*4.5. Reduced Glutathione (GSH): Oxidized Glutathione (GSSG) Ratio*

One of the most sensitive indicators of oxidative damage is the redox ratio GSH: GSSG. Influence on regulating gene expression, signaling conduction, NO metabolism, apoptosis, and impact on free radical scavenging are among the effective roles of GSH. On the other hand, the removal of ONOO- is highly dependent on the formation of oxidized glutathione (GS-SG) by GSH, which is eventually converted to GSH through NADPH-dependent glutathione reductase [45]. Various signaling pathways such as C-Jun N-terminal kinase, protein kinase B, mitogen-activated protein kinase, apoptosis signal-regulated kinase 1, NF-κB, and protein phosphatase 1 and 2A are affected due to changes in the GSH/GSSG ratio [46].

#### **5. A Brief Look at AT**

The definition given to AT today is very different from what it used to be. It was previously thought that this tissue was just a tissue with the property of storing energy in the form of lipids. Today, a new perspective has emerged on it as an endocrine tissue [21,47–49]. Of course, the feature of this tissue's storage source has helped many living things throughout history. When there is a lack or excessive consumption of nutrients, it has always been AT that has been able to help maintain the body's energy homeostasis with hyperplasia or hypertrophy in different conditions [50–53].

On the other hand, the new look at this tissue owes much to discovering the hormone leptin in 1994 as a food controller. Even earlier, in 1987, it was found that sex steroids were metabolized in this tissue, followed by the production of adipsin [54,55]. Adipsin was one of the first adipokines to be identified in cultured adipocytes based on differentiationdependent expression of its mRNA. Adipsin is an endocrine factor secreted by 3T3 fat cells [56–58]. Other important roles of adipose tissue in the body include effects on lipid and glucose metabolism, maintaining energy balance, appetite control, glucose homeostasis, insulin sensitivity, energy expenditure, inflammation, and repair of AT [59,60]. Various factors such as FFA supply, FFA esterification to triglycerides (TG), and TG degradation through the lipolysis process determine fat stores in AT. In general, the two enzymes of hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) play an important role in the lipolysis process, which indicates the formation of FFA and glycerol as a result of the separation of the TG ester bond [61]. In general, AT contains high levels of stromal vascular cells, immunity, stem, endothelial, lymphocytes, adipocytes, preadipocytes, connective tissue matrix, and nerve tissue [55,62,63]. This tissue is also divided into brown adipose tissue (BAT) and white adipose tissue (WAT). There are apparent differences between BAT and WAT, morphologically. One of these differences is related to the size of the fat storage drops. White adipose cells are placed in a large fat drop (unilocular), and brown fat cells are placed in several small fat cytoplasm drops (multilocular) [64]. Another apparent difference between these two types of tissue is the number of mitochondria in them. The number of mitochondria in BAT is much higher than in WAT but the number of mitochondria in the WAT is limited. Because WAT plays an important role in lipid metabolism processes, including beta-oxidation and the TCA cycle, maturation, and differentiation of adipocytes, the importance of mitochondrial function is highlighted [16]. In this way, the BAT can maintain body temperature, especially when it is cold. BAT owes this feature to the high number of mitochondria within it. Of course, this tissue also plays a very important role in lipid oxidation [65].

The mitochondria's inner membrane hosts a protein called uncoupling protein 1 (UCP1) that can generate heat by transferring protons to the mitochondrial matrix and separating oxidative phosphorylation and the electron transfer chain from ATP synthesis [60,66,67]. In fact, by consuming too many nutrients and being exposed to cold temperatures, UCP1 is expressed to protect the body's organisms against obesity and the cold. Sympathetic neurons stimulate UCP1 inside the BAT to lead to exotherm and energy loss. Thus, in both humans and rodents, the association between obesity and UCP1 expression is inverse [68]. Sesterins are among the vital proteins associated with obesity due to oxidative stress, which

play an important role in regulating metabolic homeostasis, suppressing ROS accumulation, and regulating the AMP-activated protein kinase (AMPK)-mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. However, reducing these proteins in the body is associated with obesity and other metabolic disorders [69]. In general, three different isoforms have been discovered for sestrin (sestrin 1–3), of which sestrin 2 is more expressed in liver and adipose tissue. It has been estimated that sestrin 2 can reduce fat accumulation in AT and improve metabolic homeostasis by suppressing ROS and mTORC1 [70].

The most important site for cold-induced exotherm in rodents is BAT. This tissue is also involved in the exotherm of fat tissue. The distribution of BAT in humans and rodents varies according to their age. In humans, there is a large amount of BAT in the body only in early infancy, and with age, the distribution of BAT in the body decreases. However, in the case of rodents, the opposite is true. Because BAT expands as rodents live longer, on the other hand, different types of BAT have been deposited in the body in a scattered manner; interscapular BAT (IBAT) is the most important and vital type [71]. The point here is that the calorific value of IBAT is consistent with the body's OS. The hypothalamus is responsible for controlling IBAT activity and is controlled primarily by the sympathetic nerves and body temperature control centers. Oxidation of FFA in IBAT provides the fuel needed for noradrenaline to activate lipolysis and heat production. However, since IBAT uses a lot of O2 to generate heat, part of the oxygen molecules are converted to free radicals, especially O2 •− by mitochondrial assemblies I and III [72]. However, WAT works more to maintain the balance of energy homeostasis and the source of fat storage and release. WAT content includes subcutaneous WAT (scWAT), visceral WAT (vWAT), and peripheral arteries, each containing fat such as the omental, gonadal, retroperitoneum, epicardial, mesenteric, and perineal. However, it should be noted that all of these reserves are responsible for regulating total energy homeostasis [16]. Most importantly, the endocrine properties of AT are embedded in WAT [66]. This means that there are many hormonal mediators, including cytokines (IL-1β, IL-6, and TNFα), adipokines (adiponectin, resistin, and leptin), and chemokines (macrophage inflammatory protein 1 (MIP1), monocyte chemoattractant protein-1 (MCP-1)), ROS and FFA from WAT [73]. However, various studies have shown that pro-inflammatory cytokines are much less expressed in scWAT than in vWAT [29].

