What does zinc do?BMJ 2002; 325 doi: https://doi.org/10.1136/bmj.325.7372.1062 (Published 09 November 2002) Cite this as: BMJ 2002;325:1062
Adequate zinc intake is critical for health. Zinc deficiency affects cells of the immune system. It causes a reduction in the number B lymphocytes and T lymphocytes (CD4 lymphocytes in particular) through increased apoptosis and also reduces their functional capacity. The functions of the macrophage, another key immunological cell that engulfs and destroys bacteria, are also compromised. The production and potency of several cytokines, the central messengers of the immune system, are also perturbed by zinc deficiency. Many of these changes occur even in the early stages of deficiency.
Zinc plays a part in the maintenance of epithelial and tissue integrity through promoting cell growth and suppressing apoptosis and through its underappreciated role as an antioxidant, protecting against free radical damage during inflammatory responses. Thus, in the case of diarrhoea, multiple functions of zinc may help to maintain the integrity of the gut mucosa to reduce or prevent fluid loss. Notably, these responses can occur within 48 hours, much more rapidly than the direct effects of zinc on cellular development.
The recommended daily allowance is only 10 mg elemental zinc, but many people in both developing and industrialised countries do not have this in their diet.1 Zinc deficiency is biochemically defined as a serum concentration of less than 9 μmol/l. However, serum zinc concentrations may not fully reflect the physiological zinc status in an individual, and individuals with apparently normal serum concentrations may benefit from daily zinc supplements.2
Benefits of supplementation
This is clearly illustrated in several randomised controlled trials of zinc supplementation. A meta-analysis indicated that daily zinc supplementation can reduce the incidence of pneumonia by 41% and diarrhoea by 18%.3 A meta-analysis of trials of adjunctive zinc supplementation in children with diarrhoea reduced the duration of the illness by 24%.4 A trial of daily zinc supplementation in otherwise healthy children from New Guinea reduced the number of cases of malaria seen at a health clinic by 38%.5
There is also evidence that zinc supplementation could offer benefit to pregnant women and their babies.6 One study showed that prenatal zinc supplementation can increase birth weight,7 and another indicated reduced incidence of diarrhoea and other morbidities in the infants.8 Babies who are small for gestational age also seem to benefit from taking daily zinc supplementation. A trial in India found that babies who received zinc from 1 month onwards were 60% less likely to die during infancy.9 Lastly, several studies indicate a potential role for zinc and supplements that contain zinc in improving immune status10 and health in elderly people.11 Zinc supplementation, therefore, seems be particularly critical during periods of immune development or degeneration: early childhood, pregnancy, and later life.
Problems caused by too much zinc
Taking too much daily zinc could also be a problem because, although it is not toxic, high doses can impair copper absorption. This can lead to copper deficiency with immunosuppression and other subtle and apparent adverse effects, especially for the mother and fetus during pregnancy.12 For this reason, doses more than twice the recommended daily allowance are not recommended and prenatal zinc supplements should contain copper,13 especially in populations with low mineral intakes.