Abstract: We found no studies that compared multiple-micronutrient supplementation (with three or more micronutrients) versus supplementation with two or fewer micronutrients. Two small studies (involving a total of 52 women) were included. One study compared multiple micronutrients with placebo and the other study compared multiple micronutrients with a group who received no supplementation. The studies were carried out in Brazil (36 adolescent mothers) and the USA (16 women) and included women with a low socioeconomic status. A lack of information in the study reports meant that risk of bias could not be adequately assessed (unclear risk of bias for many domains). There were no quantitative data for any of this review's outcomes so meta-analysis was not possible. Neither of the studies reported on the primary outcomes of interest in this review: maternal morbidity (febrile illness, respiratory tract infection, diarrhoea), adverse effects of micronutrients within three days of receiving the supplement, infant mortality (defined as a child dying before completing the first year of age). One study reported qualitatively on maternal anaemia (a secondary outcome of this review) - the study found that multiple-micronutrient supplementation was effective for recuperating from anaemia but there were no data for inclusion in our analyses. Maternal satisfaction was not reported in the included studies. Similarly, none of this review's infant secondary outcomes were reported in the included studies: clinical micronutrient deficiency; morbidity episodes (febrile illness, respiratory tract infection, diarrhoea, other), adverse effects of micronutrients within three days of receiving the supplement. We found no evidence to quantitatively assess the effectiveness of multiple-micronutrient supplementation in improving health outcomes in mother and baby. The results of this review are limited by the small numbers of studies available, small sample sizes and the studies not reporting on the outcomes of interest in this review. There is no evidence to evaluate potential adverse effects of multiple-micronutrient supplements, particularly excess dosages. There is a need for high-quality studies to assess the effectiveness and safety of multiple-micronutrient supplementation for breastfeeding women for improving outcomes for the mother and her baby. Further research should focus on whether multiple-micronutrient supplementation during lactation compared with none, a placebo or supplementation with fewer than two micronutrients is beneficial to maternal and infant health outcomes. Future studies should collect data on outcomes beyond micronutrient concentrations, for example: maternal and infant morbidity, adverse effects, maternal satisfaction, the risks of excess supplementation, and potential adverse interactions between the micronutrients and the other outcomes. This would help to bridge the gap between research on intermediary outcomes and health outcomes in order to develop sound policy in this field. Future studies could more precisely assess a variety of multiple-micronutrient combinations and different dosages and look at how these affect maternal and infant health outcomes. Larger studies with longer follow-up would improve the quality of studies and provide stronger evidence. In most of the included studies, bias could not be adequately assessed due to lack of information, therefore attention should be given to adequate methods of randomisation and allocation concealment, adequate methods of blinding of the participants, providers and the outcome assessors to improve the methodological quality of studies in this field.

Plain-language summary: The benefits and risks of multiple-micronutrient supplementation during lactation are not clear from randomised controlled studies. Key vitamins and minerals, particularly iodine, iron and zinc, are required in small amounts to ensure normal body metabolism, physical growth and development. Nutrient deficiency affects nearly one third of the world’s population, especially in low- and middle-income countries. Breastfeeding mothers need higher levels than usual in order to provide sufficient vitamins and minerals for their own health and that of their babies, particularly for normal functioning and the growth and development of the baby. Previous studies have assessed supplementation of individual micronutrients. This review looked at the use of multiple-micronutrient supplements for breastfeeding women for improving outcomes for the mother and her baby. We searched for studies on 30 September 2015 and identified two small studies (involving 52 women) for inclusion in this review. The studies were carried out in Brazil and the USA and included women who had a low socioeconomic status. The studies were poorly reported and this lack of information made it difficult to determine whether the studies were at risk of bias. Neither of the studies provided data for any of this review's important outcomes: maternal illness (fever, respiratory infection, diarrhoea), adverse effects of micronutrients within three days of taking them, infant death (defined as a child dying before reaching one year of age). Similarly, there were no data for any of the other outcomes that we were interested in. For the mother, these outcomes were maternal anaemia, and women's satisfaction. For the baby, these outcomes were micronutrient deficiency; illness episodes (fever, respiratory infection, diarrhoea, other), adverse effects of micronutrients within three days of the woman receiving the supplement. However, one of the included studies reported that multiple-micronutrient supplementation was effective for lactating women recuperating from anaemia. There is a need for high-quality studies to assess the effectiveness and safety of multiple-micronutrient supplementation for breastfeeding women for improving outcomes for the mother and her baby. Larger studies with longer-term follow-up would improve the quality of studies and provide stronger evidence. Further research should focus on whether multiple-micronutrient supplementation during lactation (compared with no supplementation, a placebo or supplementation with fewer than two micronutrients) is beneficial to the mother and her baby and any associated adverse effects of the intervention. Futher studies should report on important outcomes such as those listed in this review and consider the risks of excess supplementation. Future studies could more precisely assess a variety of multiple-micronutrient combinations and different dosages and look at how these effect outcomes for the mother and her baby.