al., 2015). Because information on usual zinc intakes is required, food intakes must be measured on two non-consecutive days or three consecutive days. It is preferable that multiple days of intake data are collected for all individuals in the population. However, if this is not possible, then at least two non-consecutive days of dietary intake data from a sub-sample(40–50) of individuals per stratum should be collected. Figure 24c.4
Figure 24c.4 Estimates of usual intake distribution for zinc for New Zealand adults obtained from 24h recall data and adjusted with replicate intake data using the refined NRC method. The y-axis (frequency of intake) shows the likelihood of each level of intake in the population. EAR, Estimated Average Requirement. Redrawn from Gibson et al. (2003).
These data permit an estimate of the day-to-day variation in zinc intake within one individual (i.e., within-person variation) to be estimated, allowing a distribution of usual zinc intakes for the population group to be generated using specialized software. Finally, the Estimated Average Requirements (EARs) for zinc, specific for age, sex, physiological status, and in some cases the phytate-to-zinc molar ratio of the diet, are needed to calculate the prevalence of inadequate intakes. If quantitative data on phytate intakes are not available, then the phytate-to-zinc molar ratio can be estimated based on the diet type. Figure 24c.4 shows an example comparing the adjusted distributions of usual zinc intakes with the observed one-day zinc intakes for New Zealand adult females aged 19–50y, using the program developed by Iowa State (ISU) and implements the ISU method (Nusser et al., 1996) for the adjustment. The adjustment process used yields a distribution with reduced variability that preserves the shape of the original observed distribution (Gibson et al., 2003). The example also shows that in this particular case, adjusting the distribution significantly reduces the proportion of individuals considered to have intakes below the EAR. An elevated risk of zinc deficiency in the population is said to exist when 25% or more of the population have zinc intakes less than the EAR.
The five main steps required to calculate this recommended dietary indicator are summarized in Box 24c.2.
- Select a representative sample of the population
- Measure food intake by using either a 1-day weighed or estimated food record or a validated 24-h recall, preferably on at least two non-consecutive days for each individual or for at least a subsample of individuals in the population (40-50 individuals for each life-stage group with a different zinc requirement).
- Calculate zinc and phytate intakes and dietary phytate-to-zinc molar ratios for each individual by using an appropriate food composition database for the country. For non-pregnant and non-lactating adults, the amount of zinc likely to be absorbed may also be calculated.
- Adjust the distribution of observed zinc intakes to represent usual zinc intakes by removing the variability introduced by day-to-day variation in an individual's zinc intake.
- Select the appropriate EAR and apply the EAR cutoff method to estimate the prevalence of usual zinc intakes below the EAR. Alternatively, in countries where local food composition values for zinc and phytate are not available, collect 24-h duplicate diet composites from each individual for the chemical analysis of zinc and phytate, again with repeats on ≥40–50 individuals per stratum as described above (IZiNCG Technical Briefs) No.3.
24c.2.2 Interpretation of zinc intakes for populations
Several expert groups have set dietary zinc recommendations that include EARs for zinc, although discrepancies exist, depending on the sources of the data, the concepts and methods used, as described in Gibson et al. (2016). For example, the EARs for zinc set by the United States and Canada are based on a fixed adjustment for zinc absorption from habitual diets for both children and adults (Food and Nutrition Board, 2001). In contrast, the European Food Safety Authority (EFSA, 2014) have generated dietary zinc requirements based on four levels of dietary phytate intake for adults (> 18y) and said to cover the average phytate intakes in European populations: (300mg/d; 600mg/d; 900mg/d; 1200mg/d). In view of the uncertainty about whether recommendations for dietary zinc based on phytate intakes can also be made for young children, the EFSA EARs for infants and children are not adjusted for phytate.
The prevalence of usual zinc intakes below the EAR is simply estimated by calculating the percentage of individuals within a specific life-stage group with usual intakes below the respective EAR. An elevated risk of zinc deficiency in the population is said to exist when 25% or more of the population have zinc intakes less than the EAR. However, to assist with the interpretation of the risk of zinc deficiency, it is recommended that estimates of the adequacy of zinc intakes be combined with biochemical data on serum zinc concentrations (IZiNCG Technical Brief) No.3.
In cases where the amount of zinc likely to be absorbed is calculated for non-pregnant and non-lactating adults ≥ 19y using the updated trivariate saturation response model of Miller et al. (2007), then the prevalence of absorbable zinc intakes below the appropriate physiological requirement for absorbed zinc should be calculated. For IZiNCG, these requirements are 1.86mg/d for adult women and 2.69mg/d for adult men ≥ 19y of age. For further details of this calculation see (IZiNCG Technical Brief) No.3.
24c.2.3 Measurement and interpretation of usual zinc intakes for individuals
Usual intakes of dietary zinc at the individual level can also be determined using quantitative dietary methods such as a weighed or estimated food records or validated 24-h recalls, provided a large number of measurement days per individual are conducted. Details on how to define the desired number of replicate days per individual are available in Gibson and Ferguson (2008). Alternatively, a dietary history or validated semi-quantitative food frequency questionnaire can be used to provide retrospective information about usual food consumption patterns, preferably over more than one month; details are available in Thompson et al. (2015). Assessment of the intake of zinc-rich foods (e.g., meat, poultry, fish), zinc-fortified foods, and high-phytate foods (e.g., unrefined cereals, nuts, and legumes) can be used to predict the intake of absorbable zinc (King et al., 2015). Inferences can be made about the adequacy of the zinc intake of an individual by comparing the difference between the estimate of the usual zinc intake of the individual and the respective values for the EAR and Recommended Nutrient Intake (RNI) for zinc. If the usual intake of the individual is greater than the RNI, then there is a high level of confidence that the usual intake is adequate, whereas if the usual intake is between the EAR and the RNI, the usual zinc intake probably needs to be improved, and if the usual intake is less than the EAR, then the usual intake very likely needs to be improved. Details on the guidelines for the qualitative interpretation of individual intakes are available from the Institute of Medicine (2000).