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 ferroportin^1,2. This process is summarized in the figure below.

Figure 12.711 Iron uptake into the enterocyte

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.

Figure 12.712 Reduction of non-heme iron by Dcytb

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

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.

Figure 12.713 Structure of phytic acid4

Other compounds that inhibit absorption are:

Polyphenols (coffee, tea)¹

Figure 12.714 Structure of gallic acid, a polyphenol⁵

Oxalate (spinach, rhubarb, sweet potatoes, and dried beans)²

Figure 12.715 Structure of calcium oxalate⁶

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.

4. http://en.wikipedia.org/wiki/File:Phytic_acid.png

5. http://en.wikipedia.org/wiki/File:Gallic_acid.svg

6. http://en.wikipedia.org/wiki/File:Calcium_oxalate.png