The positive characteristics of polyphenol applications in vivo and in vitro should be further investigated before they are systematically used in practice as supplements to the basic livestock diet.
2. Polyphenols and Their Benefits
“Let food be your medicine, and medicine your food”, said Hippocrates more than 2000 years ago, showing that the benefits of natural sources of healthy food have been appreciated since ancient times
[1814]. Plant foods, including fruits and vegetables containing active substances, play a key role in human and animal health. Plants synthesize polyphenols under stressful conditions during adaptation to their environment. Polyphenols are an important source of active substances in pharmaceutical products
[1915]. The widespread use of polyphenols as secondary metabolites is an essential part of animal and human nutrition and is of great interest to scientists because of their biological properties. In the last few decades, scientists have been paying close attention to the health benefits of polyphenols
[2016][2117]. Although the beneficial effects of polyphenols in both humans and animals have been confirmed, there are concerns regarding the potential health hazards of excessive polyphenol consumption
[2218][2319]. The most vulnerable groups are pregnant animals and their fetuses
[2420]. Therefore, it is essential to understand the impacts of plant polyphenol consumption on reproductive health.
In plants, these compounds are usually synthesized as defenses against physiological and environmental stimuli. More attention has been paid in recent years to the benefits of polyphenols to human and animal health. This has been observed based on the chemical and biological activity and the obtained results
[2521][2622][2723]. Polyphenols have an advantage over other substances due to their good availability, low toxicity, and specific activity, while their biggest disadvantage is their fast metabolism and low bioavailability. A complex mixture of polyphenols is found in food, and various factors affect their diversity, primarily environmental (e.g., rain, pedological soil composition, sun exposure) and biochemical (e.g., storage conditions, degree of maturity, and method of preparation) factors
[95]. Glycosides and aglycones of polyphenols are the most important plant secondary metabolites in both human and animal nutrition, having significant health effects
[2824][2925]. A review of the literature on polyphenols, which includes more than 20,000 papers, confirmed that a significant proportion of these molecules have inhibitory activity against enzymes, as well as antitumor, anti-inflammatory, antibacterial, and antifungal activities
[3026][3127], reflecting the extensive benefits of polyphenols for the animal community, including improvements in memory and cognition in animals and humans
[3228]. The bioavailability and kinetics of different polyphenols are very variable, so the knowledge of the fate of these compounds is quite unclear. In addition, based on their intensive metabolism in the gut and liver
[3329][3430] of the parent compound, circulating metabolites very often differ from the parent compound, which further complicates the study of in vitro biological activity in animal models. From the above, it can be concluded that understanding the kinetics and bioavailability of polyphenols is crucial to know and understand the health benefits of these compounds.
3. Division of Polyphenols and Their Sources in the Diet
The term “polyphenol” is used for compounds synthesized exclusively by the shikimin–phenylpropanoid and shikimin–polyketide pathways, which are constructed of more than one phenolic moiety and do not exhibit fundamental nitrogen functionality
[3531]. Polyphenols are plants or synthetic compounds consisting of one or more phenolic units. Most of them are glycosylated or can bind to other phenols. In addition, they can conjugate with other compounds such as glucuronic acid, galacturonic acid, or glutathione during metabolism
[3632]. The diversity and wide distribution of polyphenols in plants have led to different methods of categorizing these natural compounds
[3531][3632]. Polyphenols are classified according to their source of origin, biological function, and chemical structure. In addition, most polyphenols in plants are present in the form of glycosides with different carbohydrate units and acylated carbohydrates at different positions of the polyphenolic scaffold. The classification of polyphenols in this entry is based on the chemical structure of the aglycone. Thus, polyphenols are divided into two main groups, namely flavonoids and non-flavonoids
[3531].
3.1. Flavonoids
The class of flavonoids contains more than 4000 low molecular weight secondary plant metabolites. They are formed from aromatic amino acids
[3632][3733]. Flavonoids are classified into flavonols, flavones, flavanols, flavanones, anthocyanins, isoflavones, and proanthocyanidins (
Figure 1)
[3733][3834].
Figure 1. Classification and examples of structures of flavonoids.
