## 3 Therapeutic Relevance of Plant-Derived Antioxidants in Aging and Aging-Related Diseases
### Anti-Aging  
Aerobic life is possible thanks to oxygen, but this is what makes life not infinite at the same time. During aerobic respiration, cells are able to obtain energy through reduction-oxidation reactions [85]. The incomplete reduction of oxygen in aerobic respiration and other metabolic processes can produce highly reactive molecules, known as ROS. These molecules play an important role in cells, since they can participate in the transmission of intracellular signals, regulating diverse redox-sensitive pathways but also oxidizing and altering biomolecules [85; 86]. In 1956, [PERSON] postulated the mitochondrial free radical theory of aging (MFRTA), suggesting that the aging process is related, in part, to the accumulation of oxidative damage to cellular components [87]. Aging is a multifactorial process, characterized by a gradual loss of homeostasis and, therefore, the loss of physiological functions in which numerous elements are involved, including ROS and a widespread decrease in antioxidant defense [88; 89]. Accordingly, with aging, there is an accumulation of ROS, mainly in the mitochondria, which promotes a situation of chronic oxidative stress, favoring the appearance of oxidative damage to DNA, lipids, and proteins, inducing the degeneration of the tissues [90]. There are two main types of oxidant sources, namely, the mitochondrial source, which plays a major role in aging, and the non-mitochondrial source, which plays a role in the pathogenesis of age-related diseases [91]. The main place of intracellular oxygen consumption is in mitochondria derived from the electron transport chain. This is the reason why mitochondria seem to be the main source of endogenous oxidants involved in aging [91; 92; 93]. These reactive species include different free radicals, such as superoxides, singlet oxygen, and hydroxyl radicals, and non-free radicals, such as hydrogen peroxide [93]. All these reactive oxygen-formed species can be eliminated through the action of antioxidant defensive mechanisms. When a higher concentration of reactive species to antioxidants exists, a state of oxidative stress is established. This condition has been observed as a sign of many chronic age-related disorders, such as different metabolic disorders, Parkinson's disease (PD), kidney disease, and Alzheimer's disease (AD) [93]. An increase in ROS production has been related to an increase in cell senescence, characterized by a decrease in cell proliferation [89]. Oxidative stress also promotes mitochondrial DNA mutation, leading to the incorporation of defective subunits in the electron transport chain, and, consequently, a decrease in the synthesis of ATP. This situation contributes to the accumulation of dysfunctional mitochondria, which has been considered as one of the main causes of aging [90; 91; 94]. Antioxidants are endogenous and exogenous substances that prevent or eliminate ROS and protect against oxidative damage. Endogenous antioxidants include enzymatic and non-enzymatic molecules and are distributed within cytoplasm and cell organelles. Exogenous antioxidants are mainly found in vegetables and fruits as phytochemicals [93]. Many plants often contain a considerable amount of antioxidants (e.g., vitamins, carotenoids, terpenoids, polyphenols, alkaloids, tannins, and saponins) that can be consumed, participating in the elimination of ROS in the human body. It has been suggested that the amalgamation of antioxidant and anti-inflammatory phenolic compounds found in plants is important in aging, because the factors that increase the resistance to this oxidative stress could exert beneficial effects against aging [95; 96]. Interestingly, polyphenols are also useful in ameliorating the adverse effects of the aging process on the nervous system or brain due to the ability of these compounds to cross the blood-brain barrier [97]. The most beneficial effects come from flavonoids, specifically from anthocyanins.