Variability in conversion of β-carotene to vitamin A in men as measured by using a double-tracer study design

Background: The vitamin A activity of beta-carotene is variable and surprisingly low in women. The reasons for this are not well understood. The vitamin A activity of beta-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 beta-carotene among healthy, well-fed men living in a controlled environment. Design: We used a double-tracer test-retest design. We dosed I I healthy men orally with 30 mumol hexadeuterated (D-6) retinyl acetate (all-trans-19,19,19,20,20,20-[H-2(6)]retinyl acetate) and then with 37 mumol D6 P-carotene (19,19,19,19',19',19'-[H-2(6)]p-carotene) 1 wk later. Doses were taken with breakfasts containing 16 a fat. We measured D-6 retinol, D-6 beta-carotene, and trideuterated (D-3) retinol (derived from D-6 beta-carotene) concentrations in plasma. Areas under the plasma concentration x time since dosing curves (AUCs) were determined for D-6 retinol, D-6 beta-carotene, and D-3 retinol. Results: All men had detectable D-6 retinol concentrations in plasma. The mean (+/-SE) absorption of D-6 beta-carotene in all subjects was 2.235 +/- 0.925%, and the mean conversion ratio was 0.0296 +/- 0.0108 mol retinol to I mol beta-carotene. Only 6 of 11 men had sufficient plasma concentrations of D-6 beta-carotene and D-3 retinol that we could measure. The mean absorption of D-6 beta-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 beta-carotene. Conclusion: The vitamin A activity of beta-carotene, even when measured under controlled conditions, can be surprisingly low and variable.