There are 2 transporters for iron, one for heme iron and one for non-heme iron. The non-heme transporter is the divalent mineral transporter 1 (DMT1), which transports Fe2+ into the enterocyte. Heme iron is taken up through heme carrier protein 1 (HCP-1), and then metabolized to Fe2+. Fe2+ may be used by enzymes and other proteins or stored in the enterocyte bound to ferritin, the iron storage protein. To reach circulation, iron is transported through ferroportin1,2. This process is summarized in the figure below.
Since only the reduced form of non-heme iron (Fe2+) is taken up, Fe3+ must be reduced. There is a reductase enzyme on the brush border, duodenal cytochrome b (Dcytb), that catalyzes the reduction of Fe3+ to Fe2+, as shown below. Vitamin C enhances non-heme iron absorption because it is required by Dcytb for this reaction. Thus, if dietary non-heme iron is consumed with vitamin C, more non-heme iron will be reduced to Fe2+ and taken up into the enterocyte through DMT1.
In addition to vitamin C, there is an unidentified factor in muscle that enhances non-heme iron absorption if consumed at the same meal3. This unidentified factor is referred to as meat protein factor (MPF). The table shows how MPF can increase non-heme iron absorption.
Table 12.711 Non-heme iron absorption from chicken or beef muscle fraction3
|Mean Fe Absorption
(% of Dose)
|Egg Albumin||Whole Muscle||Whole Muscle Protein||Heme-Free Muscle Protein|
Albumin is a protein, so the egg albumin represents a non-meat protein standard for comparison. You can see that absorption is much higher with whole muscle. When only consuming muscle protein, there is a slight increase from muscle itself, and when they look at heme-free muscle iron, absorption is still higher than egg albumin3.
Inhibitors of non-heme iron absorption typically chelate, or bind, the iron to prevent absorption. Phytates (phytic acid), which also inhibit calcium absorption, chelate non-heme iron decreasing its absorption.
Other compounds that inhibit absorption are:
Polyphenols (coffee, tea)1
Oxalate (spinach, rhubarb, sweet potatoes, and dried beans)2
Calcium is also believed to inhibit iron uptake.
References & Links
1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
2. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
3. Hurrell R, Reddy M, Juillerat M, Cook J. (2006) Meat protein fractions enhance nonheme iron absorption in humans. J Nutr 136(11): 2808-2812.