Milk, human iron absorption

Even if we had accurate and comprehensive information on the trace element content of human milk, we are still faced with the enormous problem of probably large, but poorly quantitated differences in bioavailability for at least some specific trace elements between human milk and the various formulas used in infant feeding. This problem is of great importance in the young infant because of his dependence on one major dietary staple. For example, the iron status of breast fed infants is known to be relatively favorable during the first six months despite a very low iron intake from human milk and iron absorption has been shown to be especially high when given with human milk [13]. The same appears to be true for zinc [14]. 

Another small (n=10) human study in the late 1970s reported on the effect of rooibos on iron absorption 32). No detrimental effect on iron absorption was shown after the subjects had consumed traditionahfermented rooibos (200 mL containing milk and sugar) when compared with the control group consuming water. Subsequently, a more recent study pubhshed in 2005 confirmed these results that the intake of 200 mL of traditional rooibos (with milk and sugar) per day for 16 weeks by school children did not have any adverse effects on their iron status (75). 

A common baby food, strained pears, inhibited iron absorption in the presence of human milk according to Oski and Landau (55). Although the authors did not attribute the action to dietary fiber, it is a possible suspect. 

Lactoferrin s role in iron absorption was demonstrated by the addition of lactoferrin, presumably iron-saturated, to a lactoferrin free simulated human milk. Administered to adults it did not increase iron absorption (13). The addition of apolactoferrin was shown to actually inhibit iron uptake by rat and guinea-pig in everted duodenal sacs, while the addition of Fe 2- lactoferrin had no effect on uptake of iron (14). An additional study demonstrated a negative relationship between duodenal lactoferrin concentration and iron absorption in adults (15). These results would suggest that lactoferrin has no role or even a negative role in iron absorption. Further, additional studies brings into question the role, if any at all, that lactoferrin has in iron absorption. McMillan et al. (13) have shown that the iron in human milk, despite the high lactoferrin content, is more readily absorbed than iron from a simulated human milk of comparable iron content. The... 

The analysis of human milk for the distribution of iron into the various components found iron in three fractions of lipid, low molecular weight form and lactoferrin (16). The total concentration of milk iron varied from 0.26 to 0.73 mg/ml with 15 to 46% of the iron bound to the lipid fraction, and 18 to 50% found in a low molecular weight fraction. Surprisingly, only a small amount of iron was bound to the lactoferrin, which was saturated at 1-4%. These results even further complicate the role of lactoferrin in iron absorption by infants. Further experimental work needs to be done to define the role of lactoferrin in iron absorption, if any at all. 

With the exact role of lactoferrin uncertain and the mechanism of iron absorption also unknown, the concentration of iron and other trace elements in human milk is a controversal item. The data involving iron levels in breast milk date from the early fifties to the present time. During the stage of lactation, colostrum, early and mature milk samples are known to decrease in iron concentration with time, see Table II (17,18). The comparison of mature breast milk, 7 days or older, finds a range in iron concentration from 0.21 to 1.28 mg/liter. Weekly, daily and diurinal variation within a given day were demonstrated for iron, copper and zinc in human milk samples (20). Great variations were found within a given day on total yield, fat, and mineral levels (17,20). 

Iron deficiency is approximately twice as common in breastfed infants up to 30 percent have iron deficiency anemia, and more than 60 percent of the anemic infants are also iron deficient at 12 months of age (Pisacane et al., 1995), although the etiology is unclear. The iron content of human milk is low 0.5 mg/L compared with 10 to 12 mg/L in supplemented cow-milk formulas. The absorption rate, however, is considerably higher. Breastfed infants absorb up to 50 percent of consumed iron, compared with a 7- to 12-percent absorption rate for formula-fed infants (Fomon et al., 1993). The risk of iron deficiency increases after 4 months of age since most full-term infants are born with adequate iron stores to support hemoglobin synthesis through the first 4 months after birth. 

Animal origin products such as eggs, milk, fish, red meats and poultry contain low amounts of manganese. Absorption of such minerals as iron, copper, phosphorus and calcium is superior from animals products than from plant-origin foods. As reported by Kies et al. (in this book), manganese apparently is better absorbed by humans from meals containing meat and fish than from those containing plant-protein replacement products. Because of the low content... 

Recent studies have demonstrated that iron is better absorbed from human milk than from either cow s milk or formula. Furthermore, that human milk can provide sufficient iron for infants during their first year of life (10,13,17.28). Breast milk and cow s milk are equally poor in iron, containing an average of almost 1 mg/liter. Saarinen s (10) study demonstrated that infants breast fed throughout the first six to seven months of life attained greater iron stores than infants fed a cow s milk formula. The percent absorption of breast milk, cow s milk and formula is given in Table III. 

Copper deficiency occurs very rarely, but it is associated with mental aberrations. The normal diet contains one-third as much copper as iron, yet the metabolic demands are only one-fiftieth as great. Dietary copper deficiency develops in infants with diarrhea who are fed only milk. While lambs which are copper-deficient exhibit defective myelination, cerebral degeneration, and behavioral abnormalities, no such neurologic syndrome has been observed in humans. However, patients with Menkes Disease, caused by a defect in copper absorption and metabolism, are hypotonic, lethargic, and mentally retarded (Danks, 1983). 

Lactoferrin is a major component of human but not bovine milk (Bezkorovainy, 1977). Its main function in human milk appears to be as a bacterial growth inhibitor, since its iron saturation level is only 9-10%, and human milk therefore possesses a high UIBC status. In addition, lactoferrin may act as a vehicle for the absorption of iron by the human neonate (Cox et al., 1979). Human milk, because of its high lactoferrin content, exerts a much greater bacteriostatic effect than does bovine milk (Kochan et al., 1977). For this and other reasons, breast-fed human infants have a much lower incidence of gastrointestinal disorders than do bottle-fed infants (Goldman, 1973). 

The issue of bioavailability from food sources and the interactions between food groups and copper availability remains a critical question. Lonnerdal et al. demonstrated that heat treatment of cows milk formula decreases the copper bioavailability. Transitional complexes form in the milk upon heating that have a similar configuration to copper and thereby directly inhibit copper absorption. High doses of zinc also reduce copper bioavailability, as does combined iron and zinc supplementation. The dilemma is how to prepare an infant formula containing adequate copper, iron, and zinc that will meet the RDA for copper. Other nutrients dramatically affect copper absorption from foods. Soy protein-based diets promote less copper retention in tissues than lactalbumin-based diets. However, it is unclear if this effect is solely due to the soy protein composition or to the higher zinc in these soy-based formulas. In animals, phytate causes a drop in serum copper but human stable isotope studies reveal no... 

Reports of human copper deficiency are limited and suggest that severe nutrient deficiency coupled with malabsorption is required for this disease state to occur. Infants fed an exclusive cows milk diet are at risk for copper deficiency. Cows milk not only has substantially less copper than human milk but the bioavailability is also reduced. High oral intake of iron or zinc decrease copper absorption and may predispose an individual to copper deficiency. Other infants at risk include those with (1) prematurity secondary to a lack of hepatic copper stores (2) prolonged diarrhea and (3) intestinal malabsorption syndromes. Even the premature liver is capable of impressive copper storage. By 26 weeks gestational age the liver already has 3 mg of copper stored. By 40 weeks gestational age, the hepatic liver has 10-12 mg copper stored with the majority being deposited in the third trimester. Iron and zinc... 

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