**4. Phenolic compounds' effects on human health**

Epidemiological studies have repeatedly demonstrated an inverse relationship between the risk of chronic human diseases and polyphenol-rich diet consumption. Polyphenols contain phenolic groups that can accept an electron to form relatively stable phenoxyl radicals, interfering with chain oxidation reactions in cellular components. It is well established that foods and beverages high in polyphenols may enhance plasma antioxidant capacity. This increase in plasma's antioxidative capacity following consumption of polyphenol-rich foods can be explained by the presence of reducing polyphenols and their metabolites, their effects on the concentrations of other reducing agents (polyphenols' sparing effects on other endogenous antioxidants), or their effect on the absorption of pro-oxidative food components. Antioxidant consumption has been linked to decreased levels of oxidative damage to lymphocytic DNA. Similar findings have been made with polyphenol-rich foods and beverages, indicating polyphenols' protective properties. There is mounting evidence that polyphenols, as antioxidants, may protect cell constituents from oxidative damage and thus reduce the risk of developing various degenerative diseases associated with oxidative stress. **Figure 3** shows the different pharmacological functions of polyphenols.

The potential health benefits of dietary phenolics are dependent on their absorption and metabolism, which are determined by their structure, which includes their conjugation with other phenolics, their degree of glycosilation/acylation, their molecular size, and their solubility. These steps occur at various points throughout the small intestine's passage into the circulatory system and subsequent portal vein transport to the liver. Because polyphenol metabolites are rapidly eliminated from plasma, daily consumption of plant products is necessary to maintain adequate metabolite concentrations in the blood. However, it is important to keep in mind that the polyphenols found in the most abundant amounts in the daily diet are not necessarily the ones with the highest bioavailability. For example, hydroxycinnamic acids are found in high concentrations in foods, but their intestinal absorption is reduced by esterification. Additionally, differences in cell wall structures, glycoside distribution within cells, and phenolic compound binding to the food matrix can affect phenolic compound bioavailability. Epidemiological evidence to date indicates that polyphenols perform critical functions such as inhibiting pathogens and decay microorganisms, preventing triglyceride deposition, lowering the incidence of non-communicable diseases such as cardiovascular disease, diabetes, cancer, and stroke, and exerting anti-inflammatory and anti-allergic effects via processes involving reactive

**Figure 3.** *Different pharmacological functions of polyphenols.*

#### *Medicinal Plants and Phenolic Compounds DOI: http://dx.doi.org/10.5772/intechopen.99799*

oxygen species. These protective effects are partially attributed to phenolic secondary metabolites. Initially, it was believed that the protective effect of dietary phenolics was due to their antioxidant properties, which resulted in a decrease in the body's free radical levels. However, there is emerging evidence that the metabolites of dietary phenolics, which are found in the circulatory system at concentrations ranging from nmol/L to low mmol/L, exert modulatory effects in cells via their selective actions on various components of intracellular signalling cascades critical for cellular functions such as growth, proliferation, and apoptosis. Polyphenols are thought to exert their antioxidant capacity in a variety of ways, depending on the hydroxylation state of their aromatic rings, including (i) radical scavenging, (ii) chelation and stabilisation of divalent cations, and (iii) modulation of endogenous antioxidant enzymes. Phenolic acids, hydrolysable tannins, and flavonoids have anti-carcinogenic and anti-mutagenic properties because they act as antioxidants for DNA, inactivating carcinogens, inhibiting pro-carcinogen activation enzymes, and activating xenobiotic detoxification enzymes. Flavonoids and L-ascorbic acid, in particular, have a synergistic protective effect against oxidative DNA damage in lymphocytes. Chlorogenic and caffeic acids are both antioxidants in vitro and may inhibit the formation of mutagenic and carcinogenic N-nitroso compounds in vitro. Flavonoids, catechins, and their derivatives are being investigated as potential therapeutic agents in studies of degenerative diseases and brain ageing processes, and may act as neuroprotective agents in progressive neurodegenerative disorders such as Parkinson's and Alzheimer's disease. Consumption of flavonoids results in a decrease in LDL oxidation. Resveratrol, also known as trans-3,5,4′-trihydroxystilbene, is the most well-known health-promoting molecule found in grapes and red wine. It has been studied for its effects on genes, as well as the heart, breast, prostate, uterus, and immune system. Additionally, recent research indicates that resveratrol supports healthy nerves and critical brain functions, such as cognitive processes. Tannins, more commonly referred to as tannic acid, have been implicated in experimental animals in reducing feed intake, growth rate, feed efficiency, net metabolizable energy, and protein digestibility. As a result, foods high in tannins, such as betel nuts and herbal teas, are regarded as having a low nutritional value. Numerous studies, however, indicated that the primary effect of tannins was not an inhibition of food consumption or digestion, but rather a decrease in the efficiency of converting absorbed nutrients to new body substances. Tannins' anticarcinogenic and antimutagenic properties may be related to their antioxidative capacity, which is critical for cellular oxidative damage protection, including lipid peroxidation. Tannins have also been reported to have additional physiological effects, including the acceleration of blood clotting, the reduction of blood pressure, the reduction of serum lipid levels, the induction of liver necrosis, and the modulation of immune responses. Polyphenols (phenolic acid, stilbenes, tannins, isoflavones, and catechins found in green tea) have been shown to inhibit the reproduction and growth of a variety of fungi, yeasts, viruses, and bacteria, including Salmonella, Clostridium, Bacillus, or *Chlamydia pneumoniae*, *Vibrio cholerae*, and enterotoxigenic *E. coli* (ETEC). Because phenolics act as a natural defence mechanism against microbial infections, they can be used in food processing to extend the shelf life of certain foods, such as catfish fillets [4].
