Define Inhibitors and Enhancers - absorption of dietary iron?

Phytates and fibre from whole grain cereals, tannins and polyphenols in tea, oxalates in green leafy vegetables like spinach and excess calcium taken as supplements can all depress non-haem iron absorption significantly, by forming insoluble components. The Indian vegetarian diet consisting predominantly of cereals and pulses, high in phytates, has a low iron bioavailability. This is further compromised when tea is drunk with a meal, as polyphenols in tea depress iron absorption. Iron absorption from wheat has been reported to be 5%.

However, when tea is taken with a breakfast meal comprising of wheat chapattis and potato vegetable, the reported absorption has been only 1.8%. Ragi balls or sorghum breakfast with potato vegetable and tea resulted in only 0.8-0.9% absorption of iron. On the contrary, ascorbic acid is a potent enhancer of iron absorption. Addition of orange juice containing 40-50 mg ascorbic acid to a breakfast meal consisting of bread, eggs and tea was found to increase iron absorption from 3.7% to 10%. Thus, ascorbic acid can counter the inhibitory effect of tannins or phytates, producing a 2- 3 fold increase in iron absorption. Thus, ascorbic acid can enhance iron absorption in a number of ways. Firstly, it reduces insoluble feiric iron to soluble ferrous iron; secondly, ascorbic acid forms low molecular weight chelates with iron that remain soluble in the intestine; thirdly, ascorbic acid-iron chelates preferentially release the iron for absorption to the brush border. Together, these mechanisms ensure that dietary iron is well absorbed in the presence of ascorbic acid.