**2. Oxidative Stress**

A redox reaction is a type of chemical reaction that involves the transfer of electrons between two molecules. A pair of electrons transfers from a nucleophile to an electrophile, forming a new covalent bond. The redox reaction is common and vital to the basic function of life such as cellular respiration, in which sugar is oxidized to release energy, which is stored in ATP. Redox metabolisms constitute multiple metabolic pathways involved in the series of redox chemical reactions indispensable for sustaining life and, at same time, engaged in removal of electrophilic oxidative species and other harmful nucleophiles. The dynamic activities range from a single electron transfer, enzyme reaction, chemical reaction cascade, to signaling in cells, tissues, organ systems, and whole organismal levels [34]. Oxidative stress is a state caused by an imbalance between the relative levels of production of reactive oxidizing metabolites and their elimination by the enzymatic or non-enzymatic antioxidant system. The oxidative state is induced by oxidative stressors either derived from xenobiotics or produced from the activities of oxidative enzymes and essential cellular constituents [35] (Figure 1a). Oxidative stressors are linked to the aging process, neurologic disease, and psychiatric disorders including Alzheimer's disease (AD), Parkinson's disease (PD), MS, amyotrophic lateral sclerosis (ALS), and depression [36–40].

**Figure 1.** Redox homeostasis and antioxidant supplementation. (**a**) Oxidative stressors comprise of external and internal stressors, exerting oxidative chemical reactions in organism. Oxidation is an indispensable bioenergetic process to sustain life. (**b**) Reductive stressors are products of antioxidative enzymes that generate antioxidants in response to regular oxidation activity and increased oxidative stress. (**c**) Antioxidant supplementation attempts to shift the biphasic response from adverse to beneficial phase to maintain nucleophilic tone. This mechanism is called parahormesis.
