Cysteine Enhances Non-Haem Iron Absorption

Haem iron is the iron found in the blood and muscle of animals in the molecules of haemoglobin and myoglobin, respectively. Generally this form of iron is very well absorbed because it is in the ferrous (Fe2+) form that is soluble in the alkaline environment of the small intestine and recognised by iron transporters in the gut. Iron is also present in plant tissues as non-haem iron, and can be tightly bound to other plant constituents, such as tannins, and this may limit its absorption somewhat. However, the real problem with non-haem iron is that it is present in the ferric (Fe3+) oxidised form. Once liberated from the plant tissues by the action of hydrochloric acid, some ferric iron is converted to ferrous iron, and both ferric and ferrous iron remains soluble in the acid environment of the stomach. In the small intestine the ferrous iron is well absorbed as it remains soluble. However, the ferric iron complexes with other food components and forms insoluble ferric hydroxides (Fe(OH)3)) which are not absorbed.

Converting ferric iron to ferrous iron in the gut has been accomplished in studies using reducing agents such as vitamin C. The vitamin C donates electrons to the ferric iron and this reduces it to the ferrous form, improving its solubility and therefore its ability to interact with iron transporter. Concomitant consumption of red meat with non-haem iron has also been shown to improve the absorption of non-haem iron, and this is thought to occur because of the presence of sulphur containing amino acids in the protein of the meat that act as strong reducing agents. One such amino acid, cysteine, has been tested in isolation for its ability to increase the absorption of iron from plant foods. In one study1, the addition of cysteine increased non-haem iron absorption 2-fold when the iron was supplied as either whole vegetable matter, haemosiderin (a storage form of iron that is tightly bound within cells) or as a ferric salt. However, no improvements in absorption were seen if the cysteine was added to the food prior to cooking.

Dr Robert Barrington’s Nutritional Recommendation: The amount of cysteine administered with the non-haem iron sources in this study was 210 mg as cysteine hydrochloride. This is roughly the amount of cysteine present in 100 grams of fish. Therefore the results of this study may explain why fish and meat are able to enhance the absorption of iron when eaten concomitantly. Beans have a very high content of iron (especially soybeans), but the iron is poorly absorbed. Eating beans with meat may therefore substantially improve the iron absorption, as well as providing a well absorbed source of iron in the meat. For vegetarians, eating meat is not possible, and in such cases consumption of cysteine containing protein powders, vitamin C, or cysteine supplements should be considered if improved iron absorption is required. As the cysteine was in the hydrochloride form in this study, which is more acidic that free form cysteine, this may have influenced the results somewhat by creating a lower pH environment that solubilised the iron. Vitamin C may have a similar pH effect as it too would increase the acidity of the chyme.

RdB

1Martinez-Torres, C., Romano, E. and Layrisse, M. 1981. Effect of cysteine on iron absorption in man. American Journal of Clinical Nutrition. 34: 322-327
Robert Barrington

About Robert Barrington

Robert Barrington is a writer, nutritionist, lecturer and philosopher.
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