Zinc deficiency results in growth retardation, retarded sexual maturity, immune system dysfunction, skin problems, and can also exacerbate iron deficient anaemia. This is of concern because studies have shown that many individuals in the Western nations are borderline deficient in zinc. A number of factors are able to influence zinc absorption, including phytic acid, bran, hemicellulose, lignins, calcium, proteins and metal complexing agents. Because they can tip a borderline case into a full deficient state, it is important to understand the factors that can inhibit the absorption of zinc. Casein in cow’s milk has also been identified as a possible zinc binding agent, and this may explain the poor bioavailability of zinc from cow’s milk formula infant food, when compared to human milk. In fact rats fed cow’s milk can have their zinc absorption rates increased through a switch to human milk. It has been suggested from these results that zinc may bind to proteins in casein and this may then prevent the absorption of the metal ions.
Further, acrodermatitis enteropathica, an inherited human disorder of zinc absorption responds well to human milk therapy, suggesting that zinc does not bind in the same way to human milk proteins. The binding of zinc to proteins in cow’s milk has been investigated in research, with one study using an in vitro model of the gut to assess the binding of zinc to proteins in cow’s milk under simulated physiological conditions1. The results showed that at the slightly alkaline pH that might be found in the duodenum, 1 mg of casein binds 8.4 μg of zinc. However, at pH 1 no zinc bound to the casein. The zinc binding was reversible, and at pH 4.6 the zinc became unbound through casein precipitation. However, other means of causing casein precipitation did not free the zinc from its binding with casin. The binding capacity of cow’s milk (8.4 μg per mg) exceeds the amount of zinc in cow’s milk (2 to 5 μg) and with 30 mg/mL protein (80 % of which is casein) a binding capacity of upto 200 μg per mL casein has been suggested.
Therefore the binding and unbinding of zinc to casein appears to be a pH dependent phenomenon. The authors suggested that the zinc binds to the phosphopeptides created as a result of trypsin and chymotrypsin digestion of the casein. This might occur through the binding of the divalent zinc ions to the negatively charged phosphate groups, which explains the poor binding of zinc to casein in acidic condition. The bonding of zinc to these phosphopeptides was with high affinity, however, most of the zinc could be released from binding by addition of metal complexing agents such as citrate or picolinate. These metal binding agents may compete for binding to the zinc with casein, and their higher affinity for zinc may dislodge the zinc, removing it from the protein. These zinc citrate and zinc picolinate complexes may then be absorbed. However, the zinc remaining bound to the casein would not be able to undergo this process. The zinc in cow’s milk may therefore bind with high affinity to peptides within the milk.
Dr Robert Barrington’s Nutritional Recommendation: A number of factors are known to inhibit the absorption of zinc. The casein from cow’s milk appears at least under simulated gut condition to be able to bind zinc, and this may explain data from other studies showing a poor absorption of zinc from cow’s milk. As calcium can also inhibit the absorption of zinc, consuming zinc supplements away from foods containing cow’s milk is therefore recommended.
RdB
