Objective: A human intervention trial was conducted to determine the effects of consumption of carotenoid-rich vegetables on levels of retinol (ROL) and the active vitamin A metabolite all-trans retinoic acid (ATRA).
Methods: The study was divided into four periods, each lasting 2 wk: weeks 1-2, a low carotenoid period; weeks 3-4, daily consumption of 330 mL of tomato juice; weeks 5-6, 330 mL of carrot juice daily; and weeks 7-8, 10 g of dried spinach powder daily. ROL and ATRA concentrations were measured after each 2-wk study period.
Results and conclusion: This study shows that consumption of carrot juice containing high concentrations of the pro-vitamin A carotenoid beta-carotene results in slightly, non-significantly increased plasma ROL concentrations and strong, significantly increased (almost double, from 1.2 +/- 0.3 to 2.0 +/- 0.31 ng/mL) plasma concentrations of ATRA, whereas consumption of tomato juice and spinach powder results in no significant alteration in concentrations of plasma ROL and ATRA.