12.8 Zinc

Zinc

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:

• Bind to the zinc storage protein thionein. Once thionein has bound a mineral (or a metal) it is known as metallothionein.

• Be used for functional purposes.

• 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
• Polyphenols (coffee, tea)3
• Oxalate (spinach, rhubarb, sweet potatoes, and dried beans)3

Non-heme iron also inhibits zinc absorption. In supplements, zinc is found as3,4:

• 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.

For more information on vitamin K, see the Required Web Link below.

Required Web LinkZinc Fact Sheet for Health Professionals

Subsections:

• 12.81 Zinc Functions
• 12.82 Zinc Deficiency & Toxicity

References & Links

• Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.

• Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
• Bowman BA, Russell RM, editors. (2006) Present knowledge in nutrition. Washington, DC: International Life Sciences Institute Press.

Links

Zinc Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/Zinc- HealthProfessional/

Zinc Functions

Zinc is a cofactor for up to 300 enzymes in the body1. Enzymes that use zinc as a cofactor are known as metalloenzymes.

Zinc is a cofactor for the antioxidant enzyme superoxide dismutase that converts superoxide to hydrogen peroxide, as shown below.

Figure 12.811 Superoxide dismutase uses zinc as a cofactor

Alcohol dehydrogenase uses 4 zincs per enzyme. Its role in ethanol metabolism is shown in Figure 12.8122.

Figure 12.812 Ethanol metabolism3,4

Zinc is also important for the formation of zinc fingers in proteins. Zinc fingers help proteins bind to DNA2.

Figure 12.815 Structure of a zinc finger, zinc is the green atom bound in the center5

Zinc is also important for growth, immune function, and reproduction2,6.

References & Links

• Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
• http://en.wikipedia.org/wiki/File:Ethanol_flat_structure.png
• https://en.wikipedia.org/wiki/Acetaldehyde#/media/File:Acetaldehyde-2D-flat.svg
• http://en.wikipedia.org/wiki/File:Zinc_finger_rendered.png
• Singh M, Das RR. (2011) Zinc for the common cold (Review). The Cochrane Collaboration.

Zinc Deficiency & Toxicity

As can be seen on the bottom map in the link below, the risk of zinc deficiency is low in North America, but there are other places in the world where it is much more common.

Figure 12.821 Worldwide prevalence of zinc deficiency1

At particular risk are children, pregnant women, elderly and the poor1. Symptoms of zinc deficiency include2,3: Growth inhibition, Delayed sexual maturation, Dermatitis, Hair loss, Impaired immune function, and Skeletal abnormalities

In the link below you can see a picture of an infant with dermatitis caused by zinc deficiency.

Web LinkZinc Deficiency Dermatitis

Zinc toxicity is not common, but an acute toxicity results in2: Nausea, Vomiting, Intestinal cramps, and Diarrhea

Chronic toxicity can result in copper deficiency, as will be discussed in the copper section3.

References & Links

• Wessells KR, Brown KH. (2012) Estimating the Global Prevalence of Zinc Deficiency: Results Based on Zinc Availability in National Food Supplies and the Prevalence of Stunting. PLoS ONE 7(11): e50568

• Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.

Links

Zinc Deficiency Dermatitis – http://img.tfd.com/mosbycam/thumbs/50029X-fx3.jpg