Dietary iron, bioavailability

Adult males have been estimated to lose 1.0 mg of iron per day. The United States Recommended Daily Allowance for iron is 8.0 mg. What is a reasonable estimate of the bioavailability of dietary iron ... 

Dietary iron level does not seem to affect the efficiency with which dietary iron is converted into hemoglobin when ferrous sulfate (Table 1) or when ferric orthophosphate (Table 2) is the primary source of dietary iron. This is also true for white bread (Table 2) however, the source of the iron in the enriched flour used in the bread is unknown. That the efficiency of converting food iron into hemoglobin is not affected by dietary iron concentration is important to bioavailability testing because it is often difficult to formulate diets with precise amounts of iron, especially when foods are the sources of iron. 

Amine and Hegsted (1971) obtained similar carbohydrate effects studying iron absorption. Glucose is the most commonly used source of dietary carbohydrate in semipurified diets. Pennell et al. (1976) reported that beta-lactose in place of sucrose reduced the relative biological value of iron as sodium iron pyrophosphate when fed to rats. However, alpha-lactose or glucose in place of the sucrose did not affect the bioavailability of this iron source. Similarly, the source of fat can affect the bioavailability of dietary iron but, the level of dietary fat has no effect (Mahoney et al., 1980). 

The casein concentration of diets fed rats does not affect iron absorption (Amine and Hegsted, 1971, Carmichael et al., 1975) however, effect of protein source was not studied by these authors. Thus, the sources of carbohydrate and fat can markedly affect the utilization of dietary iron and should be considered as important variables in bioavailability experiments. The amount of protein, however, does not seem as critical. 

Prevention of iron deficiency in populations not sustaining chronic blood loss is possible by judicious selection of diets which enhance the bioavailability of dietary iron. The recent decades have produced significant research on the availability of iron as it is affected by various dietary components, those which enhance as well as those which inhibit iron absorption. This has allowed for the first time the quantification of dietary effects on a trace metal and the development of a model whereby the quantity of bioavailable iron in a diet may be estimated. 

Extensive research on the absorption of iron from various types of meals has allowed guidelines to be developed by which the amount of dietary iron available for absorption may be estimated. Iron is the first trace mineral to be thus treated and thus serves as a model for other nutrients (19). The model for estimating bioavailable iron is based on the concept that iron forms a) a pool of heme iron which is readily available to humans and is uneffected by other dietary components and b) a pool of nonheme iron which is of low bioavailability unless enhancing factors are present concommitantly (20). 

In view of the strong interactions with copper and protein it may be worth taking another look at the general concept of "the bioavailability of iron." For years we have considered that there is a net absorption of only about 10% of the dietary iron. 

Phytate, Wheat Bran, and Bioavailability of Dietary Iron... 

Monoferric phytate is the major fraction of iron in wheat bran, and is a highly bioavailable form of dietary iron in contrast to insoluble di- or tetra-ferric phytate. Monoferric phytate equilibrates with the miscible nonheme iron pool of a meal in extrinsic label iron absorption tests. Whole wheat bran depressed absorption by humans of nonheme iron in a meal. Dephytinized wheat bran also inhibited nonheme iron absorption by humans and the inhibition could not be clearly attributed to either the insoluble or soluble fractions of the dephytinized bran. Adult men who consumed 36 g of wheat bran per day had positive iron balances. Iron balance was not increased when dephytinized bran was consumed. The form of ferric phytate must be known to properly explain the effect of phytic acid on iron absorption. The overall meal composition must be considered to evaluate the effect of wheat bran on iron nutrition of humans. 

The effect of dietary fibers or fiber-rich foods on iron bioavailability in animals. 

Many factors have been identified as influencing the absorption of iron. In addition to changes within the host which affect iron absorption and the form of the iron salt, various dietary constituents which may increase or decrease iron bioavailability have also been studied. As diets become more plant product oriented and less iron is provided by animal products, the occurrence of these other dietary factors is also likely to change. Factors which have been implicated include the following amount of heme iron, ascorbic acid level, dietary protein,... 

Although the iron content of the diet is important, of greater nutritional significance is the bioavailability of iron in food. Heme iron, which constitutes only 6% of dietary iron, is far more available and is absorbed independent of the diet composition it represents 30% of iron absorbed. 

High levels of administered zinc limits copper uptake in humans and certain animals, and provides protection against toxicosis produced by copper in pigs and sheep. Excessive zinc in humans interferes with copper absorption from the intestine, resulting in copper deficiency, and eventually to cardiovascular diseases hi zinc intakes also decrease iron bioavailability, leading to a reduction of erythrocyte life span by 67%. Copper deficiency induced by excess dietary zinc is associated with lameness in horses, donkeys, and mules. 

The efficiency of iron absorption depends on both the bioavailability of dietary iron and iron status. Typically, 5-20% of the iron present in a mixed diet is absorbed. Dietary iron exists in two forms, heme and non-heme. Heme iron is derived from animal source food and is more bioavailable than non-heme iron, with approximately 20-30% of heme iron absorbed via endocytosis of the entire heme molecule. Iron is then released into the enterocyte by a heme oxidase. 

Nutritional significance. As one of the major whey proteins in human milk and also relatively abundant in bovine colostrum, LF is of interest as a dietary source of amino acids as well as for the bioavailability of iron. LF has an... 

Absorption Only 35-70% of oral norfloxacin is absorbed. However, 70-90% of the other fluoroquinolones are absorbed after oral administration. Bioavailability is greatest for ofloxacin and lomefloxacin. Intravenous preparations of ciprofloxacin and ofloxacin are available. Ingestion of the fluoroquinolones with sucralfate, antacids containing aluminum or magnesium, or dietary supplements containing iron or zinc can interfere with the absorption of these antibacterial agents. 

Besides the effect of dietary calcium level, the amount of iron, and perhaps the level of other metals, may affect zinc bioavailability. Solomons and Jacob (13) have shown in human subjects that increasing the iron/zinc ratio from 0 1 to 3 1 in solutions containing 25 mg of zinc and corresponding amounts of iron as ferrous sulfate produced a progressive decrease in the plasma zinc response. They further reported that the chemical form of iron was an important determinant of the interaction. Solomons (14) extensively reviewed both inhibitory factors and enhancers of zinc bioavailability found in foods. 

The uptake of iron by the human intestine is governed not only by its dietary form and companion consituents, but also by the iron condition of the individual. Iron-depleted subjects absorb all forms of iron with greater avidity than do iron-replete individuals. In human dietetics, therefore, the ascorbic content of a diet can be included in the equation for describing the bioavailability of iron from a mixed diet (35). The interaction of iron nutrition and graded intakes of ascorbic acid (< 25 mg > 25 but < 75 mg and > 75 mg) as a prediction of iron availability from a mixed North American diet is plotted in Figure 1. 

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