Vitamin A equivalence of beta-carotene in a woman as determined by a stable isotope reference method.
BACKGROUND
Quantitative information on conversion of beta-carotene to vitamin A in humans is limited.


OBJECTIVE
Our laboratory has developed a stable isotope method for studying the conversion of beta-carotene (beta-C) to vitamin A.


METHODS
Two dosage levels (a pharmacological dose, 126.0 mg beta-C-d8, and a physiological dose, 6.0 mg beta-C-d8) were used 2.5 y apart in an adult female volunteer to study dose effects on the conversion of beta-C to vitamin A. Blood samples were collected over 21 d. beta-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-d4 response in the circulation peaked at 24 hours after the beta-C-d8 dose, with a higher percent enrichment after the pharmacological dose than after the physiological dose. By using retinyl acetate-d8 as the vitamin A reference, the retinol-d4 formed from 6 mg of beta-C-d8 (11.2 mumol) was calculated to be equivalent to 1.6 mg of retinol (i.e., 3.8 mg of beta-C was equivalent to 1 mg of retinol). However, the retinol-d4 formed from 126 mg of beta-C-d8 (235 mumol) was equivalent to 2.3 mg of retinol (i.e., 55 mg beta-C was equivalent to 1 mg retinol).


CONCLUSIONS
These results provide evidence that it is feasible to use stable isotope reference method to study retinol equivalence of beta-C and that there may be a dose-dependence on bioconversion of beta-carotene to retinol.
Tang
G
G
USDA Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111, USA. GTang@HNRC.TUFTS.EDU
Qin
J
J
Dolnikowski
G G
GG
Russell
R M
RM
D000328:Adult; D002851:Chromatography, High Pressure Liquid; D003903:Deuterium / Q000032:analysis*; D004305:Dose-Response Relationship, Drug; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D008137:Longitudinal Studies; D014801:Vitamin A / Q000097:blood / Q000737:chemistry / Q000378:metabolism*; D019207:beta Carotene / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*
D019207:beta Carotene; D014801:Vitamin A; D003903:Deuterium