Vitamin A, chemical name as retinol, is the earliest vitamin being discovered. There are two types of Vitamin A: one is retinol which is the initial form of VA, it only exists in animals; another one is carotene. Retinol can be composited by β-carotene coming from plants. Inside the body, under the catalysis of β-carotene-15 and 15′-double oxogenase, β-carotene is transformed into ratinal which is returned to retinol by the performance of ratinal reductase. Thusβ-carotene is also called as vitamin precursor.