Total mortality of children receiving zinc supplementation was not significantly different from that of children in the placebo group. There was suggestive evidence of an interaction with age: there was no difference in mortality in children under 12 months of age, but in older children there was a non-significant difference of lower mortality in the zinc group than in the control group. The serum zinc concentrations in the placebo group showed high levels of zinc deficiency, which is consistent with previous data and suggests that this population was appropriate for testing of this intervention. The zinc provided in the supplements was bioavailable, as shown by the serum zinc concentrations after 12 months of supplementation, although the effect on serum zinc was not as large as has been shown in other studies of zinc supplementation.7
Selection bias is unlikely to explain our results because participation rates exceeded 96% at enrolment for both treatment groups. Similar proportions (about 8%) of children in the zinc and placebo groups moved out of the study area or were lost to follow-up after enrolment. All these children were alive when they were censored. Any differences in the mortality rates of these children by treatment group could have biased our estimates of the treatment effects. Although these children might have had a different overall risk of mortality from those who were followed-up, we think that this selection is unlikely to have differed between the treatment groups.
Compliance, whether measured as the proportion of enrolled children who continued to participate until discharge or as the proportion of days for which the supplement was taken, was lower in the zinc group, suggesting that some children could detect an unacceptable taste in the active supplements. Similar to the issue of migration, our estimates of treatment effects would be biased only if mortality risk differed within the treatment groups between children who refused and those who continued to participate. This bias cannot be ruled out, but it is likely to be small, if present at all, because most children who discontinued did so soon after enrolment. This difference in compliance might explain the slightly lower change in serum zinc status in this study as compared with previous trials of shorter duration. Imbalance in risk factors for child mortality is an unlikely explanation for our results, because the treatment groups had similar baseline characteristics.
A companion trial in a malaria-endemic area of Zanzibar22
reported no effect of zinc supplementation on overall mortality, but a beneficial effect in children of 12 months or older. This is similar to our findings, even though the characteristics of malaria in the two study populations differ (there is intense Plasmodium falciparum
transmission in Zanzibar, by contrast with infrequent Plasmodium vivax
transmission in Nepal). Although the results of these two large, community-based trials are consistent with each other, they reported a much smaller effect of zinc supplementation on mortality than did trials from Bangladesh.12,13
In view of the large sample sizes used in Nepal and Zanzibar, the true effect of zinc supplementation on mortality is probably closer to that reported in these two studies than that in the studies from Bangladesh. The non-significant 44% reduction in mortality () in the small group of children in NNIPS-4 who were known to have been born weighing less than 2000 g, together with a 68% reduction in mortality in infants who were small for gestational age in a trial in India,10
suggests that this high-risk group might benefit from zinc supplementation.
In a meta-analysis of two reports from south Asia (our study and one from Bangladesh13
) and two reports from sub-Saharan Africa (those from Zanzibar22
and Burkina Faso23
), the pooled estimated relative risk of mortality across all ages was 0·91 (95% CI 0·82−1·02) (). There was no evidence that this relative risk varied between the Asian and African studies. The pooled estimate for children younger than 12 months showed no effect of zinc supplementation on mortality (1·04, 0·90−1·21) and this was consistent by region. By contrast, zinc supplementation reduced mortality in children 12 months or older in Nepal and Zanzibar (0·82, 0·70−0·96) ().
Meta-analysis of the effect of zinc supplementation on mortality in children aged up to 48 months
The absence of a preventive effect on the number of episodes and duration of diarrhoea, dysentery, and acute respiratory infections in our study contrasts with the meta-analysis published by the Zinc Investigators Collaborative Group8
in 1999. However, most of the studies cited in that review were of selected subsets of children at high risk, including children with, or just recovering from, an episode of acute or persistent diarrhoea,24–29
and underweight or stunted children.30,31
Of the three studies that included unselected children, the largest, from Papua New Guinea,32
showed no difference in days with diarrhoea, fever, or cough between 138 children assigned to placebo and 136 assigned to zinc supplementation. In Guatemala, there were 22% fewer episodes of diarrhoea in 45 children given zinc (8·1 episodes per 100 person-days) than in 44 children given placebo (6·3 episodes per 100 person-days); the definition of diarrhoea was left to the mother, which might explain the unusually high rates.33
In the Mexican study,34
there was a lower diarrhoea attack rate in 109 children given zinc than in 108 given no zinc (0·8 vs
1·3 episodes per person-year). Since the meta-analysis in 1999, ten other studies have reported on the effect of zinc supplementation on morbidity in unselected populations. Of those trials, five showed no effect of zinc supplementation on the incidence of diarrhoea.35–39
Four studies showed reductions in diarrhoea incidence that ranged from 6% to 23% in the zinc versus placebo comparisons,13,40–42
and in one study children in the zinc group had on average one fewer day with diarrhoea than did children in the placebo group.23
Eight of these trials report data on acute lower respiratory illness, five of which showed no difference in rates between zinc and placebo groups.35,36,38,39,42
The three studies that showed differences between the zinc and placebo groups had relative risks that range from 0·72 to 1·84 for severe definitions of acute lower respiratory illness.13,40,43
Our non-specific definition might explain the absence of an effect in our results. However, the absence of an effect on either diarrhoea or acute lower respiratory illness is consistent with the heterogeneity of results from other studies, and suggests that the effects of zinc supplementation on attack rates of diarrhoea and respiratory disease vary depending on how children are selected for study. Children with specific histories of morbidity or malnutrition might benefit from zinc supplementation.
The effect of zinc supplementation on serum zinc status in this trial was lower than expected on the basis of data from shorter-term supplementation trials. There are a few potential explanations for this finding. First, the dose used in this study might have been lower than necessary to improve zinc status, which might explain the smaller effect on morbidity and mortality than in studies that used higher doses. However, morbidity results vary substantially in previous trials, even those that used higher doses than did this trial. The recommended daily allowance (RDA), as calculated by the US Institute of Medicine, for zinc in children aged 1−35 months is 2·5−3·0 mg.44
Our dose was over three times the RDA for children aged 12 months or older and about 1·5 times the RDA for those younger than 12 months. The recommended nutrient intake (RNI) from the Food and Agriculture Organization of the UN and WHO for zinc in this age-group is 1·1−8·4 mg daily, with the exact RNI depending on age and dietary bioavailability.45
The classification of bioavailability of a dietary supplement is unclear, but the doses used in this trial are 1·2−4·0 times the RNI. Second, the diet in rural southern Nepal is high in phytate, which might have restricted the absorption of the zinc supplement. The diet in our study area is similar to that in many other areas on the Indian subcontinent, so any difficulty with bioavailability would probably not be unique to our study population. However, the combination of a low dose with a diet high in phytate might have limited the absorption of zinc below that necessary for optimum effect on morbidity and mortality. Third, serum zinc concentrations must be interpreted with caution because they are reduced by infection and inflammation, which were common in our study population.
The results from this study show that zinc supplementation has no effect on the incidence of diarrhoea or respiratory infections in children of 1−3 years of age. There was a slight, non-significant reduction in mortality with zinc supplementation beyond that achieved with vitamin A supplementation. The meta-analysis supports this finding, and shows a protective effect in children of 12 months or older when estimates are pooled across studies. However, whether improvements in population zinc status via universal routine supplementation are feasible remains to be shown. Further research on the optimum dose for reduction in morbid outcomes in various settings is needed before large-scale implementation of universal supplementation pro grammes is justified.