caa a22 4500 388097140 CHVBK 20180307125306.0 cr unu---uuuuu 161130e199905 xx s 000 0 eng 10.1017/S0029665199000592 doi S0029665199000592 pii (NATIONALLICENCE)cambridge-10.1017/S0029665199000592 Optimal nutrition: vitamin A and the carotenoids [Elektronische Daten] There are two major dietary sources of vitamin A: easily absorbed retinyl palmitate in foods of animal origin, and poorly bioavailable carotenoids from plant foods. Plasma retinol is tightly controlled, probably by regulation of retinol-binding protein (RBP) formation in the liver, and only hormonal factors (e.g. oral contraceptives) and infection will alter the homeostasis. Delivery of retinol to the tissues is facilitated by the RBP-retinol complex; however, there is evidence that this mechanism can be bypassed when very high doses of vitamin A are given. Some retinyl ester may be released to tissues from chylomicrons when the latter bind to tissue lipoprotein receptors during their passage from the gut to the liver following a meal. High-dose vitamin A therapy is a means of rapidly improving vitamin A status in persons with sub-optimal vitamin A nutrition but there are dangers of toxic symptoms (e.g. teratogenicity) from excess vitamin A usage. Evidence is presented to suggest that the plasma retinol : RBP may be a guide to optimal vitamin A status, since values less than one frequently occur in less-developed countries and during infection. In contrast to plasma retinol, plasma carotenoids reflect the dietary intake of plant foods. However, absorption is limited by poor bioavailability and a saturable uptake mechanism in competition with other phytochemicals. Recent work on bioavailability suggests that the calculation of plant food vitamin A activity should be re-examined. Illness has little influence on plasma levels except by suppressing appetite. Carotenoids are generally regarded as non-toxic yet intervention studies with β-carotene in smokers have been associated with increased lung cancer and heart disease. Some carotenoids are important as vitamin A precursors, but the physiological importance of their antioxidant properties is not known and consequently the amount needed for optimal nutrition is uncertain. Copyright © The Nutrition Society 1999 Vitamin A nationallicence Retinol nationallicence Retinoic acid nationallicence Carotenoids nationallicence β-Carotene nationallicence Thurnham David I. Northern Ireland Centre for Diet and Health, University of Ulster, Coleraine BT52 1SA, UK Northrop-Clewes Christine A. Northern Ireland Centre for Diet and Health, University of Ulster, Coleraine BT52 1SA, UK Proceedings of the Nutrition Society Cambridge University Press 58/2(1999-05), 449-457 0029-6651 58:2<449 1999 58 PNS https://doi.org/10.1017/S0029665199000592 text/html Onlinezugriff via DOI 1 conference jats NATIONALLICENCE 856 40 https://doi.org/10.1017/S0029665199000592 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Thurnham David I. Northern Ireland Centre for Diet and Health, University of Ulster, Coleraine BT52 1SA, UK NATIONALLICENCE 700 1- Northrop-Clewes Christine A. Northern Ireland Centre for Diet and Health, University of Ulster, Coleraine BT52 1SA, UK NATIONALLICENCE 773 0- Proceedings of the Nutrition Society Cambridge University Press 58/2(1999-05), 449-457 0029-6651 58:2<449 1999 58 PNS CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-cambridge