Biological role of zinc
Zinc has catalytic, regulatory and structural roles in the cell (5)that influence reproduction, growth and immunity (6). More than 300 metalloenzymes require zinc as a catalyst, and -2500 transcription factors, or 8% of the human genome, require zinc for their structural integrity (7). Zinc is a type 2 nutrient that is part of general cellular metabolism (3). There is no specific tissue that acts as a zinc storage site during periods of dietary excess or from which it can be released when intakes are inadequate (3). Zinc homeostasis is thus achieved through a careful balance of absorption and excretion, and import and export at the cellular level (3). Zinc excretion appears to react more quickly to changes in intake while zinc absorption responds more slowly, but this synergistic mechanism can handle large fluctuations in zinc intake (5, 8). The body zinc pool must be sustained by a continual dietary supply that may be rapidly depleted when dietary zinc intakes are insufficient and prolonged (3).
Effects of zinc deficency
Severe clinical zinc deficiency is rare and occurs most frequently in individuals with the genetic disorder acrodermatitis enteropathica, where infants are born with impaired gastrointestinal zinc transport; in those receiving total parenteral nutrition without zinc, following alcohol abuse and with penicillamine therapy(6). Untreated severe zinc deficiency may be fatal. Symptoms of severe zinc deficiency include dermatitis, alopecia, diarrhoea, weight loss, intercurrent infections due to immune dysfunction and neurosensory disorders (6).
Mild-to-moderate zinc deficiency is more common, but may be challenging to identify as the clinical picture resembles that of a generalised poor micronutrient status. An inadequate dietary intake is likely the principal cause of zinc deficiency worldwide, and is especially common in lower-income countries. This can be due to several factors, including consumption of foods that have limited bioavailable zinc, a low total dietary zinc intake, and a high consumption of foods high in phytate that can inhibit zinc absorption (5, 6, 9). Mild-to-moderate zinc deficiency may have important health effects at a population level, including an increased susceptibility to diarrhoea and respiratory infections and an increased risk of stunting. The amount of absorbable zinc in the national food supply, and the prevalence of stunting among children under five can serve as proxy indicators of the risk of deficiency in a population. Stunted children are at a higher risk of morbidity from infection and mortality as children and non-communicable diseases in later life, and less completed schooling and a higher risk of living in poverty as adults (4). Prevention of stunting and its associated causes corresponds to the United Nations Sustainable Development Goal #2.
WHO guidelines for zinc supplementation in children and pregnant women
Zinc supplementation in the management of diarrhoea. Therapeutic zinc supplementation reduces the duration and severity of diarrhoea in children. As part of the effective management of acute diarrhoea in children, the WHO recommends that mothers, other caregivers and health workers provide children with 20 mg of zinc per day (10 mg per day for infants under the age of six months) for 10-14 days along with oral rehydration solution. https://www.who.int/elena/titles/zinc_diarrhoea/en/
Zinc supplementation in children with respiratory infection. Several studies have reported that zinc supplementation may reduce the frequency and severity of respiratory infections in children, however results have been inconsistent. https://www.who.int/elena/titles/zinc_pneumonia_children/en/
Zinc supplementation and growth in children. The available evidence is inconsistent but suggests that zinc supplementation may help to improve linear growth of children under 5 years of age. https://www.who.int/elena/titles/zinc_stunting/en/
Zinc supplementation during pregnancy. Due to the key role of zinc in cell replication, protein synthesis and growth, zinc is an essential nutrient during pregnancy. During this time, zinc requirements increase due to demands of both the mother and foetus, and an increased turnover. Zinc supplementation during pregnancy given to mothers in low-income countries may help to reduce pre-term births (10)but there is no evidence to promote its use to prevent pre-eclampsia or low-birth weight, and no specific recommendations at this time. https://www.who.int/elena/titles/zinc_pregnancy/en/
Risks from excessive zinc intekaes
Excessive zinc intake is rare in the general population.
Excessive intakes can occur in persons receiving zinc therapy for acrodermatitis enteropathica, decubitus ulcer healing, celiac disease, glucagonoma, hepatic encephalopathy, acne, and for reducing symptoms of the common cold (11). Consumption of excessive zinc for prolonged periods may interact with the absorption and/or metabolism of other trace elements in the body, particularly copper (11, 12). Copper may have a role in both humoral and cellular factors of the immune system (13), and a deficiency in copper may cause neurological effects including myelopathy, anaemia and neutropenia (11, 14, 15).