beta-Carotene in the human body: metabolic bioactivation pathways - from digestion to tissue distribution and excretion.

beta-Carotene intake and tissue/blood concentrations have been associated with reduced incidence of several chronic diseases. Further bioactive carotenoid-metabolites can modulate the expression of specific genes mainly via the nuclear hormone receptors: retinoic acid receptor- and retinoid X receptor-mediated signalling. To better understand the metabolic conversion of beta-carotene, inter-individual differences regarding beta-carotene bioavailability and bioactivity are key steps that determine its further metabolism and bioactivation and mediated signalling. Major carotenoid metabolites, the retinoids, can be stored as esters or further oxidised and excreted via phase 2 metabolism pathways. In this review, we aim to highlight the major critical control points that determine the fate of beta-carotene in the human body, with a special emphasis on beta-carotene oxygenase 1. The hypothesis that higher dietary beta-carotene intake and serum level results in higher beta-carotene-mediated signalling is partly questioned. Alternative autoregulatory mechanisms in beta-carotene / retinoid-mediated signalling are highlighted to better predict and optimise nutritional strategies involving beta-carotene-related health beneficial mediated effects.