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Short-term (intestinal) and long-term (postintestinal) conversion of ß-carotene to retinol in adults as assessed by a stable-isotope reference method^1,2,3,4

¹ From the Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston.

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

Objective: We determined the short- and long-term conversion of labeled ß-carotene to vitamin A by using a stable-isotope reference method.

Design: [²H₈]ß-Carotene (11 011 nmol, or 6 mg) in oil was given with a liquid diet (25% of energy from fat) to 22 adult volunteers (10 men, 12 women). Three days after the [²H₈]ß-carotene dose, the volunteers each took a dose of [²H₈]retinyl acetate (8915 nmol, or 3 mg) in oil with the same liquid diet. Blood samples were collected over 56 d.

Results: The 53-d area under the serum [²H₄]retinol response curve (from the [²H₈]ß-carotene dose) was 569 ± 385 nmol · d, and the 53-d area under the serum [²H₈]retinol response curve (from the [²H₈]retinyl acetate dose) was 1798 ± 1139 nmol · d. With the use of [²H₈]retinyl acetate as the vitamin A reference, the [²H₄]retinol formed from [²H₈]ß-carotene (11 011 nmol) was calculated to be equivalent to 3413.9 ± 2298.4 nmol retinol. The conversion factor of ß-carotene to retinol varied from 2.4 to 20.2, and the average conversion factor was 9.1 to 1 by wt or 4.8 to 1 by mol. This conversion factor was positively correlated with body mass index (r = 0.57, P = 0.006). The postabsorption conversion of ß-carotene was estimated as 7.8%, 13.6%, 16.4%, and 19.0% of the total converted retinol at 6, 14, 21, or 53 d after the [²H₈]ß-carotene dose, respectively.

Conclusion: The quantitative determination of the conversion of ß-carotene to vitamin A in humans can be accomplished by using a stable-isotope reference method. This approach provides in vivo metabolic information after a physiologic dose of ß-carotene.

Key Words: [2H8]ß-Carotene • [2H8]retinyl acetate • humans • retinol equivalence • stable isotope • mass spectrometry

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