On the other hand, different hormones affect the storage and release of WAT fat. For example, after a meal, TG storage is highly dependent on insulin action. While in fasting and when different body organs (skeletal muscle and liver) need energy, the TG stored in WAT is broken down by catecholamines into FFA and G [65]. Furthermore, beige or brown (brown-in-white) fat cells are a new type of exothermic fat cell within WAT. However, these cells have both white and brown fat cells at the same time. They are more similar to fat white cells in terms of growth and more similar to BAT in terms of function and morphology [64,74].

#### **6. Metabolic Disorders in AT**

One of the practical and important factors in maintaining animals' and humans' body weight is maintaining fat homeostasis in WAT. This depends a lot on the proper and adequate performance of the WAT-derived materials [75,76]. Excess fat due to the long-term balance of positive energy contributes to OS in adipocytes, obesity, and subsequent obesityrelated metabolic disorders such as hyperglycemia, insulin resistance and cardiovascular disease. [55,65]. Thus, systemic OS is closely related to obesity. During obesity, the concentration of OS indicators such as high-sensitivity C-reactive protein (CRP) and oxidized LDL increases [33]. Obesity results from increasing the size and volume of body fat cells and has adverse effects on the health of living organisms [76,77]. On the other hand, the mismatch between height and body weight due to excessive fat accumulation is named obesity [78]. Today, the people of developed countries are facing obesity-related health problems, but the people of developing countries are grappling with this global dilemma [79].

It is estimated that by 2030, half of all retirees in the United States will suffer from chronic obesity and related diseases, especially cardiovascular disease. This has numerous

negative effects on the global health system due to the costly and time-consuming treatment of obesity [80]. Countless BMI studies have accepted more than 30 as obesity, but several studies have shown that BMI is not a good indicator of obesity. These studies cited the inability of this criterion to measure lean mass. These studies have shown that the measure of waist-to-pelvis or waist circumference may be a more accurate and accurate indicator of BMI in calculating fat distribution [16,81]. According to statistics published by the WHO, the prevalence of this disease is likely to reach more than one billion people in the world by 2030, which is a worrying statistic [79,82]. On the other hand, there is a direct and positive relationship between BMI and indicators of oxidative damage to proteins (advanced oxidation protein products (AOPP), lipids (MDA or 8-iso-PGF2α), and DNA (8-OHdG) during obesity) [30].

Oxidative DNA damage due to prolonged exposure to OS impairs mitochondrial function and leads to excessive fat accumulation and subsequent insulin resistance [16]. In recent years, obesity has become widespread due to lifestyle changes such as increased consumption of nutrients, especially fast foods, and decreased physical activity (environmental factors). Of course, the role of genetics is also felt to some extent in the development of this disease, but its effect is not as tangible as environmental factors [83]. There should be a strong emphasis on body weight control as a practical way to prevent obesity-related diseases. This usually happens with lifestyle modifications and focuses on eating healthy and adequate nutrients and engaging in regular exercise [84]. Various studies have shown that obesity is associated with increased OS, decreased antioxidant activity, and insulin resistance in AT [85]. Numerous studies have shown that fat accumulation in obesity is closely related to an increase in ROS and subsequent OS. Therefore, obesity-induced OS plays an important role in disrupting adipokines regulation and amplifying inflammatory signals and even leads to changes in cellular composition and premature aging [84]. It is estimated that with excessive consumption of nutrients, WAT expands (size increases) by 10% per year, which is known as fatty remodeling and the penetration of immune cells into AT [21]. AT remodeling leads to the rapid spread of obesity, which is usually accompanied by changes in the size (hypertrophy) and number (hyperplasia) of fat cells [16,51].

Obesity is the cause of various other diseases such as T2D, dyslipidemia, cardiovascular disease, atherosclerosis, and hypertension [53]. Clinical studies have also shown that BMI is directly related to OS by-products such as protein carbonylation products or lipid peroxidation. By causing OS in AT, adipocytokines' secretion (IL-1β, IL-6, and TNFα) is disrupted and eventually leads to obesity and associated diseases [85]. Of course, the function of other cells and tissues in the body, including beta pancreatic cells, vascular endothelial cells, and myocytes, is affected by obesity due to adipose tissue [21].

#### **7. Antioxidants**

As mentioned earlier in this article, oxidative damage can be defined as an imbalance between ROS production and antioxidant defense, leading to overproduction of ROS. The result of this imbalance is a change in cellular redox status. In vivo, antioxidant defense systems play an important role in restoring cellular redox status, especially under normal and stress-free conditions [86]. To combat OS, the body uses enzymatic antioxidant systems (SOD, CAT, peroxidase (POD), peroxiredoxin (Prxs), and glutathione peroxidases (GPX)) and non-enzymatic (carotenoids, tocopherol, and ascorbic acid) [85]. When the body is under pressure using various stressors, especially fat accumulation in AT, these antioxidant defenses alone may not be sufficient and require the use of antioxidant supplements [87–91].
