Optimal nutrition: vitamin A and the carotenoids

Abstract

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.

Bioconversion of Carotenoids in Five Fruits and Vegetables to Vitamin A Measured by Retinol Accumulation in Rat Livers

“…Furthermore, while liver retinol significantly increased during the repletion period (except for parsley), plasma retinol did not increase during this period due to any treatment. These results are in agreement with Thurnham and Northrop-Clewes (1999), who suggested that there is no relation between retinol content in plasma and diet, unless the liver retinol pool is depleted noticeably. It seems that a homeostatic process is involved in maintaining the retinol levels in plasma, despite the retinol depletion in liver, as suggested by Zacaria-Rungkat et al (2000).…”

A comparison of lutein and zeaxanthin concentrations in formula and human milk samples from Northern Ireland mothers

“…Breast milk concentrations of lutein and zeaxanthin appear to differ greatly between individuals, as is also seen in plasma (Thurnham & Northrop-Clewes, 1999). Jackson et al (1998) and Macias and Schweigert (2001) also reported a high between-subject variation in his study of milk carotenoid concentrations.…”

Influence of habitual diet on antioxidant status: a study in a population of vegetarians and omnivores

“…The results of this study indicate that there were no differences between vegetarians and omnivores in the level of cellular endogenous antioxidants, such as in the activities of GPX, SOD and GST in erythrocyte, in the level of GSH in erythrocytes and in the plasma levels antioxidants nutrients (vitamin C, retinol and a-tocopherol) despite the increased dietary intakes of these antioxidants by the vegetarian group. The reason for the lack of difference in the antioxidant vitamin status between the two groups might partly be due to homeostasis and/or saturation of retinol and a-tocopherol (Thurnham and Northrop-Clewes, 1999), and partly to plasma vitamin C level reflecting current vitamin C intake rather than the habitual intake (Bates and Thurnham, 1997). Moreover, since this was a 'convenient cohort', the healthy volunteer effect, particularly in the omnivore group could not be ruled out.…”