Nutritional iron requirements and food iron absorption.

To prevent nutritional iron deficiency, sufficient iron must be absorbed from the diet to meet the normal physiological requirements. Daily iron losses in males are about 1 mg (14 micrograms kg-1), while the average additional requirements incurred in women include menstruation (0.6 mg), pregnancy (2.7 mg) and lactation (less than 0.3 mg). Requirements during pregnancy are not evenly distributed and increase to between 5-6 mg in the last trimester of pregnancy, which is more than can be absorbed from even an optimal diet. While the amounts absorbed are affected by the iron content of the diet, the composition of the latter is even more relevant. About one-quarter of the iron in haem proteins is absorbed regardless of the other components in the diet, while non-haem iron absorption is subject to the interplay of promoting and inhibiting substances in the diet. Thus diets rich in enhancers of non-haem iron absorption, chiefly meat and/or ascorbic acid, have high iron bioavailability (about 3 mg d-1) while diets in which inhibitors, such as polyphenols and phytates, predominate are poor sources of iron (less than 1 mg d-1). Examination of the relative proportions of promoters and inhibitors of iron absorption in individual foodstuffs and the measured iron absorption from them may be useful in predicting the overall iron bioavailability from mixed diets.