KIN 343 - 6.3 Phosphorus Bioavailability

Unit 6.3: Phosphorus Bioavailability

1. Forms of Phosphorus

  • Inorganic Phosphorus

    • Primarily absorbed as inorganic phosphate (Pi) from food.

    • Absorption Efficiency: 90%

    • Active transport through NAPI 2b transporter.

  • Organic Phosphorus

    • Consumed primarily as organic phosphate but less efficiently absorbed (50%-70%).

    • Requires cleavage of inorganic phosphate from organic phosphate by enzymes (alkaline phosphatase and phospholipase C).

2. Absorption Mechanisms

  • Inorganic Phosphate

    • Highly efficient absorption process in GI tract.

    • Assumed passive, possibly transcellular or paracellular.

  • Organic Phosphate

    • Absorption can be impaired due to dependence on enzymatic reactions that may not reach completion.

    • Lower bioavailability compared to inorganic forms.

3. Importance of Phosphorus Sources

  • Milk

    • Contains about a third of its phosphate as free inorganic phosphate, making it valuable for nutrient refeeding.

4. Effects of Phytate on Bioavailability

  • Phytate

    • Major phosphorus repository in grains and nuts (80% of phosphate in grains).

    • Lowers bioavailability due to interaction with cations like calcium and magnesium.

    • Mammals lack the enzyme (phytase) needed to hydrolyze phytate into absorbable forms.

  • Hydrolysis Necessity

    • Hydrolysis by microorganisms (e.g., yeast and bacteria) can make phytate phosphorus bioavailable.

    • Unsuitable forms of phytate form insoluble complexes with cations, further reducing bioavailability.

5. Dietary Form and Cation Interactions

  • Dietary Forms

    • Phosphate can be ingested through various forms – as phospholipids or as part of phytate.

    • Phospholipids are readily absorbable due to enzymatic cleavage (phospholipase C).

  • Cation Interactions

    • Divalent cations (calcium, magnesium, aluminum, iron, zinc) can inhibit phosphorus absorption by forming insoluble salts.

    • This binding reduces the bioavailability of phosphorus by preventing its absorption through passive or active transport.

6. Clinical Relevance: Phosphate Management

  • Renal Failure and Hyperphosphatemia

    • In patients with renal failure, excess phosphate causes hyperphosphatemia.

    • Treatment includes administering calcium to create insoluble salts with phosphate, promoting excretion of both calcium and phosphate rather than dietary limitation.

    • This mechanism is crucial to manage phosphorus levels effectively in such patients.

Summary of Key Points

  • Phosphorus is absorbed most efficiently in its inorganic form.

  • Organic phosphate absorption is less efficient and is constrained by various factors, including the presence of phytate.

  • Dietary sources of phosphorus and their interactions with minerals play a significant role in bioavailability.