**3. Grape Polyphenols: An Effective Tool**

Adapted diet and physical activity are known for several years as central countermeasures to avoid the deleterious physiological effects of obesity and aging on tissue homeostasis and to promote a healthy life. Treatment of obesity is difficult, and initially based on lifestyle change, diet recommendations, and increased physical activity [77] but, whereas it is actually effective, it is associated with very low compliance. Aging is inevitable but it is desirable to age in a healthy way, therefore the purpose of our society is now to increase the rate of healthy aging to avoid harmful consequences on skeletal muscle mass and function and to limit frailty and dependence. As stated above, oxidative stress and chronic inflammation are core mechanisms associated with obesity and aging. Among the numerous natural chemical compounds tested for their antioxidant and anti-inflammatory properties, grape polyphenols present great interest.

Attention to the importance of dietary intake of polyphenols was ignited by the phenomenon called the 'French Paradox', first described by Serge Renaud from the University of Bordeaux in 1992. According to his observations, the French population when compared to other Western populations whose diet is rich in saturated fatty acids (e.g., the American population), shows a much lower incidence of coronary heart disease and associated mortality [78]. In fact, adherence to the Mediterranean diet which includes mainly plant-derived foods and red wine consumption has been associated with a lower risk of chronic diseases and mortality [79] and a lower frailty index in older adults [80]. The first

explanation put forward to explain these associations was the moderate consumption of red wine. Undeniably, among fruits, grapes (but also red wine, grape seeds, and grape pomace) contain high amounts of polyphenols [81] (Table 1), although not being the richest source. This does not affect its great biological interest due to the exceptional variety of polyphenol families and molecules of known beneficial activity on human health that can all be found in it (Table 2).

**Table 1.** Polyphenols characterization, total polyphenol content (TPC) of major well-known vegetal sources of dietary polyphenols. TPC is expressed in mg/100 g of fresh weight (FW). Data in the table were extracted from the PhenolExplorer database [82–84].



**Table 2.** Main effects of polyphenols present in grapes on mechanisms involved in obesity and aging.


#### **4. Structure and Function of Grape Polyphenol**

Recently, the polyphenols intake has been estimated to be 1607 mg/d in a French wellbalanced diet [81] which is above than previously found in epidemiological studies [116]. Grape is one of the major fruit crops produced worldwide and wine is the most widespread alcoholic beverage consumed. Moreover, the by-products of wine production are also a rich resource of biologically active molecules, which need to be emphasized since they possess the same phytochemicals of wine without the deleterious effect of alcohol [117].

Accumulating evidence shows that grape polyphenols regulate several mechanisms to prevent oxidative stress and inflammatory-mediated diseases [118] (Figure 1). Thanks to the phenolic groups, polyphenols can neutralize ROSP due to a transfer of electrons and/or hydrogen atoms to form phenoxyl radicals which are relatively more stable, thanks to resonance stabilization [42]. Polyphenols can also activate the endogenous antioxidant system via the ancestor Nrf2- Kelch-like ECH-associated protein 1 (KEAP1) signaling pathway [119,120]. Then, in aged muscle, grape polyphenols can increase the antioxidant enzymes such as catalase, SOD, glutathione reductase, and GPx leading to a reduction of muscle lipid damage associated with improvement in muscle function [121]. Resveratrol, and grape polyphenols, can increase the endurance capacity of muscle, which is related to increase reliance on mitochondrial lipid oxidation [107,122]. Polyphenols can increase mitochondria biogenesis and function due to their ability to activate sirtuin 1 (SIRT1) a class III histone deacetylase, NAD+-dependent (Figure 1). SIRT1 regulates several important

