Dietary supplements for the lactating mother: influence on the trace element content of milk

Milk production is a complex process where nutritional factors interact with structural hormonal and behavioural influences. In recent years important advances have been made in understanding the role of the nutritional status of lactating women on the outcome of breastfeeding. Many questions remain unanswered about the exact requirement of trace elements for lactating mothers. The effect of dietary zinc, copper and iodine supplements on the milk concentration of these micronutrients was studied. The supplementation trial employed a specific balanced nutritional supplement prepared for the nursing mothers. The study was carried out on women living in Ferrara and its surrounding area. The population under study was healthy Italian mothers, of good socioeconomic status, and their normal infants. In total, 32 women were enrolled in the study and 22 completed it. The infants (9F, 13M) were full-term, healthy singletons and were put to breast within 12 h of birth. All women who finished the study completed a 3 d dietary record. Nutrient analysis revealed the following mean daily dietary trace element intake in the lactating mothers: zinc = 12 mg, copper = 1.4 mg and iodine = 145 microg. The zinc and copper dietary intake was in agreement with the daily intake proposed for nursing Italian mothers, while the daily intake of iodine was below the recommended intake of 200 microg. The breastfeeding mothers were placed in 2 groups, with 7 primiparas and 4 multiparas per group: lactating women eating a traditional Italian diet without vitamin and mineral supplements, and lactating women enrolled in the nutrification programme and given a nutritional supplement to their traditional diet. The supplement (PerMamma Abbott) provided 20mg zinc sulfate, 2mg copper sulfate and 116 microg potassium iodide. These quantities cover about 60-90% of the recommended intake for nursing Italian mothers. Samples of 10 ml of milk were collected at 3, 30, 90d postpartum. Zinc milk concentrations declined significantly over the study period for all lactating subjects, without differences in the rate of decline between the women who started supplementation during lactation and those who did not. Copper did not change during the first month of lactation, then declined at day 90 in supplemented and unsupplemented women, without significant differences between the two groups. An early sharp decline in milk iodine occurred in all lactating subjects, independently of iodine supplementation. After the first month of lactation breast milk iodide levels remained stable in all subjects under study. No significant differences between the two study groups were observed. The lack of correlation between the iodide level in breast milk and maternal dietary intake of iodine is not in agreement with previously published reports. The present results indicate that in healthy, well-nourished lactating Italian women, whose diet is adequate, the levels of zinc, copper and iodine in milk are not influenced by short-term supplementary intakes and that the milk levels of the trace elements studied are maintained over different levels of intake. Further research and examination by longitudinal studies are needed to establish the exact relationship between the amount of iodine furnished to the nursing mother and the iodine content of human milk. The role of compensatory homeostatic mechanisms which act during lactation needs further consideration and closer scrutiny.