Plant polyphenols and human health

Plant polyphenols are a group of compounds found in many plants and their parts with several hydroxyl groups on aromatic ring, also known as phenol, structures.  They are the secondary plant metabolites, meaning the substances that have little or no role in photosynthesis, respiration, or growth and development, but which may accumulate in surprisingly high concentrations. They usually provide protection to plants against many herbivores and microbes. They also provide color, taste (such as bitter, tart, astringent, and more), and structural rigidity and strength (with lignin) to plants and their parts, such as the fruits, vegetables, seeds, bran, bark, pulp, juice, nectar, and more. Some of the well known plant polyphenolics are tannins, lignins, lignans, anthocyanins, quercetin, and resveratrol, among others. While lignin is the most abundant phenolics in nature, condensed tannins (also known as proanthocyanidins) are also ubiquitously present in virtually all families of plants, and may comprise up to 35% of the dry weight of certain leaves.

Polyphenols were initially and are often considered antinutrients, compounds that interfere with the absorption of other nutrients, such as protein, as a mechanism of plant defense against herbivores. While it is true, many of the polyphenolics or phytochemicals are equally beneficial for our health and nutrition. Unlike the traditional vitamins, phytochemicals as dietary components are not essential for short term well-being. While our body has specific mechanisms for the accumulation and retention of vitamins, most of the phytochemicals are treated as non-nutrient xenobiotics and metabolized so as to eliminate them efficiently. Regardless, they are high in antioxidants and accord protection against oxidative stress in our daily lives. 

Interest in plant polyphenols have increased tremendously over the past several years. Numerous studies are conducted each year to elucidate the effect, actions, and mechanisms by which plant polyphenols exert protective health effects in humans. As a result, there is considerable evidence that diets rich in fruit and vegetables can reduce the incidence of non-communicable diseases of the chronic nature, such as diabetes, metabolic syndrome, cardiovascular disease, cancer, Alzheimer’s, and urinary tract disease, among others. Although most of the protective effects were initially attributed to their high antioxidant capacity and the resultant decrease in oxidative stress, research has emerged lately that they also exert modulatory effects on different components of the intracellular signaling pathways vital for such cellular functions as growth, proliferation and apopotosis.  

Several thousand polyphenols are known to be present in higher plants, while several hundred have been found in edible plants. The health effects of polyphenols depend on their type, amount consumed, bioavailability, and some other factors such as the physiological state and other foods that are consumed along with polyphenols. Some of the foods that are high in plant polyphenols include fruits, vegetables, spices, red wine, green tea, and cocoa etc. They are also present in other foods such as sorghum, millet, barley, dry beans, peas, black beans, and other legumes. Among the fruits, berries are high in polyphenols. Among the by-products, seeds, skins, and different types of bran contain high concentrations of polyphenols.