There is very little information on the effects of the age at introduction of complementary foods on iron or zinc status of breast-fed infants. These two nutrients have been identified as the most likely limiting nutrients among EBF infants during the first 6 months of life . Although the adequacy of certain vitamins during the period of exclusive breastfeeding may also be of concern, depending on maternal diet and nutritional status, these concerns can generally be addressed by assuring that the mother's intake is adequate (e.g. for vitamin A, B6, or B12). By contrast, the concentrations of iron and zinc in human milk are not altered by maternal supplementation.
During the first 6 months, infant iron status is largely dependent on iron stores at birth, which are influenced by gestational age, birth weight, maternal prenatal iron status, and the timing of clamping of the umbilical cord. After birth, infant iron needs are influenced by the rate of growth and certain types of infections. Thus, although full-term, normal birth weight infants whose mothers had adequate prenatal iron status can generally maintain adequate iron status through >6 months of exclusive breastfeeding, certain subgroups of infants may be at risk of iron deficiency prior to 6 months.
In the first controlled trial in Honduras, very few of the EBF infants with a birth weight of >3 kg had low hemoglobin (5% < 103 g/l) or plasma ferritin (0% < 12 ^g/l) at 6 months of age, but in those with a birth weight of <3 kg, the EBF infants were at higher risk of iron deficiency than the infants who received complementary foods (~49 vs. 27% for low hemoglobin; ~26 vs. 10% for low ferritin) . This is not surprising given that the complementary foods were fortified with ferrous sulfate. However, the provision of free iron-fortified complementary foods during the age interval of 4-6 months did not eliminate iron deficiency at 6 months. Evidence from a separate study of iron supplementation in the same population  indicates that iron supplements given to high-risk infants are likely to be more efficacious for preventing iron deficiency than feeding iron-fortified complementary foods prior to the age of 6 months. In the second Honduras trial, with term, low birth weight infants, there was a significant interaction effect between the provision of complementary foods and iron supplementation. Among infants not given medicinal iron drops, iron status was higher in the group given iron-fortified complementary foods than in the EBF group. However, in those given medicinal iron drops, iron status was higher in the EBF group, suggesting that complementary foods interfered with iron utilization . Given the recommendation that low birth weight infants should receive iron supplements beginning in early infancy, these results suggest that exclusive breastfeeding for 6 months, together with iron supplementation, is likely to optimize iron status for such infants.
There is a paucity of data on the effects of age at introduction of complementary foods on the iron status of breast-fed infants in other countries. In a small observational study of breast-fed infants in Italy , the EBF group had significantly higher hemoglobin concentration than the MBF group (117 vs. 109g/l) at 12 months of age. Although iron-fortified cereals are usually one of the first complementary foods given to infants in the US, the bioavailability of the electrolytic iron typically used in such foods is estimated to be quite low . Among the breast-fed cohort (n = 173) in an intervention trial in Chile in which infants were randomly assigned to receive iron-fortified (55 mg electrolytic iron/100 g dry cereal) or unfortified rice cereal at 4 months , there was no significant difference in iron status (e.g. serum ferritin) between groups at 8 months of age, but by 12 months the group receiving unfortified cereal had a lower iron status and was more likely to be anemic (10.8 vs. 1.4%). This suggests that the iron in the cereal was absorbed, but that the breast-fed infants did not benefit from the extra iron until after 8 months of age.
As is the case for iron, the zinc concentration of human milk is relatively low, and it is thought that low stores of zinc at birth may predispose certain subgroups of infants to zinc deficiency . In disadvantaged populations, zinc supplementation during infancy has generally had positive effects on growth and morbidity . However, in the Honduras study of low birth weight infants, there was no impact of complementary foods on plasma zinc, even though the mean zinc intake of infants in the complementary foods group was twice that of the EBF group . In a randomized trial in the US in which infants at 5 months of age were given either beef or iron-fortified rice cereal as the first complementary food , no effect on growth, development or biochemical indices of iron or zinc status at 9 months was observed despite significant differences in zinc intake at 5-7 months. Thus, there is no evidence that complementary feeding prior to 6 months would enhance zinc status. High-risk infants, however, may benefit from zinc supplementation.
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For many years, scientists have been playing out the ingredients that make breast milk the perfect food for babies. They've discovered to day over 200 close compounds to fight infection, help the immune system mature, aid in digestion, and support brain growth - nature made properties that science simply cannot copy. The important long term benefits of breast feeding include reduced risk of asthma, allergies, obesity, and some forms of childhood cancer. The more that scientists continue to learn, the better breast milk looks.