Polyphenols are another class of compounds which are regularly listed in the active constituents of plants with medicinal, dietary or other beneficial applications. They are often referred to together with flavones and flavonoids with which they have common structural characteristics. The term polyphenols is often used rather loosely and there is no universal agreement on what should be included but, whatever the criteria, all of these compounds contain more than one phenol unit.
Phenol itself is a simple chemical compound, also known as carbolic acid, and is not generally found in nature by itself but there are many modifications of it which all comprise the class of compounds called phenols. Some of these are quite simple compounds and while most of them are industrial products, many are found in nature such as the amino acid tyrosine and salicylic acid, which is derived from the bark of the willow tree and is a precursor for aspirin. Phenols are also found in plant products such as olive oil where they play an important role in the oil’s taste and nutritional profile.
Polyphenols are much more widely found in nature and number in the thousands. They have more complicated structures than the simple phenols and although they are often fascinating from a chemical point of view, their structures are not very meaningful to the non-specialist, so here are just a couple of the more simple and possibly more familiar ones. Curcumin is found in turmeric and quercetin is found in a wide range of seeds, fruits, vegetables etc.
Polyphenols are secondary metabolites, i.e. they are produced by the metabolic processes occurring in an organism but do not play a role in the normal functioning and development of that organism. Secondary metabolites act as means of defence against herbivores, microbes, viruses or competing plants, as signal compounds to attract pollinating or seed dispersing animals, and as protection of plants from ultraviolet radiation and oxidants. Plants produce a more varied selection of secondary metabolites than animals, seemingly because they cannot rely on physical mobility to protect themselves.
All polyphenols absorb UV radiation and very many of them also absorb visible light. The UV absorbing property is thought offer a first line of defence in plants by acting as a sunscreen against the damaging effects of UV radiation on a plant’s DNA, proteins and membranes. This property also means that they are highly visible to pollinators, for example, many species of which are able to “see” beyond the normal visible spectrum into the UV range, while the pigmentation of flowers, which is often due to polyphenols, is also important in attracting herbivores which can aid in seed dispersal. On the other hand, polyphenols often have a very bitter of astringent taste and can therefore act as a defence against undesirable herbivores.
One of the most important roles of polyphenols is as antioxidants and the protection they provide against the undesired effects of reactive oxygen species (ROS) which are produced by the metabolism of oxygen. ROS are essential for the proper functioning of organisms but when the normal mechanisms for their regulation fail, they can cause damage to DNA and cells, a situation known as oxidative stress. In such cases, antioxidants such as polyphenols can assist in bringing ROS levels in a plant organism down to normal levels. Since oxidative stress is not confined to plants but is also an important risk factor in animals, the importance of foods with available polyphenols is immediately apparent. The degree of this availability is, however, is not always easy to evaluate since polyphenols are often in a form such that they are released during the digestive process. However, it is probably due to this method of release that the ideal (and cheapest) way to get the necessary quantities is via the consumption of foods such as fruits, vegetables, nuts etc rather than from supplements whose effectiveness is often not properly supported by the available evidence.
One Comment