**10. NAFLD and OS**

Various factors such as central obesity, IR, T2D, over nutrition, lack of exercise, and other metabolic syndrome parameters predispose multiple diseases such as non-alcoholic fatty liver disease (NAFLD) [126,127]. NAFLD is usually characterized by fat accumulation in the liver tissue, and oxidative stress plays a crucial role in its formation and development. It covers a wide range of liver-related diseases such as steatosis, steatohepatitis, liver fibrosis, liver cirrhosis, and even hepatocellular carcinoma [128,129]. For NAFLD, there are non-progressive forms (non-alcoholic fatty liver disease (NAFLD) or simple steatosis) and progressive and aggressive forms (non-alcoholic steatohepatitis (NASH)). Hepatocellular carcinoma (HCC) and cirrhosis are considered as consequences of NASH [130–132]. Macrophages and Kupffer cells can stimulate pro-inflammatory mechanisms and then satellite cell activity at the liver surface by secreting inflammatory cytokines such as IL-6, TNF-α, and IL-β. In such inflammatory conditions, conditions are provided for increasing the deterioration of insulin resistance and the development of liver fibrosis (Figure 3) [133]. In general, in patients with NAFLD, lipids' storage capacity in the liver tissue is so high that it leads to hepatocyte dysfunction and even death [134].

**Figure 3.** Various factors such as central obesity, insulin resistance (IR), type 2 diabetes (T2D), overnutrition, lack of exercise, and other metabolic syndrome parameters predispose multiple diseases such as NAFLD. Macrophages and Kupffer cells can stimulate pro-inflammatory mechanisms and then satellite cell activity at the liver surface by secreting inflammatory cytokines such as IL-6, TNF-α, and IL-β. In such inflammatory environments, conditions are provided for increasing the deterioration of IR and the development of liver fibrosis. It should be noted that the increased flow of FAs to the liver through the bloodstream, the synthesis of de novo hepatocytes, and impaired clearance through β-oxidation lead to the accumulation of TAG droplets in hepatocytes. Various proteins such as FATP, transmembrane proteins, FABP, caveolins, and FAT/CD36 can accelerate the absorption of FA by increasing the proliferation of FFA in blood vessels. It is noteworthy that these proteins' expression can improve by an HFHSD. On the other hand, in fasting conditions, FFAs are mainly produced during the lipolysis process by beta-adrenergic receptor agonists [126–133].

Numerous studies have shown that the mechanisms associated with the pathogenesis of obesity and NAFLD are the same [135]. A two-hit theory can usually explain the pathogenesis of NAFLD. The first theory is explained when triacylglycerol (TAG) droplets accumulate in hepatocytes and lead to simple hepatic steatosis development. In the second theory, NAFLD's pathogenesis is attributed to increased oxidative stress, IR, lipid peroxidation, and endoplasmic reticulum inflammation [136,137]. It should be noted that the increased flow of FAs to the liver through the bloodstream, the synthesis of de novo hepatocytes, and impaired clearance through β-oxidation lead to the accumulation of TAG droplets in hepatocytes. TG synthesis in the liver is mainly due to the lipids produced by de novo lipogenesis (DNL), dietary lipids, and carbohydrates. TG synthesis is dependent on the uptake of FFAs from the plasma by the liver [138]. DNL is the process by which exogenous energy sources or endogenous carbohydrates can synthesize lipids. Three steps are defined for this process. First, FAs can be synthesized through acetyl-CoA subunits produced during glycolysis and carbohydrate metabolism. Then, to form long-chain unsaturated FAs, FA elongation and desaturation must occur. Finally, the FAs formed from the

previous step are assembled to convert to TG and very-low-density lipoproteins (VLDLs). When the balance between TG synthesis and degradation is lost, the conditions for NAFLD are created [130]. Various proteins such as fatty acid (FA) transporter protein (FATP), transmembrane proteins, FA binding protein (FABP), caveolins, FA translocase (FAT)/CD36 can accelerate the absorption of FA by increasing the proliferation of FFA in blood vessels. It is noteworthy that these proteins' expression can improve by a high-fat, high-sugar diet (HFHSD). On the other hand, in fasting conditions, FFAs are mainly produced during the lipolysis process by beta-adrenergic receptor agonists [139].

AT is severely affected by NAFLD because it is a source of FAs storage, and the secretion of adipokines is impaired. AT acts like a double-edged sword. This means that some hormones secreted by AT, such as adiponectin and visfatin, have protective effects against NAFLD; however, the hormones resistin and leptin contribute to hepatic development of steatosis and IR [140,141].

ROS production in hepatic mitochondria results from excessive oxidation of fatty acids, which ultimately causes OS in liver tissue. Proteins, DNA, and lipids are susceptible to OS and are easily damaged by activating pro-inflammatory cells such as Kupffer cells and stimulating the release of inflammatory cytokines. Furthermore, the expression and activity of antioxidant enzymes are usually inhibited by ROS overload, and thus, the liver's antioxidant capacity undergoes a declining trend. Finally, NAFLD occurs as a result of OS and chronic inflammation. Researchers should try to reduce OS to improve NAFLD disease [142].

Approximately 25% of adults worldwide are affected by this disease. This trend is increasing, and the number of these patients increases every year. According to a metaanalysis study, the global prevalence of NAFLD has risen to 25.2% in the last 20 years and has caused concern among the public [143]. On the other hand, no effective treatment for this disease has been achieved despite significant medical advances. Currently, the only treatment approach is lifestyle changes (diet and exercise) and bariatric surgery [144]. Because there are substantial differences between different communities in terms of lifestyle and diet, various studies have shown that the prevalence of NAFLD in Eastern societies is lower than in Western societies [145]. At the systemic level, there is impaired control of food intake resulting in hyperalimentation, intestinal dysbiosis leading to gastrointestinal hormone secretion, IR, gut dysfunction, abnormal adipokine, and activation of pro-inflammatory factors [146].

