KIN 343 - 9.4 Zinc Bioavailability

Zinc Bioavailability

Enhancers of Zinc Absorption

  • Ligands: These are substances that can increase zinc solubility and enhance its absorption at the brush border. Examples include:

    • Citric acid

    • Amino acids (Histidine, Cysteine, Lysine, Glycine)

  • Dual Mechanism:

    1. Solubility Maintenance: Ligands help keep zinc soluble in the small intestine.

    2. Transport Suggestion: Zinc may bind to amino acids, potentially allowing it to "piggyback" into enterocytes via amino acid transporters.

    • This hypothesis is not fully established and remains debated in current research.

Factors Affecting Zinc Absorption

  • Zinc Status: Low dietary zinc levels and low intracellular zinc levels enhance absorption.

    • Zip 4 Transporter: Carrier-mediated absorption mechanism (facilitated diffusion) is increased under conditions of low zinc status.

  • Absorption Efficiency: Ranges from 10% to 80% of dietary zinc, largely influenced by dietary intake levels.

    • Increased synthesis of Zip 4 transporter at low zinc levels improves zinc uptake and subsequent delivery to the bloodstream.

Environmental Factors for Zinc Absorption

  • pH Levels: Better solubility and absorption of zinc occur in lower pH or more acidic conditions.

Inhibitors of Zinc Absorption

  • EGCG (Epigallocatechin Gallate): A polyphenol that binds to multiple zinc ions, potentially reducing zinc absorption by sequestering it.

  • Oxalate and Phytate:

    • These compounds bind to cations including zinc, calcium, and magnesium, forming insoluble complexes that decrease absorption.

    • Phytase in Yeast: Can hydrolyze non-complexed phytate, enhancing zinc absorption.

  • Competing Divalent Cations:

    • Calcium, magnesium, and non-heme iron compete with zinc for absorption through shared transport mechanisms.

    • High calcium levels can form complexes that also bind phytate with higher affinity than zinc, potentially preserving zinc availability.

Interaction of Calcium with Phytate

  • Calcium's Role: High levels of calcium may obstruct zinc absorption but may also bind to phytate, freeing up zinc for better bioavailability in certain contexts (e.g., enriched bread).

    • This suggests a dual effect of calcium: competitive inhibition but also potential enhancement of zinc availability under specific dietary conditions.