Vitamin A equivalence of β-carotene in a Woman as determined by a stable isotope reference method

Summary

Background: Quantitative information on conversion of β-carotene to vitamin A in humans is limited.

Aim of the study: Our laboratory has developed a stable isotope method for studying the conversion of β-carotene (β-C) to vitamin A.

Methods: Two dosage levels (a pharmacological dose, 126.0 mg β-C-d ₈, and a physiological dose, 6.0 mg β-D-d ₈) were used 2.5 y apart in an adult female volunteer to study dose effects on the conversation of β-C to vitamin A. Blood samples were collected over 21 d. β-C and retinol were extracted from serum and isolated by high performance liquid chromatography. The retinol fraction was derivatized to a trimethylsilyl ether which was analyzed by gas chromatograph/mass spectrometry with electron capture negative chemical ionization.

Results: The retinol-d ₄ response in the circulation peaked at 24 hours after the β-C-d ₈ dose, with a higher percent enrichment after the pharmacological dose than after the physiological dose. By using retinyl acetate-d ₈ as the vitamin A reference, the retinol-d ₄ formed from 6 mg of β-C-d ₈ (11.2 μmol) was calculated to be equivalent to 1.6 mg of retinol (i. e., 3.8 mg of β-C was equivalent to 1 mg of retinol). However, the retinol-d ₄ formed from 126 mg of β-C-d ₈ (235 μmol) was equivalent to 2.3 mg of retinol (i. e., 55 mg β-C was equivalent to 1 mg retinol).

Conclusion: These results provide evidence that it is feasibile to use stable isotope reference method to study retinol equivalence of β-C and that there may be a dose-dependence on bioconversion of β-carotene to retinol.