Ascorbic acid iron bioavailability

Effect of Heat Processing on Bioavailability of Added Iron. Several studies in Table III measured directly the effect of heat processing on added iron. These studies compared processed foods to a control group of identical unprocessed food. Studies in Table 111 utilizing unprocessed controls include 15, 19, and 23. Other studies did not employ an unprocessed control, but used a reference dose to enable comparisons from study to study. Reference doses of ferrous sulfate (most animal assays) or ferrous ascorbate (most human tests) were frequently used. Preparation of ferrous ascorbate, usually a 2 1 molar ascorbic acid iron solution, has been detailed by Layrisse et al. (25). These controls enabled measurement of variation in iron absorption from subject to subject, important in view of greater absorption of an iron deficient versus an iron replete subject. When a reference dose was fed as a radiolabeled salt (55Fe), and on alternate times the test diet was fed with a different radiolabel (59Fe), errors due to variation in subject absorption were eliminated, as each subject served as its own control. The different availabilities of various iron sources from baked enriched rolls were established in this manner (17). 

Camire (2002) showed that texturization does not seem to have a great effect on mineral retention and bioavailability. Others have reported increased retention of ascorbic acid in rice- and maize-based snacks (Hazell and Johnson, 1989 Plunkett and Ainsworth, 2007), increased iron diffusibility and absorption of iron-complexed protein (Poltronieri et al, 2000 Watzke, 1998), and no difference in iron and zinc absorption in human subjects fed textured bran-flour (Fairweather-Tait et al, 1989). 

Gastric acid and ascorbic acid facilitate the absorption of iron. Therefore, bioavailability of iron ingested with food is considerably decreased and also enteric-coated iron preparations are absorbed to a lesser extend. Fixed combinations with ascorbic acid increase the absorption of iron by at least 30%. However such increased uptake seems to have little advantage over a modest increase of dose. 

Iron bioavailability may be increased in the presence of meat (Politz and Clydesdale 1988). This is the so-called meat factor. The exact mechanism of this effect is not known, but it has been suggested that amino acids or polypeptides that result from digestion are able to chelate nonheme iron. These complexes would facilitate the absorption of iron. In nitrite-cured meats some factors promote iron bioavailability (the meat factor), particularly heme iron and ascorbic acid or erythor-bic acid. Negative factors may in-clude nitrite and nitrosated heme (Lee and Greger 1983). 

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. 

A few trends are evident from the heat-processing data in Table III. Processing increased bioavailability of added iron when the process involved heating a predominantly aqueous food (i.e., wet-heat processing), as well as when ascorbic acid was added before heating. A greater bioavailability resulted after the processing of canned liquid milk-based infant formula (13),... 

Iron bioavailability is affected by valence state, form, solubility, particle size, and com-plexation which in turn may be affected by the food matrix. Complexation of iron has been found to have either a positive or negative effect on availability, with such compounds as ascorbic acid and fructose increasing availability and oxalates, phytates, phosphates and food fibers perhaps decreasing availability. Availability has also been shown to be directly correlated to acid solubility. We have found that acidity tends to increase ionization as well as favoring the ferrous state which has greater solubility at... 

No discussion of complexation of iron would be complete without mentioning ascorbic acid. Studies too numerous to list have clearly defined the positive effects of ascorbic acid on increasing the bioavailability of non-heme iron to both animals and humans. The interactions of Vitamin C and iron have recently been reviewed by lynch and Cook (31) However, upon examination it would seem that the effects of ascorbic are due to factors which include, but are not limited to, complex formation. 

If one was willing to accept some simplification, the most important factors involved in the efficiency of ascorbic acid in iron bioavailability might be listed as follows ... 

Since the effects of pH and reduction potential on iron bioavailability are to be discussed next it would seem logical to include a discussion of ascorbic acid in this section. 

Such chemical changes will obviously effect bioavailability and perhaps explain some of the results reported. For instance, Brise and Hallberg (kO) determined that 200-500 mg ascorbic acid more than tripled the bioavailability of 30 mg of iron administered as ferrous sulfate while 100 mg or less had little effect. Similarly, Cook and Monsen (Ul) determined that the increase in iron absorption from a semisynthetic meal was directly proportional to... 

In order to explain the effects observed by Nojeim and Clydesdale (1 3), I would like to propose a mechanism for the action of ascorbic acid in the presence of iron which might explain how it could act as both a reducing agent and a prooxidant as well as perhaps shedding some more light on its role in the chemistry and thus the bioavailability of iron. 

It seems apparent then, that the final form of iron in a food system should be directly influenced by the reduction potential of that system and anything which affects the reduction potential might affect the bioavailability of iron in the system. The enhancement of bioavailability of iron by the reducing compounds ascorbic acid and fructose is well known and has been discussed. However, it should be reemphasized that the addition of these, and other reducing agents, may in part increase iron bioavailability by their effect on redox potential as well as by their ability... 

The levels of many other compounds (i.e. ascorbic acid, protein and calcium) in the diet can moderate the effects of phosphorus on zinc and iron utilization. Probably the most important of these factors is calcium. If both inorganic calcium and phosphorus salts are added to a diet, the bioavailability of iron and probably zinc from that diet will be depressed probably. 

One way to Increase the bioavailability of iron in the diet is to increase the content of factors stimulating its absorption. Ascorbic acid is known to increase the absorption of iron. The Norwegian recommendation of ascorbic acid in the diet was until recently 30 mg per day. But we have accepted the Swedish recommendation of 60 mg per day partly because the bioavailability of the iron could be improved. 

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,... 

Ferric iron must be reduced to Fe for absorption by the mucosal cells in the proximal duodenum, the primary site of absorption. The most effective enhancer of iron absorption is ascorbic acid, especially with food of low iron bioavailability where a 10-fold, or greater, enhancement has been observed (8). Inhibitors of iron absorption include tea, soy protein, bran, and other fibers. Gastric and intestinal juices assist in solubilizing and releasing bound Fe in food. Absorbed Fe is reoxidized to Fe by ferroxidase(s) before it is bound by apotransferrin or apoferritin (storage). Ferroxidase activity has been demonstrated with xanthine oxidase and cemloplasmin [11-13]. 

Non-heme iron exists in plant products and its bioavailability is compromised by the concurrent ingestion of tannins, phytates, soy, and other plant constituents, that decrease its solubility in the intestinal lumen. Bioavailability of non-heme iron is increased by concurrent ingestion of ascorbic acid and meat products. Nonheme iron is reduced from the ferric to the ferrous form in the intestinal lumen and transported into enterocytes via the divalent metal transporter (DMT-1). Once inside the enterocyte, iron from heme and nonheme sources is similarly transported through the cell and across the basolateral membrane by the ferroportin transporter in conjunction with the ferroxidase hephaestin after which it can be taken up by transferrin into the circulation. The regulation of iron across the basolateral membrane of the enterocyte is considered the most important aspect of iron absorption. 

Dietary recommendations for infants are based on the iron content and bioavailability of human milk. The iron in infant formula is much less bioavailable (10%) than that of human milk and is thus present in greater concentrations than that of human milk. Infants who are not breast-fed should consume iron-fortified formula. Complementary foods offered after 6 months of age can potentially meet iron needs if they have a high content of meat and ascorbic acid. This is rarely the case in developing or developed countries, and fortified infant cereals and iron drops are often introduced at this time in developed countries. In developing countries where diets are poor in bioavailable iron, iron-fortified weaning foods are not commonly consumed, and iron supplements are rarely given to infants and children. 

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