Zinc Metabolism – Functions, Absorption and Disorders

What Is Zinc Metabolism?

Zinc metabolism encompasses all biochemical processes by which the body absorbs, transports, stores in tissues, and excretes the essential trace element zinc (Zn). Zinc is involved in more than 300 enzymatic reactions and plays a central role in immune function, cell division, wound healing, protein synthesis, and growth and development.

Absorption of Zinc

Zinc is primarily absorbed in the small intestine – particularly in the jejunum – from dietary sources. The absorption rate varies depending on dietary zinc content and ranges between 15 and 40 percent of ingested zinc. Absorption occurs via specific transport proteins, mainly ZnT transporters (Zinc Transporters) and ZIP transporters (Zrt-/Irt-like Proteins).

• ZnT transporters facilitate the export of zinc out of cells or into organelles.
• ZIP transporters are responsible for importing zinc into the cytoplasm of the cell.

Absorption is influenced by several factors: zinc from animal sources (e.g., meat, seafood) is better absorbed than zinc from plant sources. Phytates (phytic acid), found in whole grains and legumes, inhibit zinc absorption by forming insoluble complexes with zinc.

Transport in the Blood

After absorption, zinc is transported via the portal vein to the liver. In the bloodstream, approximately 98 percent of zinc is protein-bound:

• About 80 percent is bound to albumin.
• A smaller fraction is bound to alpha-2-macroglobulin and other plasma proteins.
• Only about 1–2 percent exists as free, ionized zinc.

The liver plays a central role in distributing and regulating zinc levels in the body. It also produces the transport protein metallothionein, which can bind and store zinc.

Storage and Distribution in the Body

The human body contains approximately 2–3 grams of zinc in total. Unlike iron, there is no specific zinc storage depot (such as ferritin), which is why regular dietary zinc intake is particularly important. Zinc is present in virtually all body cells, with the highest concentrations found in:

• Muscle tissue (approximately 60% of total body zinc)
• Bone (approximately 30%)
• Skin, liver, kidneys, pancreas, and eyes
• The prostate gland (particularly high concentration)

Within cells, zinc is stored mainly in the cytoplasm, the nucleus, and various organelles, often bound to metallothionein or other zinc-rich proteins.

Biological Functions of Zinc

Zinc performs structural, catalytic, and regulatory roles in the body:

• Catalytic function: Zinc is a component of over 300 enzymes, including alcohol dehydrogenase, carbonic anhydrase, and DNA polymerase.
• Structural function: Zinc stabilizes the structure of proteins and cell membranes, for example in so-called zinc fingers – specialized protein domains involved in gene regulation.
• Regulatory function: Zinc is involved in the regulation of gene expression, immune defense, and signal transduction.

Excretion of Zinc

Zinc is primarily excreted via feces, as a large portion of unabsorbed zinc along with zinc secreted into bile and pancreatic juice leaves the body through the intestine. A smaller amount is excreted via urine, sweat, skin cells, and semen. The body adaptively regulates zinc excretion: when dietary zinc intake is low, fecal excretion is reduced to maintain zinc levels.

Disorders of Zinc Metabolism

Zinc Deficiency

Zinc deficiency can result from insufficient dietary intake, increased requirements (e.g., during pregnancy and growth phases), malabsorption, or chronic disease. Symptoms include growth retardation, impaired immune function, poor wound healing, hair loss, and loss of taste and smell. A rare genetic disorder of zinc metabolism is acrodermatitis enteropathica, in which intestinal zinc absorption is severely impaired.

Zinc Excess (Zinc Toxicity)

Excessive zinc intake – usually from high-dose supplements – can cause nausea, vomiting, abdominal cramps, and impaired copper absorption, as zinc and copper compete for the same transport proteins. Chronic zinc excess can lead to copper deficiency and associated neurological symptoms.

Zinc Intake Recommendations

The World Health Organization (WHO) recommends a daily zinc intake of approximately 4.9–14 mg for adults depending on dietary composition and bioavailability. In Germany, the German Nutrition Society (DGE) recommends 7 mg per day for adult women and 10 mg per day for adult men. Good dietary sources of zinc include meat (especially red meat), seafood (especially oysters), cheese, nuts, legumes, and whole grains.

References

1. World Health Organization (WHO): Trace Elements in Human Nutrition and Health. Geneva: WHO Press, 1996.
2. Wessels I, Maywald M, Rink L: Zinc as a Gatekeeper of Immune Function. Nutrients. 2017;9(12):1286. doi:10.3390/nu9121286
3. Deutsche Gesellschaft für Ernährung (DGE): Reference Values for Nutrient Intake – Zinc. Bonn, 2021. Available at: https://www.dge.de