**1. Introduction**

Plant metabolites can be divided into two groups: primary metabolites, which are involved in the nutrition and the essential metabolic processes (e.g. carbohydrates, lipids and proteins); and secondary ones, which have an important function in the interaction between a plant and

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its environment, such as pigments or defensive compounds. Secondary metabolites included a group of compounds known as phenolic; in this group, we can find the flavonoids.

Flavonoids are a large group of natural substances with variable structures present almost in all growing parts of the plants, being reported as the most abundant plant pigment along with chlorophyll and carotenoids, also providing fragrance and taste to fruits, flowers and seeds, which makes them attractants for other organisms [1, 2]. These compounds are also one of the largest groups of secondary metabolites [3]. Besides their relevance in plants, flavonoids are important for human health because of their high pharmacological activities. Recent interest in these substances has been stimulated by the potential health benefits arising from the anti‐ oxidant activities of these polyphenolic compounds.

In this chapter, we focused in the antioxidant, antibacterial, antiviral and anti‐inflammatory activities of flavonoids, which according to many studies are conferred mainly by the content of hydroxyl groups attached to base structures of these compounds. Biochemical actions of these compounds depend primarily on the presence and position of their substituent groups, which can affect the metabolism of each one [4]. One of the most important characteristics of flavonoids is that they often occur in the glycosidic form, which possibly let them take place in the gastrointestinal tract [5].

However, they attract attention due to their antioxidant activity and reduce free radical for‐ mation and also scavenge free radicals [6]. Flavonoids have other important biological activi‐ ties such as protect skin from UV light exposure, protect DNA from damage, strengthening of capillaries, anti‐inflammatory effect and protective action against radiation, moistening, softening, soothing, antiseptic and other. Due these properties, flavonoids can be used as ingredients in the production of cosmetics and pharmaceutical products [7, 8].

Nowadays, flavonoids are major bioactive compounds known for their potential health ben‐ efits, which have been used against many chronic diseases such as cancer, antiviral, inflam‐ mation, cardiovascular and neurodegenerative disorders; it is widely assumed that active dietary constituents are antioxidant nutrients present in fruits and vegetables [9].
