Vitamin A is an umbrella term for several fat-soluble substances such as retinol, retinyl palmitate and beta-carotene. Its various metabolites are essential for vision, cell differentiation, epithelial barrier function and immune function.
Levels of Vitamin A (beta carotene)
Vitamin A is an essential micronutrient that plays an important role in a wide range of physiological processes, including vision, immune response, cell differentiation and proliferation, intercellular communication and reproduction.It is a term that collectively encompasses retinol and its active metabolites, including retinal, retinyl ester and retinoic acid.
Vitamin A is obtained through the diet in two forms, since we cannot synthesize this vitamin. Preformed vitamin A (retinol and retinyl ester) is obtained from animal sources such as meat, dairy products and fish. Provitamin A (beta-carotenoid) is obtained from colored fruits and vegetables. Both forms of ingested vitamin A must be converted to retinal and retinoic acid after absorption to support biological processes.
Beta-carotenoids and carotenoids in general act as antioxidant substances that protect the body against reactive oxygen species or ROS and play an important role in the prevention of multiple conditions, including cardiovascular disease and diabetes. Studies suggest that carotenes have a protective effect on non-alcoholic fatty liver disease and lung cancer.
Vitamin A deficiency replaces normal epithelium with stratified, keratinizing epithelium in the eyes, periocular glands, respiratory tract, alimentary tract and genitourinary tract. Excess vitamin A causes acute and chronic adverse health effects.
Vitamin A deficiency is much more common worldwide than vitamin A toxicity. The World Health Organization (WHO) estimates that 3 million children develop clinical vitamin A deficiency annually, compared to the estimated 200 cases of vitamin A toxicity diagnosed annually.
The formation of vitamin A from provitamin A depends on the efficiency of absorption and the efficiency of conversion of the major active metabolites of vitamin A such as retinal, retinol and retinoic acid. Interindividual differences in the absorption and conversion of carotenoids to the biologically active forms of vitamin A have been observed, and studies suggest that certain single nucleotide polymorphisms in genes encoding enzymes involved in the absorption and conversion of beta-carotene may explain some of this variability.
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