Dietary zinc supplementation reduces the incidence of childhood pneumonia by 41%and the prevalence of diarrhoea by up to 25%according to a systematic review of 10 randomised controlled trials all performed in the developing world (Journal of Pediatrics 1999;135:689-97).
This compares favourably with other preventive interventions for diarrhoea, such as sanitation and breast feeding, and is more effective than any other intervention to prevent pneumonia.
Zinc deficiency is common in young children in the developing world and is associated with reduced immunocompetence and increased rates of serious infectious diseases. Several trials in poor countries have shown the benefit of zinc supplementation in reducing infection (BMJ 1998;317:369), but these have varied in the magnitude of the effect and the presence of a differential effect by age and sex. Some trials were underpowered to detect the effects on infrequent outcomes, and others remain unpublished.
A pooled analysis was conducted by the Child Health Research Project, a group of researchers from Johns Hopkins School of Public Health and the World Health Organisation, who had access to the original trial data. Trials were included if they provided oral supplements containing at least half the US recommended daily allowance of zinc for children, and if morbidity surveillance was carried out for at least four weeks. Two sets of trials were identified—those in which zinc was given continuously, and those giving only a short course.
For the zinc supplemented children in the seven continuous trials, the pooled odds ratios for diarrhoeal incidence and prevalence were 0.82 (95%CI 0.72 to 0.93) and 0.75 (0.63 to 0.88) respectively. Supplemented children had an odds ratio of 0.59 (0.41 to 0.83) for incidence of pneumonia.
No significant variations in the effects were seen in the subgroups of children stratified by age, sex, and weight, and nor was there a significant difference between short course and long term supplementation.
The authors conclude that “the development of effective and feasible interventions to improve the zinc status of developing country populations is essential.” One such intervention, zinc fortification of bread, was shown in a randomised controlled trial to reduce diarrhoea, respiratory illnesses, and skin infections in Turkish schoolchildren (Cereal Chemistry 1995;73:424-6).
Dr Robert Black, of Johns Hopkins School of Public Health and co-author of the study, said: “Zinc fortification is potentially a powerful tool for settings which produce commercial food, and the idea has been acceptable to food manufacturers. If there's no commercial food, increasing zinc intake is possible by reducing the amount of dietary phytates, which interfere with zinc absorption. This can be done by soaking or fermenting food. Long term, it is possible that plant breeding could be used to increase zinc or reduce phytate content.”
But several questions still remain before zinc therapy can be incorporated into diarrhoeal disease control programmes, including the optimal dosing regime and duration of therapy. Dr Shammim Qazi, from the Division of Child Health and Development of the World Health Organisation, said: “At present the WHO is not recommending zinc supplementation as routine. We are waiting for the results of larger trials, and we are planning a trial ourselves.”
The Child Health Research Project's Special Report, Zinc for Child Health, is at http://ih.jhsph.edu/chr/publicat.htm