The basic part of the structure of flavonoids is the core, which consists of 15 carbon atoms arranged in three rings (C6-C3-C6) or a diphenylpropane skeleton, designated A, B, and C (
Figure 1). Flavonoids are usually found as glycosylated derivatives in plants and contribute to the attractive colors of the flowers, leaves, and fruits
[3935]. The flavones apigenin and luteolin are commonly found in cereals and aromatic herbs (parsley, rosemary, and thyme), while their hydrogenated analogues hesperetin and naringin are found almost exclusively in citrus fruits. The flavonols quercetin and kaempferol are abundant in vegetable peels and fruits, with the exception of onions. Isoflavones are the most abundant in legumes, such as soybeans, black beans, and chickpeas. The flavanols catechin, epicatechin, epigallocatechin, and their gallate esters are ubiquitous in plants. Anthocyanidins and their glycosides (anthocyanins) are natural pigments and are most abundant in berries and red grapes. Flavonoids play various roles in the ecology of plants. Because of their attractive colors, flavones, flavonols, and anthocyanidins can serve as visual signals for pollinating insects. Because of their bitterness, catechins and other flavonols can provide a defense system against insects that are harmful to plants. They can also act as stress protectants in plant cells by trapping the ROS produced by the photosynthetic electron transport system. In addition, due to their favorable UV absorption, flavonoids protect plants from the sun’s UV radiation and remove the ROS produced by UV rays
[3935].
3.2. Non-Flavonoids
Non-flavonoids can be classified into lower molecular weight compounds such as phenolic acids, lignans, and stilbenes, and more complex structures such as tannins (
Figure 2).
Figure 2. Classification of non-flavonoids and examples of their chemical structures.
The structural characteristics of simpler non-flavonoids are described below.
3.2.1. Phenolic Acids
Phenolic acids are abundant in food and are divided into two classes: benzoic acid derivatives and cinnamic acid derivatives. Phenolic acids and flavonoids are the most abundant polyphenols in foods—they account for about one-third and two-thirds of the total sources, respectively
[3632][3733].
The content of hydroxybenzoic acid in edible plants is generally low, except in some red fruits, black radish, and onions, which may have concentrations of several tens of mg/kg fresh mass. Hydroxycinnamic acids are more common than hydroxybenzoic acids and consist mainly of p-coumaric, caffeic, ferulic, synaptic, chlorogenic, and rosmarinic acids
[3834].
3.2.2. Stilbenes
Stilbenes are a small and important class of non-flavonoid polyphenols characterized by a 14-carbon skeleton. They are built of two benzene rings connected by an ethylene bridge (
Figure 1)
[3733]. In the central part of the structure, two aromatic rings are connected to ethene, and ethene hydrogen can be in the
cis and
trans positions. In nature, stilbenes occur most frequently in the form of
trans stereoisomers. To date, more than 400 different stilbene compounds have been identified, most of which are derived from
trans resveratrol (3,5,4′-trihydroxy-
trans stilbene). Due to the complexity of qualitative–quantitative stilbene analysis, most studies have focused on simple stilbenes, such as resveratrol, piceid, pterostilbene, and piceatannol.
The knowledge on stilbenes mainly relates to the protection of plants against biotic (phytopathogenic microorganisms and herbivores) and abiotic (e.g., radiation and tropospheric ozone) stress. In this way, they repel attacks by having a direct toxic effect on the pathogen, while on the other hand they act as antioxidants and protect the cells from oxidative stress
[3733].
3.2.3. Lignans
Lignans are diphenolic compounds that contain a 2,3-dibenzylbutane structure formed by the dimerization of two cinnamic acid residues (
Figure 1). The richest source is flaxseed, which contains secoisolariciresinol and matairesinol. One of the most common forms of lignans is secoisolariciresinol (2-(4-hydroxy-3-methoxybenzyl)-3-(3-metoxybenzyl)butene-1-4-diol) diglycoside. Seicoisolariciresinol and matairesinol ingested with food are converted by the intestinal microflora into mammalian lignans, enterodiol and enterolactone, which are absorbed via the enterohepatic circulation. Mammalian lignans have a chemical structure similar to natural estrogen, and are thought to act as selective modulators of estrogen receptors and to have anticancer activity
[4036].