iron absorption

Last reviewed 10/2023

• body iron stores - absorption is increased in iron deficiency and reduced in iron overload

• rate of erythropoiesis - absorption is increased by increased erythropoiesis, independent of body stores

• diet (1):
• substances which form soluble complexes with iron facilitate absorption, for example vitamin C
• those which form insoluble complexes inhibit it, for example, phytate
• the presence of large quantities of trace minerals in the diet can retard absorption

• chemical state of iron:
• ferrous (Fe2+) iron is more easily absorbed than ferric (Fe3+) iron and this change is largely dependent on the presence of luminal gastric acid and ceruloplasmin within the intestinal mucosal cell
• iron in meat as haem is more easily absorbed than non-haem iron e.g. in vegetables; it is also less prone to the influence of gastric pH

Notes:

• Haem and non-haem iron
  • There are two major sources of food iron: haem iron and non-haem iron.
    • The two forms of iron in the diet are absorbed with different efficiency
      • Haem iron
        • heme iron is derived primarily from hemoglobin and myoglobin in animal protein sources
        • is readily bio-available, since it is absorbed intact within the porphyrin ring and is not influenced by most inhibitory factors in the diet. The non-haem iron in food enters an exchangeable pool that is markedly affected by inhibitory iron-binding ligands
          • organic (haem) iron must be hydrolysed from any protein to which it is attached and it is then absorbed relatively easily but slowly. The overall absorption of iron from meat may be 20-25%. The most efficient absorption takes place in the duodenum, and is inversely related to the iron store level.
      • Non haem iron
        • Non-heme iron is found mainly in enriched cereals and pasta, beans, and dark green leafy vegetables
        • Some forms of nonhaem iron, such as ferritin and hemosiderin, only partially enter the exchangeable pool and are poorly absorbed.
        • Non-haem iron must be solubilized and hydrolysed before absorption is possible. Hydrochloric acid in the stomach performs this function and also converts any ferric iron in food to its absorbable ferrous state. This reaction is facilitated by ascorbic acid (vitamin C). Other factors enhancing the absorption of inorganic iron include citric acid, lactic acid, fructose and peptides derived from meat. All of these form ligands with the ferrous iron, maintaining its solubility and thus facilitating absorption
          
          
• factors in the diet that affect the absorption of iron
  • although uptake of heme iron by enterocytes is affected little by consumption of other foods, absorption of nonheme iron is relatively inefficient, and can be altered substantially by co-consumption of certain dietary elements
    • factors known to enhance nonheme iron absorption include animal protein, copper, and vitamin C
      
      
    • factors known to inhibit nonheme iron absorption include vegetable protein, phytic acid, oxalic acid, zinc, calcium, eggs, tea, and coffee (2)
      

Reference:

• 1) Am J Clin Nutr. 2000 May;71(5):1147-60
• 2) Menzie CM et al. Obesity-related hypoferremia is not explained by differences in reported intake of heme and nonheme iron or intake of dietary factors that can affect iron absorption. J Am Diet Assoc. 2008 Jan;108(1):145-8.