Relative contribution of α-carotene to postprandial vitamin A concentrations in healthy humans after carrot consumption

Abstract

Background: Asymmetric α-carotene, a provitamin A carotenoid, is cleaved to produce retinol (vitamin A) and α-retinol (with negligible vitamin A activity). The vitamin A activity of α-carotene-containing foods is likely overestimated because traditional analytic methods do not separate α-retinol derivatives from active retinol.Objective: This study aimed to accurately characterize intestinal α-carotene cleavage and its relative contribution to postprandial vitamin A in humans after consumption of raw carrots.Design: Healthy adults (n = 12) consumed a meal containing 300 g raw carrot (providing 27.3 mg β-carotene and 18.7 mg α-carotene). Triglyceride-rich lipoprotein fractions of plasma were isolated and extracted, and α-retinyl palmitate (αRP) and retinyl palmitate were measured over 12 h postprandially via high-performance liquid chromatography-tandem mass spectrometry. The complete profile of all α-retinyl esters and retinyl esters was measured at 6 h, and total absorption of α- and β-carotene was calculated.Results: αRP was identified and quantified in every subject. No difference in preference for absorption of β- over α-carotene was observed (adjusting for dose, 28% higher, P = 0.103). After absorption, β-carotene trended toward preferential cleavage compared with α-carotene (22% higher, P = 0.084). A large range of provitamin A carotenoid conversion efficiencies was observed, with α-carotene contributing 12-35% of newly converted vitamin A (predicted contribution = 25.5%). In all subjects, a majority of α-retinol was esterified to palmitic acid (as compared with other fatty acids).Conclusions: α-Retinol is esterified in the enterocyte and transported in the blood analogous to retinol. The percentage of absorption of α-carotene from raw carrots was not significantly different from β-carotene when adjusting for dose, although a trend toward higher cleavage of β-carotene was observed. The results demonstrate large interindividual variability in α-carotene conversion. The contribution of newly absorbed α-carotene to postprandial vitamin A should not be estimated but should be measured directly to accurately assess the vitamin A capacity of α-carotene-containing foods. This trial was registered at clinicaltrials.gov as NCT01432210.

Keywords: bioconversion; carrot; vitamin A; α-carotene; α-retinol.

© 2017 American Society for Nutrition.

Figures

FIGURE 1

A fraction of the β- and α-carotene absorbed by the enterocyte is enzymatically cleaved to retinal and α-retinal (in the case of α-carotene). These products are then enzymatically reduced and esterified to fatty acids before being packaged into chylomicrons. BCO1, β-carotene oxygenase 1; LRAT, lecithin retinol acyltransferase.

FIGURE 2

Mean baseline-corrected plasma TRL concentrations of αRP and RP over the course of 12 h after consumption of a carrot-containing meal. Concentrations of analytes are represented as means ± SEMs, n = 12. TRL, triglyceride-rich lipoprotein.

FIGURE 3

The total carotenoid absorbed in each subject, including that cleaved to form retinyl esters/α-retinyl esters (A). Percentage of total carotenoid converted to retinoids relative to carotenoid absorbed (B). The percentage of α-carotene contribution to the newly formed retinyl ester in the same subject, respectively (C). Black bars represent β-carotene and white bars represent α-carotene. The line in panel C denotes the expected contribution of α-carotene (25.5%) to retinyl esters, assuming 100% absorption and conversion according to the provided dose. Note that the subject numbers in each panel represent the same individual (i.e., subject 1 in panel A is subject 1 in panels B and C).

FIGURE 4

Esterification profile of various retinyl (gray bars) and α-retinyl (white bars) species, expressed as a percentage of total retinyl and α-retinyl esters, respectively, at 6 h after the consumption of the carrot test meal. Values are expressed as means ± SEMs, n = 12. Asterisks indicate a significant difference between the percentage of α-retinyl and retinyl species of the same type (Wilcoxon’s Signed Rank test, P < 0.05).