Iron Absorption

Reference from the joint report of FAO/WHO expert consultation on Human Vitamins and Minerals verbatim.

20. Hallberg, L. 1981. Bio-availability of dietary iron in man. Ann. Rev. Nutr., 1: 123-147.

21. Hallberg, L. 1979. Dietary heme iron absorption. A discussion of possible mechanisms for the absorption-promoting effect of meat and for the regulation of iron absorption. Scand. J. Gastroenterol., 14: 769-779.

22. Hallberg, L., Hulthén, L. & Gramatkovski, E. 1997. Iron absorption from the whole diet in men: how effective is the regulation of iron absorption? Am. J. Clin. Nutr., 66: 347-56.

23. Hallberg, L. 1993. Inhibition of haem-iron absorption in man by calcium. Br. J. Nutr.,

69:533-540.

With respect to the mechanism of absorption, there are two kinds of dietary iron: heme iron and non-heme iron (20). In the human diet the primary sources of heme iron are the haemoglobin and myoglobin from consumption of meat, poultry, and fish whereas non-heme iron is obtained from cereals, pulses, legumes, fruits, and vegetables. The average absorption of heme iron from meat-containing meals is about 25 percent (21) The absorption of heme iron can vary from about 40 percent during iron deficiency to about 10 percent during iron repletion (22). Heme iron can be degraded and converted to non-heme iron if foods are cooked at a high temperature for too long. Calcium is the only dietary factor that negatively influences the absorption of heme iron and does so to the same extent that it influences non-heme iron (Table 41) (23).

24. Hallberg, L. 1974. The pool concept in food iron absorption and some of its implications. Proc. Nutr. Soc., 33: 285-291.

25. Hallberg, L. 1985. Factors influencing the efficacy of iron fortification and the selection of fortification vehicles. In: Clydesdale FM, Wiemer KL, eds. Iron fortification of foods. p. 17-28. New York: Academic Press Inc. p. 17-28.

Non-heme iron is the main form of dietary iron. The absorption of non-heme iron is influenced by individual iron status and by several factors in the diet. Dietary factors influencing iron absorption are outlined in Table 41. Iron compounds used for the fortification of foods will only be partially available for absorption. Once iron is dissolved, its absorption from fortificants and food contaminants is influenced by the same factors as the iron native to the food substance (24, 25).

26. Hallberg, L. & Björn-Rasmussen, E. 1981. Measurement of iron absorption from meals contaminated with iron. Am. J. Clin. Nutr., 34: 2808-2815.

27. Hallberg, L. 1983. Iron absorption from some Asian meals containing contamination iron. Am. J. Clin. Nutr., 1983, 37:272-277.

Iron originating from the soil (e.g., from various forms of clay) is sometimes present in considerable amounts on the surface of foods as a contaminant originating from dust on air-dried foods or from water used in irrigation. Even if the fraction of iron that is available is often small, contamination iron may still be nutritionally important because of the great amounts present (26, 27).

28. Wollenberg, P. & Rummel, W. 1987. Dependence of intestinal iron absorption on the valency state of iron. Naunyn-Schmiedeberg´s Arch. Pharmacol., 36: 578 - 582.

Reducing substances (i.e., substances that keep iron in the ferrous form) must be present for iron to be absorbed (28). The presence of meat, poultry, and fish in the diet enhance iron absorption. Other foods contain factors (ligands) that strongly bind ferrous ions, that subsequently inhibit absorption. Examples are phytates and certain iron-binding polyphenols.