Vitamin A: functions and food sources

Vitamin plays a central role in many essential biological processes. Vitamin A (retinol) functions in reproduction, growth, the maintenance of skin and mucous membranes and the visual process.

Vitamin A is needed to process incoming light to visual images and to keep the eye’s surface healthy. Vitamin is known to be involved in fetal development and in the regulation of proliferation and differentiation of many types of cells throughout life. It plays a role in immune function, both as a cell regulator and by helping maintain the skin and mucous membranes.

Vitamin A is normally transported in the blood linked to a specific protein, retinol binding protein (RBP). Specific proteins on cell surfaces and within cells are also involved with intracellular transport of the vitamin.

Vitamin A is fat soluble and is primarily stored in the liver, where RBP is synthesized. The liver holds over 90 percent of the body’s vitamin A reserves, with the rest deposited in fat tissue, lungs and kidneys.

In a well nourished person, vitamin A stores are generally sufficient to last many months on a vitamins A-deficient diet before signs of deficiency appear.

The initial symptoms of vitamin A deficiency are night blindness and keratinization of hair follicles. Continued deficiency leads to damage to eye tissue and irreversible blindness.

The US recommended Daily Allowance (RDA) of vitamin A for adults is 5000 IU (1000 retinol equivalents). Main dietary sources of vitamin A are the carotenoids from fruits and vegetables. These carotenoids do seen to have a separate function as antioxidants in addition to their pro-vitamins A function.

Rich dietary sources of retinol (preformed vitamin A) include dairy products, eggs and organ meats. Some carotenoids (found in deep-yellow and dark green vegetables) can be converted to vitamin A during digestion. In the US diet, approximately half of the vitamin A activity is derived from B-carotene and other carotenoids.
Vitamin A: functions and food sources
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