The multifaceted nature of retinoid transport and metabolism

Since their discovery over a century ago, retinoids have been the most studied of the fat-solublevitamins. Unlike most vitamins, retinoids are stored at relatively high concentrations in the body to bufferagainst nutritional insufficiency. Until recently, it was thought that the sole important retinoid deliverypathway to tissues involved retinol bound to retinol-binding protein (RBP4). More recent findings, however,indicate that retinoids can be delivered to tissues through multiple overlapping delivery pathways, involvingchylomicrons, very low density lipoprotein (VLDL) and low density lipoprotein (LDL), retinoic acid boundto albumin, water soluble β-glucuronides of retinol and retinoic acid, and provitamin A carotenoids. Thisreview will focus on explaining this evolving understanding of retinoid metabolism and transport within thebody.