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28: Zinc (Chapter 24c)

Last updated

Mar 11, 2025
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- 27.9: Other indices of copper status (24b.8)
- 28.1: Zinc (24c.1)

Rosalind S. Gibson
University of Otago

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Abstract

Multiple physiological functions are affected by zinc deficiency because of the involvement of zinc in numerous metabolic processes as a catalytic, structural, and regulatory ion. As a consequence, in zinc deficiency numerous nonspecific changes occur, including impairments in growth, immune function, appetite, taste acuity, and possibly neurobehavioural function. Zinc homeostasis in the body is tightly controlled over a wide range of zinc intakes, primarily by adjusting losses of endogenous fecal zinc. Such strict homeostatic control leads to difficulties in the assessment of zinc insufficiency and excess. The BOND Expert Zinc Panel recommended three indicators to identify populations at elevated risk of zinc deficiency — the prevalence of usual zinc intakes below the estimated average requirement, the percentage with low plasma / serum zinc concentrations, and the percentage of children < 5y who are stunted (i.e., HAZ < –2SD).Reference limits for plasma / serum zinc (depending on age, sex, time of day, fasting status) are available to identify populations at risk of zinc deficiency, provided the correct protocols are followed for the collection, separation, and analysis of plasma zinc. Moreover, in view of the poor specificity of impaired linear growth, assessment of this functional indicator should be made in combination with at least one of the other indicators. Assessment of marginal zinc deficiency in individuals is more difficult because of the absence of frank clinical signs and the lack of a reliable sensitive and specific biomarker. The recommended approach combines a medical history, qualitative dietary and clinical assessment, with plasma / serum zinc providing supplementary data. The BOND Expert Zinc Panel classified other potential zinc biomarkers into three groups: (1) potentially useful (hair zinc, urinary zinc, and neurobehavioral function); (2) emerging biomarkers (nail zinc, zinc-dependent proteins, oxidative stress and DNA integrity; zinc kinetics, taste acuity), and (3) not useful (zinc-dependent enzymes, erythrocyte and leukocyte zinc); strengths and weaknesses of each are discussed.

