Vitamin A

A deficiency of vitamin A is associated with increased morbidity and mortality, most likely because of increased severity of infections [85].Vita-min A and related retinoids maintain the integrity of epithelial boundaries and the production of mucosal secretions. Some of the immunological abnormalities following vitamin A inadequacies include a reduced number of leukocytes, reduced lymphoid organ weights, reduced circulating levels of complement, impaired T-cell functions, and decreased resistance to immunogenic tumours.

Vitamin A supplementation studies showed decreases in respiratory infections and neutrophil counts and increases in reticuloendothelial system function, lymphocyte proliferation, tumour resistance, graft rejection and cytotoxic T-cell activities. Supplementation above that required to maintain normal vitamin A stores results in adjuvant effects. Excess vitamin A increases antibody and cell-mediated immune responses, stimulates Kupffer cells and potentiates some types of liver toxicity and gouty arthritis [86]. The inhibition of T-cell apoptosis by retinoic acid may contribute to its adjuvant effect [87].

The vitamin A precursor p-carotene is generally considered an antioxidant with activities that are independent of its provitamin A function. Some of its reported benefits include protecting host cells and tissues from oxidation by the respiratory burst and promoting lymphocyte proliferation, T-cell functions, cytokine production and cell-mediated cytotoxicity. However, Bates [88] cautioned that carotenoids can exhibit both pro-oxidant and antioxidant activity.

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