Dietary iron requirements are dependent on the amount and availability of food iron ingested. On the basis of recent studies of food iron absorption employing extrinsic tag techniques, the availability of heme iron has been defined and estimates of the availability of nonheme iron based on the amounts of enhancing substances appear possible. A model has been developed whereby the availability of iron in a given meal may be estimated. Calculations are made on a meal basis of 1) the amount of heme iron and its availability, and 2) the amount of nonheme iron and its availability as influenced by the meal's content of enhancing factors. Examples of these calculations are provided.

The ability of various animal proteins to enhance the absorption of dietary nonheme iron was evaluated by performing multiple radioiron absorption measurements in 70 volunteer subjects. Protein equivalent substitutions of nine animal foods were made in two basic test meals. The first was a standard meal of high iron availability (mean absorption, 8.3%) containing beef muscle as the animal protein. The second was a semisynthetic meal of low iron availability (mean absorption, 1.4%) containing ovalbumin as the protein source. Two categories of animal protein were defined. Substitution of beef, lamb, pork, liver, fish, and chicken for the egg ovalbumin in the sannisynthetic meal resulted in a significant, 2-fold to 4-fold increase in iron absorption whereas no increase was observed with milk, cheese, or egg. Reciprocal findings were obtained when these foods were substituted for the beef contained in the standard meal. All sources of animal proteins are not equivalent in their effect on nonheme iron absorption.

A sizable segment of the population was found to be taking large quantities of vitamin C to reduce the number or severity of upper respiratory infections. To determine the affect of this supplementation on iron balance, multiple radioiron absorption tests were performed in 63 male subjects. The increase in iron absorption from a semisynthetic meal was directly proportional to the amount of ascorbic acid added over a range of 25 to 1,000 mg. The ratio of iron absorption with/without ascorbic acid at these two extremes was 1.65 and 9.57, respectively. The relative increase was substantially less when the test meal contained meat. A large dose of vitamin C taken with breakfast did not effect iron absorption from the noon or evening meal. A telephone survey of 100 individuals revealed that 67 were taking supplemental ascorbic acid in doses ranging as high as 2 g daily. The average intake of supplemental ascrobic acid in this population was 280 mg daily. If taken only with breakfast, this level of supplementation would produce a nearly 2-fold increase in the amount of iron absorbed daily. If taken in divided doses with each mean, the increase in iron absorption would be more than 3-fold.

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