processes such as gene expression, metabolism, and oxidative stress response via deacetylation of lysine groups [123]. More specifically, SIRT1 activates PGC-1α, mitochondrial transcription factor A (TFAM), nuclear receptor peroxisome proliferator-activated receptor (PPAR), and nuclear respiratory factor (NRF), all involved in mitochondria biogenesis and function. Moreover, SIRT1 not only modulates mitochondrial function but also regulates FOXO protein acetylation which in turn modulates manganese SOD and catalase expression increasing the defense against oxidative stress [124]. The energy sensor, AMPK, could also activate SIRT1 since it regulates the cellular levels of NAD+. Grape polyphenols can phosphorylate and activate AMPK [122]. Then, polyphenols can increase lipid oxidation since it is well known that AMPK controls malonylCoA level a major inhibitor of the CPT and lipid entry into mitochondria [125] but also AMPK controls mitochondrial biogenesis via the deacetylation and activation of PGC-1α by SIRT1 [107]. Indeed, grape polyphenols supplementation increases muscle PGC-1α alpha mRNA and maintains CPT1 mRNA after fructose ingestion in first-degree relatives of T2D patients, suggesting a conserved lipid oxidation capacity which could explain the body weight gain decrease in this group compared to placebo [126]. The inflammation pathway is also negatively regulated by SIRT1 through NFkB acetylation explaining in part the anti-inflammatory activity of polyphenols. On the other part, resveratrol has been found to alleviate obesity-induced skeletal muscle inflammation due to a shift in macrophage toward an anti-inflammatory profile and a decrease in TLR4 expression [127]. In the context of muscle atrophy, resveratrol has demonstrated an ability to decrease atrogin-1 and MuRF-1 expression depending on SIRT1 activity [128]. Then, polyphenols but mostly resveratrol have been described as exercise mimetic compounds according to their positive regulation of muscle mass [129]. Polyphenols are secondary metabolites of vegetables and fruits (largely in berries) with important physiological and defense functions as pigments and antioxidant molecules. They are accumulated in the berries, especially in the solid parts (mainly skin and seeds), in response to various internal and external stimuli (growth, free radicals excess, ultraviolet (UV) radiations, fungi, insects, and bacteria attack) [130]. Then, polyphenols content in grapes is highly variable depending on the cultivar, maturity, and fermentation processes during wine production [131]. They play an important role in maintaining the organoleptic characteristics of wines (color, taste, and astringency), and thanks to their antimicrobial activity they are used as preservatives in the food and cosmetic industries [132,133]. By highlighting their antioxidant properties [134,135], the French paradox stimulated the exploration of the effects that polyphenols from different sources could have on various diseases [136]. As we stated at the outset, the study of grape polyphenols on muscle is still in its infancy, and therefore studies specifically demonstrating the effects of polyphenols extracts obtained from grapes are still scarce. That is why in this section, we will report studies on main polyphenols (individual molecules or groups of molecules) found in grape, that can be also isolated from other sources [137]. In fact, even if isolated from other sources, these polyphenols show the same chemical structure, and we reasonably assume that they could exert the same mechanism of action. For each subfamily, we will cite studies focusing on their effects on obesity, aging, T2D, inflammation, and oxidative stress, with a particular look at pathways affecting muscle. The chemical structure of polyphenols is characterized by the presence of one or more hydroxyl groups and one or more aromatic rings with six carbon atoms.

Polyphenols are a wide class of compounds with more than 8000 molecules isolated and described [138]. Thus, for easier understanding, polyphenols are divided into two big families according to the number of aromatic rings, the way they are linked to each other, and the position and oxidation state of the hydroxyl groups: flavonoids and non-flavonoids, each of which contains other classes (Figure 2).

**Figure 2.** Polyphenols classification.

#### *4.1. The Flavonoids*

Flavonoids are known as plant pigments but also for their antioxidant, antimicrobial, and light-screening functions [139]. Moreover, thanks to their properties they are successfully used in the pharmaceutical and food sectors as preservatives and pigments [140]. Depending on the chemical structure, oxidation degree, or unsaturation of the heterocyclic ring C, flavonoids can be further classified into six mean groups, notably: flavanols, flavonols, anthocyanes, flavones, isoflavones, flavanones [141]. These molecules are generally water-soluble, and they can be found in glycosylated or aglycone form. Their basic structure is the flavone ring.
