KIN 343 - 7.4 Vitamin A Bioavailability

7.4 Vitamin A Bioavailability

Overview of Vitamin A Bioavailability

  • Focus on the bioavailability of vitamin A from carotenoids, particularly beta-carotene monooxygenase polymorphism.

  • Previous discussions included the role of fats in vitamin absorption in relation to both carotenoids and retinoids.

Importance of Fats for Absorption

  • Fats stimulate the release of lipases and bile, essential for digestion.

  • My cells are vital carriers for vitamin A, aiding in packaging and transportation into enterocytes.

  • Chylomicrons, composed of fats, cholesterol esters, and phospholipids, are crucial in vitamin A metabolism.

Role of Beta-Carotene Monooxygenase

  • Variability in enzyme activity due to polymorphisms affects vitamin A bioavailability from carotenoids.

  • Polymorphisms can result in significantly different capabilities in converting beta-carotene into retinoids.

Study on Polymorphisms in Beta-Carotene Monooxygenase

  • Vitamin A deficiency leads to preventable blindness, especially in children.

  • Increased demand for vitamin A in women of childbearing age during lactation necessitates effective supplementation.

  • Retinoids, while well-absorbed, can be toxic at higher doses, particularly dangerous for children and pregnant women.

Carotenoids as a Safer Supplement

  • Carotenoids are considered Generally Recognized as Safe (GRAS) with no established upper intake limits.

  • Supplementing populations at risk for deficiency (notably children and women) with beta-carotene is common practice.

  • Concerns about efficacy arise, as individuals differ in their ability to convert beta-carotene to retinoids.

Genetic Variability in Response to Carotenoids

  • Approximately 30% to 50% of the population are poor responders to carotenoids due to genetic variations in the beta-carotene monooxygenase gene.

  • Poor responders can produce up to 90% less vitamin A from beta carotene.

  • A specific polymorphism involves an alanine-to-valine substitution at position 379 of the beta-carotene monooxygenase enzyme.

Impact of the 379th Amino Acid Polymorphism

  • Different alleles create variations in enzyme activity, affecting vitamin A conversion efficacy.

  • Studies show that the valine variant has a 30% lower activity compared to the alanine variant when breaking down beta carotene to retinal.

  • Example measurement: 1000 picomoles of retinal formed by alanine type versus 660 picomoles by valine type in a given timeframe.

Clinical Implications of Variability

  • Women with the valine variant produce less retinal from the same beta-carotene intake than those with the alanine variant.

  • The blood of poor responders shows higher beta-carotene and lower retinal levels, leading to potential accumulation and visible signs of excess (e.g., orange skin).

Conclusions from Studies

  • The risk of hypervitaminosis A from carotenoid supplementation is very low; excess carotenoids remain non-toxic.

  • However, the variability in conversion efficiency suggests that the standard dosing to treat deficiency may not be effective for everyone.

  • Individuals with less efficient enzyme variants may require significantly higher doses of beta-carotene to achieve sufficient retinal levels.