Abstract: We included eight trials (709 participants); seven were from middle-income countries of Asia, Africa, Europe, and Latin America where zinc deficiency is likely to be a public health problem. Four trials compared the effect of zinc-fortified staple foods with unfortified foods (comparison 1), and four compared zinc-fortified staple foods in combination with other nutrients/factors with the same foods containing other nutrients or factors without zinc (comparison 2). The interventions lasted between one and nine months. We categorised most trials as having unclear or high risk of bias for randomisation, but low risk of bias for blinding and attrition. None of the studies in comparison 1 reported data on zinc deficiency. Foods fortified with zinc increased the serum or plasma zinc levels in comparison to foods without added zinc (mean difference (MD) 2.12 µmol/L, 95% confidence interval (CI) 1.25 to 3.00 µmol/L; 3 studies; 158 participants; low-quality evidence). Participants consuming foods fortified with zinc versus participants consuming the same food without zinc had similar risk of underweight (average risk ratio 3.10, 95% CI 0.52 to 18.38; 2 studies; 397 participants; low-quality evidence) and stunting (risk ratio (RR) 0.88, 95% CI 0.36 to 2.13; 2 studies; 397 participants; low-quality evidence). A single trial of addition of zinc to iron in wheat flour did not find a reduction in proportion of zinc deficiency (RR 0.17, 95% CI 0.01 to 3.94; very low-quality evidence). We did not find a difference in serum or plasma zinc levels in participants consuming foods fortified with zinc plus other micronutrients when compared with participants consuming the same foods with micronutrients but no added zinc (MD 0.03 µmol/L, 95% CI -0.67 to 0.72 µmol/L; 4 studies; 250 participants; low-quality evidence). No trial in comparison 2 provided information about underweight or stunting. There was no reported adverse effect of fortification of foods with zinc on indicators of iron or copper status. Fortification of foods with zinc may improve the serum zinc status of populations if zinc is the only micronutrient used for fortification. If zinc is added to food in combination with other micronutrients, it may make little or no difference to the serum zinc status. Effects of fortification of foods with zinc on other outcomes including zinc deficiency, children’s growth, cognition, work capacity of adults, or on haematological indicators are unknown. Given the small number of trials and participants in each trial, further investigation of these outcomes is required.

Plain-language summary: We included eight trials (709 participants); seven were from middle-income countries of Asia, Africa, Europe, and Latin America where zinc deficiency is likely to be a public health problem. The effect of fortification of foods with zinc on incidence of zinc deficiency is uncertain. Fortification of foods with zinc may slightly improve the blood zinc levels of populations if zinc is the only micronutrient used for fortification. Zinc added to food in combination with other micronutrients may make little or no difference to blood zinc levels. The fortification of foods with zinc may make little or no difference on incidence of underweight or stunting. The effects of fortification of foods with zinc on other outcomes, including wasting and weight/height, are unknown. Fortification of foods with zinc does not seem to have any adverse effect on indicators of iron or copper status. Most studies included in this review involved a small number of participants and were judged to be at low or unclear risk of bias. There were also some inconsistencies in the results across different studies.