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

Abstract

Background: Blood ␤-carotene and vitamin A responses to oral ␤-carotene are variable in humans. Some individuals are characterized as responders and others as low-or nonresponders. A better understanding of the conditions that produce the variability is important to help design public health programs that ensure vitamin A sufficiency. Objective: Our objective was to assess variability in absorption and conversion of ␤-carotene to vitamin A in vivo in humans by using a novel double-tracer [hexadeuterated (D 6 ) ␤-carotene and D 6 retinyl acetate] approach. Design: Eleven healthy women were housed at the US Department of Agriculture Western Human Nutrition Research Center metabolic unit for 44 d, where they consumed diets adequate in vitamins and minerals except for carotenoids. After an adaptation period, the women were given 30 mol D 6 retinyl acetate orally, followed 1 wk later with 37 mol D 6 ␤-carotene (approximately equimolar doses). Time-dependent plasma concentration curves were determined for D 6 retinol, D 6 ␤-carotene, and trideuterated (D 3 ) retinol (derived from D 6 ␤-carotene). Results: Mean (± SE) absorption of D 6 ␤-carotene was 3.3 ± 1.3% for all subjects. The mean conversion ratio was 0.81 ± 0.34 mol D 3 retinol to 1 mol D 6 ␤-carotene for all subjects. However, only 6 of the 11 subjects had plasma D 6 ␤-carotene and D 3 retinol concentrations that we could measure. The mean absorption of D 6 ␤-carotene in these 6 subjects was 6.1 ± 0.02% and their conversion ratio was 1.47 ± 0.49 mol D 3 retinol to 1 mol D 6 ␤-carotene. The remaining 5 subjects were low responders with ≤ 0.01% absorption and a mean conversion ratio of 0.014 ± 0.004 mol D 3 retinol to 1 mol D 6 ␤-carotene. Conclusion: Variable absorption and conversion of ␤-carotene to vitamin A both contribute to the variable response to consumption of ␤-carotene. Our double-tracer approach is adaptable for identifying efficient converters of carotenoid to retinoid. Am J Clin Nutr 2000;71:1545-54.

Bioequivalence of β‐carotene and retinol

“…The study clearly indicated that there is lower bioconversion of β-carotene to retinol with a higher dose of β-carotene. The lower dose of β-carotene used (6 mg) was closer to the amounts obtained from an average vegetablecontaining diet and less than one-third of that used by Lin et al 68 and Hickenbottom et al 69 . Thus the study of Tang et al 70 suggests that bioconversion of β-carotene to retinol is better from smaller doses and would support the view that the method used by van Lieshout et al 65 has advantages over the other methods used in measuring the bioefficacy of β-carotene.…”

“…The same method was used in both studies and the main objective was to assess variability of β-carotene conversion to retinol, in 11 well-nourished women 68 and 11 men. 69 The tracers used were hexadeuterated ( 2 H 6 ) retinyl acetate and 2 H 6 β-carotene.…”

“…68,69,71 The exception is the study by van Lieshout et al, 65 where small amounts of 13 C 10 β-carotene and 13 C 10 retinol tracers were given daily, and bioequivalency was obtained from the ratio of plasma concentrations of 13 C 5 retinol to 13 C 10 retinol on day 21 when equilibration was achieved. The ratio obtained for bioefficacy was 0.79 mol retinol per mole β-carotene and this result is very close to those obtained from the vitamin A depleted subjects (Table 2).…”

β-Carotene conversion products and their effects on adipose tissue

“…Low converters have a low postprandial conversion efficiency after supplementation, reflected by the retinyl ester/ b-carotene ratio measured in the chylomicron fraction [7]. Approximately, 45% of the western population can be classified as low converter [30,47]. It has been established that conversion efficiency is depending at least in part on the vitamin A status [57], which is explained by the existence of a negative feedback of RA on BCMO1 mRNA expression [3,66].…”

“…It has been established that conversion efficiency is depending at least in part on the vitamin A status [57], which is explained by the existence of a negative feedback of RA on BCMO1 mRNA expression [3,66]. Besides, many studies have highlighted the huge interindividual variability in conversion efficiency, with a coefficient of variation being as high as 221% [7,12,30,47]. Furthermore, according to [77] who investigated the kinetics of a labelled b-carotene dose [77], low responders would also display a lower conversion efficiency compared to normal responders (Fig.…”