Iron Fortification of Food: Its Measurement by the Extrinsic Tag Method

The assimilation of fortification iron has been studied in 228 subjects by the extrinsic tag method. The first study was made comparing the absorption from ferric and ferrous iron salts given with a complete meal. Labeled wheat enriched with 2 mg of iron as ferric chloride was given with a complete meal, and the same study was repeated in the same subjects with wheat enriched with ferrous sulfate. The same protocol was followed in the second study, but the meal contained no meat. Ferric chloride was absorbed, as well as ferrous sulfate in both studies. Multiple absorptions were performed from test meals containing foods of either animal or vegetal origin tagged biosynthetically with ⁵⁵Fe (intrinsic tag) to which was mixed iron as ⁵⁹FeCl₃ (extrinsic tag) in doses from 0.5 to 60 mg of iron. With test meals of labeled maize given along or with unlabeled meat, the ratio of extrinsic to intrinsic tag absorption was close to unity along a wide range of absorption percentage and was independent of the dose of iron fortification administered. It is concluded that fortification iron added as a soluble iron salt mixed with the nonheme iron pool of the diet and its absorption reflects the absorption of the nonheme iron. Similar studies in which increasing doses of ⁵⁹FeCl₃ were added to a test meal of tagged animal meat showed a progressive fall in the ratio of extrinsic to intrinsic tag absorption, indicating that assimilation of dietary iron occurs from two independent pools—heme and nonheme iron. Absolute absorption of fortificaiton iron added to maize increased from 0.07 to 0.34 mg iron with an increase in the dose of nonheme iron from 2.5 to 62 mg. Over a similar dose range of fortification iron added to animal muscle, absorption increased from 0.17 to 3.3 mg iron. These studies indicate that food iron fortification is likely to be effective only in individuals who take animal protein as part of their diet.