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Variability in conversion of ß-carotene to vitamin A in men as measured by using a double-tracer study design^1,2,3

¹ From the Department of Nutrition, University of California, Davis (SJH, JRF, YL, SRD, and AJC), and the Western Human Nutrition Research Center, US Department of Agriculture Agricultural Research Service/Pacific West Area, Davis, CA (BJB and TRN).

Background: The vitamin A activity of ß-carotene is variable and surprisingly low in women. The reasons for this are not well understood. The vitamin A activity of ß-carotene in men is still uncertain. Contributions of dietary factors compared with individual traits are largely unknown.

Objective: Our objective was to measure the intrinsic variability in the vitamin A activity of ß-carotene among healthy, well-fed men living in a controlled environment.

Design: We used a double-tracer test-retest design. We dosed 11 healthy men orally with 30 µmol hexadeuterated (D₆) retinyl acetate (all-trans-19,19,19,20,20,20-[2H₆]retinyl acetate) and then with 37 µmol D₆ ß-carotene (19,19,19,19',19',19'-[2H₆]ß-carotene) 1 wk later. Doses were taken with breakfasts containing 16 g fat. We measured D₆ retinol, D₆ ß-carotene, and trideuterated (D₃) retinol (derived from D₆ ß-carotene) concentrations in plasma. Areas under the plasma concentration x time since dosing curves (AUCs) were determined for D₆ retinol, D₆ ß-carotene, and D₃ retinol.

Results: All men had detectable D₆ retinol concentrations in plasma. The mean (±SE) absorption of D₆ ß-carotene in all subjects was 2.235 ± 0.925%, and the mean conversion ratio was 0.0296 ± 0.0108 mol retinol to 1 mol ß-carotene. Only 6 of 11 men had sufficient plasma concentrations of D₆ ß-carotene and D₃ retinol that we could measure. The mean absorption of D₆ ß-carotene in these 6 subjects was 4.097 ± 1.208%, and the mean conversion ratio was 0.0540 ± 0.0128 mol retinol to 1 mol ß-carotene.

Conclusion: The vitamin A activity of ß-carotene, even when measured under controlled conditions, can be surprisingly low and variable.

Key Words: Carotene • vitamin A activity • ß-carotene • retinol • stable isotope • tracer • men

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