Vitamin A is an extremely important and easily deficient fat-soluble vitamin necessary for the human body to maintain normal metabolism and function. It is not a single compound, but a series of retinol derivatives including retinol, retinal, retinoic acid, retinyl acetate and retinyl palmitate. Vitamin A only exists in animals, and is found in large quantities in fish, especially cod liver oil. Plants do not contain vitamin A, but many vegetables and fruits contain provitamin A - carotene, which can be broken down into vitamin A in the small intestine. Vitamin A has a wide range of important physiological functions in the human body, and is essential for maintaining healthy vision, skin and mucosal integrity, immune function, growth and development, and reproduction.