Vitamins are essential organic compounds required in small amounts for the normal functioning of the body. With a few exceptions (such as Vitamin D, which is synthesised in the skin upon exposure to sunlight), the human body generally cannot synthesise vitamins in adequate quantities. Therefore, they must be supplied through the diet. In contrast, the human body naturally produces DNA (during cellular replication), hormones (via endocrine glands), and enzymes (via protein synthesis).