Advances in vitamin A research in 1999 and 2000 have improved the understan
ding the molecular processes through which beta -carotene and other provita
min A carotenoids are converted to vitamin A, the roles of cellular retinoi
d-binding proteins that serve as retinoid chaperones during metabolism, the
regulation of retinoid transport, and the nature and regulation of several
enzymes required for the absorption, storage, activation, and inactivation
or degradation of retinoids. Not only has a clearer picture emerged of spe
cific molecular processes, but it is also becoming evident that whole-body
retinoid homeostasis is facilitated by close communication among organs due
to the rapid interorgan recirculation of retinoids, and by the "autoregula
tion" by retinoic acid of several enzymes and retinoid-binding proteins tha
t mediate retinoid homeostasis. Curr Opin Gastroenterol 2001, 17:184-192 (C
) 2001 Lippincott Williams & Wilkins, Inc.