Role of Ascorbic acid and Other Absorption Promoters

Ascorbic acid is the most potent enhancer of iron absorption (7, 28-32). By adding substantial quantities of ascorbic acid to a plant-based meal, iron absorption may be increased as much as 6-fold (28). This influence is most pronounced in meals that contain high levels of phytates and polyphenols (31). The solubilizing effect of ascorbic acid counterbalances the negative consequences of dietary fiber and phytic acid (7).
In iron deficient individuals, vitamin C greatly enhances iron absorption from foods when consumed in the same meal (33). In individuals with normal iron status, vitamin C has a lesser effect on improving iron absorption. The absorption-enhancing effect is much smaller when the vitamin C is taken 4 to 8 hours before the meal. Both synthetic and dietary ascorbic acid enhance iron absorption (7). For meals consumed in the morning, the iron-enhancing effect of vitamin C is high. With meals consumed later in the day, the effect may be somewhat dampened.
When food is held at warm temperatures, ascorbic acid is oxidized and the iron absorption enhancing effect is decreased (33, 34). Ascorbic acid also increases the iron bioavailability of those iron compounds used in fortification (2). Citric, malic or tartaric acids found in fruits and vegetables improve iron absorption two- to four-fold (35). Lactic acid, found in sauerkraut also enhances iron absorption (35). There is limited information about the influence of vitamin C on zinc bioavailability.
PolyphenolsPolyphenols commonly present in many vegetables include phenolic acids, flavonoids and their polymerization products (35). There are numerous kinds of phenolic compounds in beverages such as tea and coffee, herbal teas, cocoa and red wine. These compounds form insoluble complexes with iron and may exist as an "iron-tannin" complex and thus inhibit iron absorption (5,15). The relative order of polyphenol content per cup of beverage follows the following order: black tea>coffee>cocoa>herbal teas. There is reason to believe that in iron deficient individuals, drinking polyphenol-containing cocoa, tea or coffee beverages along with a phytate-containing wheat-, rice- or maize-based meal may further compromise iron status. For clients with severe iron deficiency, use of herbal teas may be the preferred option. Similarly, phenolics present in vegetables such as butter beans, spinach and lentils may strongly inhibit iron absorption from a composite meal