Iron requirements and bioavailability of dietary iron

Over the past few decades so much knowledge has been gained about iron needs, dietary iron availability and adequacy that it is now one of the best defined nutrients in these respects. Development of new, accurate methods for the measurement of both the losses of iron from the body and the absorption of iron from the diet has significantly contributed to this situation. Present knowledge of iron needs is summarized. Specific to iron are the much higher needs in women than in men and the great variation in needs between different women due to a marked physiological variation in menstrual iron losses and to the effects of pregnancies. Iron availability is discussed separately for heme and non-heme iron (the major type of food iron). Heme iron in small amounts is, on average, better absorbed than non-heme iron. The absorption of heme iron is influenced very little by the iron status of the subject and by the other food components in the diet with the exception of meat which stimulates absorption. On the other hand, the absorption of non-heme iron is markedly influenced both by the iron status of the subject and a great number of dietary factors. The absorption of iron from the diet is thus determined more by meal composition than by the amount of iron present in the diet. The great variation in absorption between different meals is illustrated and the importance of various factors influencing non-heme iron absorption is also demonstrated. Whilst the mode of food preparation itself influences iron absorption, meat or fish and ascorbic acid are some principal food constituents that enhance absorption of iron. On the other hand, several factors like tannins, phytates, phosphates, soya protein products and various "dietary" fibres have been reported to inhibit non-heme iron absorption. A consideration of the nutritional adequacy of iron high-lights the importance of methods of evaluation, particularly the usefulness of the "bioavailable nutrient density" (BND) approach for different meals; BND for iron represents the amount of iron absorbed per 1'000 kcal (4'180 kJ). The main problem in iron nutrition in Western countries today is that arising from the combination of a low-energy intake, especially in women having the highest iron needs, with a conservation in the choice of meals/meal composition. Provision of bioavailable dietary iron to meet needs has hence not been adjusted to "modern life".