Final answer:
The RDA for vitamin A is expressed in Retinol Activity Equivalents (RAE), taking into account the activity of both preformed vitamin A and provitamin A carotenoids like β-carotene. This vitamin is essential for vision and immune function and is stored in the liver.
Step-by-step explanation:
The recommended dietary allowance (RDA) for vitamin A is expressed in Retinol Activity Equivalents (RAE), which account for the activity of both preformed vitamin A and provitamin A carotenoids like β-carotene.
β-carotene is a potent precursor of vitamin A that can be found in orange vegetables such as carrots and sweet potatoes. It is converted to active vitamin A through a process involving the enzyme β-carotene 15, 15'-oxygenase, mainly in the liver in humans.
Vitamin A is essential for numerous bodily functions, including vision and immune function, and is transported in the body bound to proteins such as retinol-binding protein (RBP) and prealbumin.
The vitamin is stored in the liver as palmitate esters and is derived from dietary sources through the conversion of β-carotene to vitamin A and from the hydrolysis of retinyl esters to retinol in the intestine.
It is important to maintain an adequate intake of vitamin A to support overall health, but excessive intake can lead to conditions such as hypervitaminosis due to the fat-soluble nature of vitamin A, which allows it to accumulate in the body's lipid stores.