One of the effective strategies in the prevention and treatment of NAFLD is nuclear factor erythroid-derived 2-like 2 (Nrf2), which as a transcription factor consists of a highly protected basic region-leucine zipper (bZIP) structure and is mainly a member of the Cap "n" Collar (CNC) family. Activation of cellular antioxidant enzymes, regulation of lipid metabolism, and insulin sensitivity improvement are the essential cytoprotective effects of Nrf2. Hence, many researchers have tried to identify Nrf2 activators to improve NAFLD [147].

The Kelch-like-ECH-associated protein 1 (Keap1)-Nrf2-antioxidative response element (ARE) signaling pathway has been considered an essential antioxidant mechanism due to its effect on improving the oxidative stress response [148]. The Nrf2 gene includes six highly protected epichlorohydrin (EHC) domains (Nrf2-EHC homology, Neh) called Neh1-6. The C-terminus Neh1 subtends a protected bZIP DNA region that binds to musculoaponeurotic fibrosarcoma protein (Maf) to create a heterodimer. This heterodimer eventually binds to DNA and can detect ARE. On the other hand, Neh2 is composed of two vital regions, ETGE and DLG, which, by binding to KEAP1, can contribute to the strong binding of Nrf2 to the cytoplasm [129]. C-terminus is the site of another Nrf2 domain, Neh3, which participates in the transcriptional activity of ARE after binding with chromo-ATPase/helicase DNA-binding protein (CHD6). The other two Nrf2 domains, Neh4 and Neh5, initiate the transcription process when interacting with the cyclic adenosine monophosphate response element (CREB)-binding protein (CBP) [149]. Finally, the last Nrf2 domain, Neh6, and being rich in serine are used to Nrf2 decompose independent

of KEAP1 [150]. The expression of Nrf2 in homeostatic conditions and combination with KEAP1 in the cytoplasm is considered a mediator for the degradation and ubiquitinoylation of Nrf2. However, when exposed to oxidative or electrophilic stress, KEAP1 modulates cysteine residues and ultimately releases Nrf2. On the other hand, the Nrf2 protein isolated from KEAP1 returns to the cell nucleus and is dimerized to bind to AREs, along with bZIP proteins such as Maf [151], and then promotes the expression of ARE-mediated downstream target genes containing antioxidant enzymes. The most critical antioxidant proteins targeting Nrf2 are HO-1, GSH, and NAD(P)H quinone oxidoreductase 1 (NQO1). It should also be noted that Nrf2 plays a pivotal role in suppressing the progression of NAFLD, maintaining cellular homeostasis, and protecting against oxidative or electrophilic stresses [152].

#### **11. Roles of Nutraceuticals as an Antioxidant in Reducing Oxidative Stress**

Nutraceuticals are not recognized traditionally as a nutrient but have physiological health benefits in the human body. Plant-derived nutraceuticals are well-known for their direct or indirect antioxidant activities, which relates to scavenging or eliminating free radicals during cellular metabolism. They can interact with the oxidized species at both cellular and molecular levels by regulating gene expression, epigenetic controls, and protein and DNA repair. Previously it was reported that nutraceuticals have potential properties in immunity modulation, gene expression, and various signaling process regulation [153–158]. The nutraceuticals can be prepared from the foods available in the local market, for example, ginger, garlic, avocado, and onion, in the form of polyphenols, carotenoids, sulforaphane and other isothiocyanates, glucosinolate, phytosterol, etc. [159]. They can increase the level of heme oxygenase (HO) 1, total glutathione, and other phase 2 enzymes by activating the transcription Nrf2. Moreover, treatment for a certain period with nutraceuticals may also improve the lipid profile and can reverse the harmful effects of obesity on blood lipids [160]. For example, curcumin is a potential nutraceutical, reduces macrophage infiltration in WAT, increase adiponectin in AT, decreases NF-κB activity, therefore reduces the expression of inflammatory markers and OS [161]. To date, the use of nutraceuticals, bioactive compounds or exercise could be an additional strategy in reducing obesity and related diseases [162,163].

#### **12. Conclusions**

OS affects various tissues, such as adipose tissue, skeletal muscle, and heart, in the body. In this study, we specifically examined adipose tissue response to OS. As mentioned in the text, this tissue is disrupted by various factors such as overconsumption of nutrients and sedentary lifestyle. This disorder eventually leads to lipid accumulation in adipose tissue and reduced energy expenditure. Of course, various treatments have been introduced for this disorder. However, most of them face limitations that are fully explained in the text. On the other hand, numerous studies have proven the effectiveness of diet, especially the use of antioxidant supplements, on the improvement of obesity caused by OS. The results of this treatment are inconsistent but have fewer side effects than other treatments such as medication and surgery. Further studies are needed because the results of the studies are contradictory. In future studies, researchers will investigate the effect of taking antioxidant supplements on heart and skeletal muscle tissues.

**Author Contributions:** S.T. conceived the review and drafted the manuscript. K.S. and R.T.R. made some additions to the text, revised the manuscript and approved the final version. All authors have read and agreed to the published version of the manuscript.

**Funding:** The publication was supported by the Scientific Research (A) (20H00574) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

**Acknowledgments:** We gratefully appreciate our colleagues and laboratory team for research progress and discussion.

**Conflicts of Interest:** The authors declare no conflict of interest.