28.1: Zinc (24c.1)
This page highlights the importance of zinc in health, particularly for immune function and growth in adults and children. Zinc's absorption is affected by dietary factors like phytic acid and iron. Deficiencies can lead to serious health problems, particularly in vulnerable populations. The page discusses the challenges of assessing zinc status, emphasizing the need for multiple indicators and careful measurement protocols.
28.2: Dietary Zinc Intake (24c.2)
This page highlights the importance of assessing dietary zinc intakes to estimate exposure and prevalence of deficiencies, especially in low-income countries. Methods include weighed food records and 24-hour recalls, focusing on foods affecting zinc bioavailability. Analyzing food consumption aids in predicting absorbable zinc, and comparing usual intake to the Estimated Average Requirement (EAR) and Recommended Nutrient Intake (RNI) evaluates adequacy.
28.3: Serum zinc (24c.3)
This page discusses serum and plasma zinc concentrations, emphasizing their interchangeability and the significance of serum zinc as a biomarker for deficiency. Factors affecting serum zinc include dietary intake, age, gender, hormonal influences, and various health conditions. Accurate measurement is crucial, requiring proper sample handling and consideration of inflammation effects.
28.4: Linear growth (24c.4)
This page discusses the importance of linear growth as a bioindicator for zinc deficiency, particularly in relation to childhood stunting. It emphasizes the correlation between low height-for-age and inadequate zinc intake, advocating for the measurement of linear growth along with other biomarkers. Standardized measurement methods and WHO reference data are highlighted for accurate interpretation.
28.5: Urine zinc (24c.5)
This page discusses urinary zinc excretion, which ranges from 0.3–0.6mg daily and is influenced by dietary intake and supplementation. A systematic review found it to be a reliable biomarker for zinc supplementation in well-nourished adults but indicates decreased excretion only in cases of very low intake. Various clinical and lifestyle factors affect zinc excretion.
28.6: Hair zinc concentrations (24c.6)
This page explores the integration of zinc into hair during growth and its potential as a long-term biomarker for zinc exposure. It highlights the challenges in using hair zinc levels, including unstandardized sampling and reference values, with a suggested cutoff indicating risk of deficiency. Methodology for measuring hair zinc concentrations is detailed, emphasizing the importance of representative samples, proper washing techniques, and reliable analytical methods.
28.7: Nail zinc (24c.7)
This page discusses nail zinc as an emerging biomarker for zinc exposure, which is less researched than hair zinc. Normal levels range from 80 to 200µg/g, with elevated levels in those exposed to zinc or taking supplements. Nails may indicate long-term zinc exposure, but health correlation studies yield mixed results. The BOND Zinc Expert Panel advocates for further research.
28.8: Metallothionein concentrations in circulating blood cell types (24c.8)
This page discusses metallothionein, a metal-binding protein crucial for zinc homeostasis, predominantly found in the liver and kidneys. Serum levels indicate liver status and stress response, while erythrocyte levels relate to dietary zinc but vary with erythropoiesis. Advances in immunoassays enhance research on metallothionein as a potential biomarker for zinc status, though more studies and reference materials are needed to confirm their utility.
28.9: Metallothionein mRNA expression in circulating blood cell types (24c.9)
This page discusses how zinc induces metallothionein gene expression through MTF-1, based on a review of sixteen studies. It highlights a correlation between dietary zinc intake and metallothionein levels in blood cells—higher intake boosts expression while lower intake reduces it. However, shifts in expression do not correspond with plasma zinc levels, suggesting metallothionein may be a more sensitive biomarker for zinc status.
28.10: Zinc transporter mRNA expression in circulating blood cell types (24c.10)
This page discusses the role of zinc transporters ZIP 1-14 and ZnT 1-10 as potential biomarkers for zinc status in blood cells. A systematic review revealed inconsistent changes in their expression following zinc supplementation or depletion, indicating a need for further research to establish their reliability under typical dietary conditions.
28.11: Oxidative stress and DNA integrity (24c.11)
This page discusses oxidative stress, which results from an imbalance between reactive oxygen species and antioxidant activity, risking cellular damage. Zinc is highlighted for its role in antioxidant defense and DNA integrity, with dietary zinc linked to reduced DNA strand breaks. However, changes in oxidative stress markers are not definitive indicators of zinc status, leading to their classification as emerging biomarkers.
28.12: Kinetic markers: pool sizes and turnover rates (24c.12)
This page discusses the identification of an exchangeable zinc pool (EZP) that is essential for zinc metabolism, with a turnover rate of approximately 12.5 days and influenced by factors like diet, age, and sex. It highlights how dietary restrictions reduce EZP size, while supplementation increases it. Plasma zinc turnover rates also adjust with zinc depletion, indicating a need for further research to confirm these as biomarkers of zinc status.
28.13: Taste acuity (24c.13)
This page discusses the connection between diminished taste acuity, or hypogeusia, and marginal zinc deficiency. It highlights research indicating that low zinc levels in both children and adults can impair taste perception, especially for salt. Although aging affects taste, its relationship with zinc levels is unclear.
28.14: Zinc-dependent enzymes (24c.14)
This page discusses the importance of zinc in metallo-enzymes but notes the lack of reliable biomarkers for zinc status in humans. Research on alkaline phosphatase and other enzymes has produced inconsistent results, with a systematic review showing no significant impact of zinc on alkaline phosphatase levels. Consequently, experts consider these enzymes inadequate for evaluating zinc exposure or status, attributing variability in responses to multiple alkaline phosphatase isozymes.
28.15: Erythrocyte zinc (24c.15)
This page discusses the concentration of zinc in erythrocytes versus plasma, noting its prevalence in carbonic anhydrase. The BOND Zinc Expert Panel found erythrocyte zinc ineffective for assessing zinc status due to varied responses and low sensitivity. It highlights the absence of standardized measurement units and interpretive criteria for erythrocyte zinc levels, complicating research comparisons.
28.16: Leukocyte zinc (24c.16)
This page discusses the zinc levels in leukocytes, noting that they are higher than in erythrocytes, particularly in monocytes and lymphocytes. However, the BOND Zinc Expert Panel considers leukocyte zinc levels unreliable for assessing zinc nutrition due to inconsistent results, methodological issues, potential contamination, insufficient blood volume requirements for testing, and unclear interpretive criteria.
28.17: Salivary zinc (24c.17)
This page discusses the study of zinc concentrations in saliva as indicators of zinc status and their association with the protein gustin, important for taste. It highlights inconsistent research findings, challenges like controlling saliva flow and diurnal variations, and concludes that the use of salivary zinc as a reliable biomarker has been abandoned due to these complications.
28.18: Multiple biomarkers (24c.18)
This page discusses the challenges of assessing zinc deficiency due to the lack of a reliable biomarker. WHO recommends using usual zinc intakes, serum zinc concentrations, and children's growth measurements as key indicators for evaluation. Public health strategies may vary based on resources, and for individual assessments, a combination of medical history, diet evaluation, and clinical assessments is advised, as biomarkers alone may not provide an accurate reflection of deficiency.

Abstract

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