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Updated: Sep 20, 2026
Formulation of Vitamin A Tracer Doses for Administration to Human Subjects using Retinol Isotope Dilution Technique
Published on: March 31, 2026
Short-term (intestinal) and long-term (postintestinal) conversion of beta-carotene to retinol in adults as assessed
Guangwen Tang1, Jian Qin, Gregory G Dolnikowski
1Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111, USA. guangwen.tang@tufts.edu
Background:
Quantitative information on the conversion of beta-carotene to vitamin A in humans is limited.
Objective:
We determined the short- and long-term conversion of labeled beta-carotene to vitamin A by using a stable-isotope reference method.
Design:
[(2)H(8)]beta-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 [(2)H(8)]beta-carotene dose, the volunteers each took a dose of [(2)H(8)]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 [(2)H(4)]retinol response curve (from the [(2)H(8)]beta-carotene dose) was 569 +/- 385 nmol. d, and the 53-d area under the serum [(2)H(8)]retinol response curve (from the [(2)H(8)]retinyl acetate dose) was 1798 +/- 1139 nmol. d. With the use of [(2)H(8)]retinyl acetate as the vitamin A reference, the [(2)H(4)]retinol formed from [(2)H(8)]beta-carotene (11,011 nmol) was calculated to be equivalent to 3413.9 +/- 2298.4 nmol retinol. The conversion factor of beta-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 beta-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 [(2)H(8)]beta-carotene dose, respectively.
Conclusion:
The quantitative determination of the conversion of beta-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 beta-carotene.
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