Deficiencies in each of the micronutrients zinc, iron, vitamin A, folate, vitamin B12, vitamin D and iodine can promote increased susceptibility to infections, birth defects, blindness, reduced growth, cognitive impairment, poor school performance and even death, with Stevens et al (2022) estimating that worldwide such deficiencies affect over half of preschool-aged children and two-thirds of non-pregnant women of reproductive age. Recently, however, there has been renewed focus on the micronutrient zinc.
What is zinc and how much do we need?
Zinc is a trace element whose properties include regulation of gastrointestinal, immune, integumentary, reproductive and central nervous systems (Corbo and Lam, 2013). The body harbours two pools of the element: the first contains slow-exchanging zinc, mainly located in muscle and bone; the second, a rapid-exchanging zinc pool, is found in the blood, the gastrointestinal tract, the liver and other internal organs (De Benedictus et al, 2022). The rapidexchange pool is more reactive to the amount of zinc absorbed from food, and this pool is the first to be depleted through low dietary zinc intake.
Good sources of zinc include meat, shellfish and dairy foods like cheese, and according to the NHS (2020) our daily zinc requirements are 9.5 mg/day ‘for men (aged 19-64 years)’, 7 mg/day for women, and it warns ‘do not take more than 25 mg of zinc supplements a day unless advised to by a doctor.’ Further, in the context of nutrition, ‘zinc deficiency is common after bariatric surgery and is an indicator of protein malnutrition early after surgery’ (Sallé et al, 2010).
Pregnancy and breastfeeding
De Benedictis et al (2022) investigated zinc intake and bioavailability in 258 pregnant and non-pregnant women based in Ireland and reported that ‘low zinc intake may be prevalent in as much as 87% of women, including 47% of pregnant women.’ The authors noted that although the Irish dietary reference value for zinc is 10 mg/day, the zinc concentrations needed almost double during pregnancy, and that vegetarian diets may provide insufficient bioavailable zinc due to the lack of zincrich foods like meat and fish, and the presence of high phytate concentrations in plant-based sources that can inhibit zinc absorption in the small intestine.
When breast milk zinc concentrations were determined in 390 samples from Palestinian women, Shawahna (2022) suggested that the prevalence of low breast milk zinc concentrations found in the study might be partly explained by the Palestinians’ phytate-rich diet of mainly rice, cereals and maize.
Children and zinc
In experimental studies, chronic inflammation has been shown to cause a significant decline in serum zinc concentrations, with Mirza and Sevim (2016) citing evidence that relatively low zinc concentrations were found in children with bronchial asthma compared to a control group, ‘so the changes in trace element status may be the effect of the chronic disease state and not the cause of the disease per se.’ Mirza and Sevim (2016) describe the case of a 2-year-old boy who presented with severe oedema and erythema, crusting on both lower eyelids, and an erythematous rash on his perioral region. He was found to have a low plasma zinc concentration, and ‘was started on a regimen of zinc supplementation, and at 4 weeks’ follow-up there was nearly complete resolution of the lesion’ (Mirza and Sevim, 2016).
Older people
Although anaemia is a significant comorbidity for older adults it cannot be fully ascribed to iron deficiency, with MacDonnell et al (2021) noting that low-grade inflammation and other micronutrient deficiencies also contribute to the condition. In their cross-sectional study of 285 residents (>65 years) in 16 New Zealand aged-care facilities, MacDonnell et al (2021) found that 32% of participants exhibited anaemia, although less than 2% had either depleted iron stores or iron deficiency, with plasma zinc and selenium deficiencies affecting 72% and 38% of participants, respectively. These findings underline the role of multiple micronutrients when interpreting the aetiology of anaemia.
Depression
In terms of a possible role of zinc in depression, Wang et al (2018) state: ‘Collectively, empirical evidence most strongly supports a positive association between zinc deficiency and the risk of depression and an inverse association between zinc supplementation and depressive symptoms.’ However, association does not necessarily equate with causation. Professor Gautam Gulati -Consultant Forensic Psychiatrist at the University of Limerick - told Practice Nursing that there is limited research in respect of micronutrients and severe mental illness. ‘As a practitioner, one is cognisant that in cases of treatment-resistant depression for example, it's worth investigating micronutrient deficiency as these can make the illness harder to treat’. Professor Gulati also makes the point that the importance of good nutrition in treating mental illness ‘cannot be over emphasised. A balanced diet, that offers the right nutrients is invaluable and should be discussed with all patients.’
‘Zinc is a trace element whose properties include regulation of gastrointestinal, immune, integumentary, reproductive and central nervous systems.’
Wounds and a role in COVID-19 therapy
Lin et al (2017), in reviewing the role of zinc in wound healing, comment on zinc's major role in the regulation of every phase of the wound healing process: ranging from cell membrane repair, oxidative stress, coagulation, inflammation and immune defence, tissue re-epithelialization, angiogenesis, to fibrosis and scar formation. More specifically, Lin et al (2017) note that due to the loss of zinc during injury, zinc therapy has been used in wound care to enhance healing in zinc-deficient patients. For example topical zinc sulphate, typically at an optimal 3% concentration, promotes wound healing through its antioxidant effect, and other types of application include 1% zinc chloride and zinc oxide.
Samad et al (2021) not only highlight the fact that zinc is ‘an agent of antiviral immunity and an enhancer of both inherent and acquired immunity’, with zinc deficiency predisposing patients to viral infections like herpes simplex, the common cold, and hepatitis C; they also report that the zinc-containing compound pyrithione has reportedly been shown to inhibit SARS-coronavirus RNA polymerase activity, and preliminary research suggests that pyrithione may stimulate zinc's efficiency in its antiviral ability against COVID-19.
Finally
As Stevens et al (2022) demonstrate, the role of micronutrients in human health is of vital importance. The example of zinc serves to illustrate the diverse range of bodily mechanisms and processes whose optimal operation depends on this vital element.