Dear Colleagues,

Zinc is a trace mineral essential to all forms of life because of its fundamental role in gene expression, cell development, and replication. Severe or clinical zinc deficiency was defined last century as a condition characterized by short stature, hypogonadism, impaired immune function, skin disorders, cognitive dysfunction, and anorexia. Although severe zinc deficiency is considered rare, mild-to-moderate zinc deficiency is likely prevalent throughout the world today. Lack of consensus on indicators of zinc deficiency has hampered efforts to document prevalence of zinc deficiency. For this reason, it is important to attempt to quantify the prevalence of zinc deficiency and its contribution to the global burden of disease.

Millions of people throughout the world may have inadequate levels of zinc in the diet due to limited access to zinc-rich foods (animal products, oysters, and shellfish) and the abundance of zinc inhibitors, such as phytates, common in plant-based diets. Zinc deficiency is largely related to inadequate intake or absorption of zinc from the diet, although excess losses of zinc during diarrhea may also contribute. The distinction between intake and absorption is important, because although some intakes of zinc may be acceptable, the levels of inhibitors (e.g., as phytates) in the diet may mean that inadequate amounts of zinc are absorbed. For this reason, zinc requirements for dietary intake are adjusted upward for populations in which animal products, the best sources of zinc, are limited, and in which plant sources of zinc are similarly high in phytates. Because zinc is not well conserved in the body and because zinc deficiency is directly related to dietary zinc intake, an indirect approach to quantify the prevalence of zinc deficiency would be to examine the adequacy of zinc in the diet in various regions worldwide.

Dietary surveys are conducted in many countries, but few such surveys exist in developing countries. Even when dietary intake data are available, incomplete information on the content of zinc and its bioavailability in local foods has made calculation of zinc bioavailability and physiological status challenging. In this regard, the assessment of marginal zinc status is problematic (current common markers: urine, serum, hair), as there is no universally accepted single measure to assess zinc status (Lowe et al., 2009, 2011; Gibson et al., 2016). Recently, it was again suggested that serum zinc concentrations are unrelated to zinc intake from either diet or supplements. This is not the first report that serum zinc concentrations are unrelated to zinc intakes. Three recent meta-analyses came to the same conclusion (Lowe et al., 2012; Moran et al., 2012; King, 2018). Hence, the need to develop additional robust indicators of zinc status and to further expand the already known clinical markers, for which limited data of reliability exists, is evident.

Furthermore, recent zinc animal model (Gallus gallus) studies followed by human studies showed that enzymes involved in metabolizing linoleic acid (specifically, the conversion of linoleic acid to dihomo-γ-linolenic acid, or the LA:DGLA ratio) may also be sensitive to modest changes in dietary zinc (Reed et al., 2014, Chimhashu et al., 2018; Knez et al., 2017, 2018). Yet again, the need to move beyond the three target indicators of zinc nutrition (i.e., zinc intake, serum zinc, and stunting) is evident when assessing the zinc status of populations. Metabolic studies show that tissues use multiple mechanisms to conserve zinc if the dietary supply is low. It also appears that some functions are more readily sacrificed than others. In the future, a zinc assessment model that combines key metabolic indicators of zinc status along with routine measurements needs to be developed to better diagnose the zinc status of populations.

Hence, the aim of the current Special Issue is to further explore dietary approaches that are aimed to alleviate zinc dietary deficiency and present and discuss strategies to monitor physiological zinc status in sensitive populations.

Dr. Elad Tako
Guest Editor

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• Zinc deficiency
• Biomarkers of Zinc status
• Monitoring Zinc status
• Improve dietary zinc absorption
• Zinc dietary inhibitors