Assay of iron bioavailability

The bioavailability of iron from any source (e.g., iron supplement, food or meal composite) is considered to be that portion of the total iron which is metabolizable. Philosophically, this concept is important because the amount of iron utilized by avian and mammalian species is directly associated with iron need. When assaying iron bioavailability, it is therefore necessary to use an organism whose need will exceed the amount provided. In animal assays of iron bioavailability, iron need is assured by a growth phase and/or creation of iron deficiency through feeding an iron deficient diet and phlebotomy. Because healthy subjects are usually used in human assays of iron bioavailability (Cook et al., 1981 Cook and Monson, 1976 Radhakrishman and Sivaprasad, 1980), it is inappropriate to compare the data obtained from animal and human assays. In fact it is questionable if assays of iron bioavailability yield good information on the quantities of metabolizable iron available when healthy human subjects are used. 

In ten cases, Che corrected values were less than the uncorrected ones. Because of this inconsistency and because correction does not reduce variability within nor among experiments, attempting to correct for the iron contribution of Che basal ingredients Co Che hematinic response does not seem to improve this assay of iron bioavailability. 

The bioavailability of iron from several organic phosphorus-containing compounds appears to be good. The iron in ferripoly-phosphate protein powder (13) and ferric glycerol phosphate Q, 6) was found to be 92-100% as bioavailable as ferrous sulfate in heme repletion assays with anemic rats and chicks. Morris and Ellis (14) have reported that the iron in monoferric phytate was utilized by rats as well as the iron in ferrous ammonium sulfate. While Lipschitz, et al. (15) have reported that dogs absorbed radio-labelled iron from a small dose (1.5 mg iron) of monoferric phytate one-half as well as they absorbed iron from ferrous sulfate. 

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). 

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