A total of 201 studies were included in this review: 121 studies were of infants and children, 79 were of women and one study had both women and children in the study population; 125 trials were RCTs and the rest were quasi-experimental and before-after studies. For the RCTs, randomisation and allocation concealment were adequate in 41 studies, attrition rates were reported in 110 studies and blinding was adequately reported in 62 studies. The quality of evidence ranged from very low to moderate on GRADE.
Children: The analysis showed that iron fortification significantly increased haemoglobin concentration and serum ferritin levels and reduced anaemia. The analysis showed a significant impact of zinc fortification on increasing serum zinc concentration. Non-significant impacts were observed for growth, weight gain, serum alkaline phosphatase, haemoglobin levels and serum copper levels.
Pooled results showed that vitamin D fortification significantly increased serum concentration of 25-hydroxy-vitamin D3 and reduced serum parathyroid hormone concentration; vitamin A fortification had significant impacts on serum retinol concentration and haemoglobin levels. Iodine fortification had a significant effect on median urinary iodine concentrations.
The pooled analysis showed a significant impact of dual fortification (iron and iodine) on haemoglobin concentration and on reducing the prevalence of anaemia. The analysis showed a significant effect of multiple micronutrients fortification on haemoglobin levels, serum ferritin concentration and on reducing anaemia prevalence. Multiple micronutrients fortification had a non-significant impact on height for age, weight for age and weight for height.
Women: Pooled results showed that iron fortification significantly improved haemoglobin concentration, serum ferritin levels and prevalence of anaemia. Folate fortification had a significant impact in reducing neural tube defects, spina bifida and anencephaly. Iodine fortification had a significant impact on median urinary iodine concentrations and on the incidence of hypothyroidism.
The analysis showed that vitamin D and calcium fortification had a significant impact on vitamin D3 levels (increased) and serum parathyroid hormone levels in postmenopausal women but not in women of reproductive age. The pooled analysis showed that multiple micronutrients fortification significantly improved haemoglobin levels, serum ferritin, serum zinc and serum retinol.
There was a lack of evidence for the impact of fortification strategies on morbidity and mortality outcomes in women and children. Most analyses were found to have high heterogeneity.
Further subgroup analyses were reported in the review.