Food and nutrition.pdf

106 Function of Foodsource of iron. Human milk contains about 2 mg of iron per litre and cows'milk only half this amount.Absorption and UtilizationAbsorption of iron takes place mainly in the upper portion of the smallintestine. Most of the iron enters the bloodstream directly and not throughthe lymphatic system. Evidence indicates that absorption is regulated tosome extent by physiological demand. Persons who are iron deficient tendto absorb iron more efficiently and in greater quantities than do normalsubjects.Several other factors affect iron absorption. For example, tannins,phosphates and phytates in food reduce iron absorption, whereas ascorbicacid increases it. Studies have indicated that egg yolk, despite its relativelyhigh iron content, inhibits absorption of iron - not only the iron from theegg yolk itself, but also that from other foods.Healthy subjects normally absorb only 5 to 10 per cent of the iron intheir foods, whereas iron-deficient subjects may absorb twice that amount.Therefore, on a diet that supplies 15 mg of iron, the normal person wouldabsorb 0.75 to 1.5 mg of iron, but the iron-deficient person would absorbas much as 3 mg. Iron absorption generally increases during growth andpregnancy, after bleeding and in other conditions in which the demandfor iron is enhanced.Of greatest importance is the fact that the availability of iron fromfoods varies widely. Absorption of the haem iron in foods of animal origin(meat, fish and poultry) is usually very high, whereas the non-haem ironin foods such as cereals, vegetables, roots and fruits is poody absorbed.However, people usually eat meals, not single individual foods, anda small amount of haem iron consumed with a meal where most of theiron is non-haem iron will enhance the absorption of all the iron. Thusthe addition of a quite small amount of haem iron from perhaps fish ormeat to a large helping of rice or maize containing non-haem iron willresult in much greater absorption of iron from the cereal staple. If thismeal also includes fruits or vegetables, the vitamin C in them will alsoenhance iron absorption. However, if tea is consumed with this meal, thetannin present in the tea will reduce the absorption of iron.RequirementsThe dietary requirements for iron are approximately ten times thebody's physiological requirements. If a normally healthy man or postmenopausalwoman requires 1 mg of iron daily because of iron losses,then the dietary requirements are about 10 mg per day. Thisrecommendation allows a fair margin of safety, as absorption is increased