Many animal products are good sources of zinc and are estimated to account for 70% of the zinc North Americans’ consume1. An estimated 15-40% of consumed zinc is absorbed2. Zinc is taken up into the enterocyte through the Zir-and Irt-like protein 4 (ZIP4). Once inside the enterocyte, zinc can:
1. Bind to the zinc storage protein thionein. Once thionein has bound a mineral (or a metal) it is known as metallothionein.
2. Be used for functional purposes.
3. Bind to the cysteine-rich intestinal protein (CRIP) where it is shuttled to a zinc transporter (ZnT). After moving through the basolateral membrane, zinc primarily binds to the circulating protein albumin3.
These functions are represented in the figure below.
Figure 12.81 Fates of zinc once it is taken up into the enterocyte
The zinc attached to albumin is transported to the liver through the portal vein. There is not a major storage site of zinc, but there are pools of zinc in the liver, bone, pancreas, and kidney1. Zinc is primarily excreted in feces.
There are some similarities between zinc and iron absorption. Increased zinc consumption results in increased thionein synthesis in the enterocyte. As a result, more zinc is bound to thionein (forming metallothionein) and not used for functional uses or transported into circulation, as represented by the thick and thin arrows in the figure below.
Figure 12.82 Fate of zinc under high zinc status
The enterocytes are then sloughed off preventing the bound zinc from being absorbed.
Figure 12.83 Enterocytes are sloughed off and excreted in feces.
There are a number of inhibitors of zinc absorption:
Phytate (phytic acid), which inhibits calcium and iron absorption, also binds to and inhibits zinc absorption3.
Figure 12.84 Phytic acid structure4
Polyphenols (coffee, tea)3
Figure 12.85 Structure of the polyphenol gallic acid5
Oxalate (spinach, rhubarb, sweet potatoes, and dried beans)3
Figure 12.86 Structure of calcium oxalate6
Non-heme iron also inhibits zinc absorption.
In supplements, zinc is found as3,7:
Zinc oxide - 80% zinc
Zinc chloride - 23% zinc
Zinc sulfate - 23% zinc
Zinc gluconate - 14.3% zinc
Zinc oxide is the least bioavailable form, but since it is 80% zinc, it is commonly used in supplements7.
References & Links
1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
2. Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
3. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
7. Bowman BA, Russell RM, editors. (2006) Present knowledge in nutrition. Washington, DC: International Life Sciences Institute Press.