Flavonoids are a group of secondary plant substances that are chemically derived from the basic structure of flavan (2-phenylchroman). They consist of two aromatic rings connected by a tetrahydropyran ring. Most of the flower pigments belong to the flavonoids. There are about 8,000 such compounds in nature. They are formed by various oxidation states in the oxygen-containing ring, different substitutions on the aromatic rings and the addition of sugars (glycoside formation). Flavonoids are universally present in plants and therefore also in human food. Animals cannot produce flavonoids; they absorb them from food.