Composition, Microbiota, Mechanisms, and Anti-Obesity Properties of Rice Bran

Rice is a major cereal crop and a staple food for nearly 50% of people worldwide. Rice bran (RB) is a nutrient-rich by-product of rice processing. RB is rich in carbohydrates, fibers, proteins, lipids, minerals, and several trace elements (phosphorus, calcium, magnesium, potassium, and manganese). The extraction process and storage have influenced RB extracts and RB oil’s quality. The RB composition has also varied on the rice cultivars. The color of RB indicates the richness of the bioactive compounds, especially anthocyanins. γ-oryzanol, tocopherols, tocotrienols, and unsaturated fatty acids are major components of RB oil. It has been established that RB supplementation could improve the host’s health status. Several preclinical and clinical studies have reported that RB has antioxidant, anticancer, anti-inflammatory, anticolitis, and antidiabetic properties. The beneficial biological properties of RB are partially attributed to its ability to alter the host microbiome and help to maintain and restore eubiosis. Non-communicable diseases (NCDs), including heart disease, diabetes, cancer, and lung disease, account for 74% of deaths worldwide. Obesity is a global health problem and is a major reason for the development of NCDs. The medical procedures for managing obesity are expensive and long-term health supplements are required to maintain a healthy weight. Thus, cost-effective natural adjuvant therapeutic strategy is crucial to treat and manage obesity. Several studies have revealed that RB could be a complementary pharmacological candidate to treat obesity. A comprehensive document with basic information and recent scientific results on the anti-obesity activity of RB and RB compounds is obligatory. Thus, the current manuscript was prepared to summarize the composition of RB and the influence of RB on the host microbiome, possible mechanisms, and preclinical and clinical studies on the anti-obesity properties of RB. This study suggested that the consumption of RB oil and dietary RB extracts might assist in managing obesity-associated health consequences. Further, extended clinical studies in several ethnic groups are required to develop dietary RB-based functional and nutritional supplements, which could serve as an adjuvant therapeutic strategy to treat obesity.


Introduction
Oryza sativa (rice) is a common food crop worldwide and is cultivated in several countries. Rice is one of the major sources of carbohydrates, especially in low-income countries [1]. Rice bran (RB) is the by-product of the rice milling process and a major food waste. RB contains fat (12 to 25%), starch (10 to 20%), protein (10 to 16%), celluloses (10 to 12%), hemicellulose (8 to 11%), fiber (6 to 15%), reducing sugar (3 to 8%), ash (6.5 to 10%), and phenolic acids, γ-oryzanol, and tocopherols [2,3]. RB has minerals such as magnesium, iron, sp. indica from different regions of China were evaluated for their phenolic acids content. The results showed that 12 phenolic compounds are present in all rice varieties. O. japonica has a higher phenolic content than other studied varieties [58]. The free, bound, and total phenolic and flavonoid compounds differ in chemical composition and antioxidant activities in defatted RB and their soluble and insoluble fibers were evaluated by Zhao et al. The soluble fiber from defatted RB has low total phenolic and total flavonoid contents. The insoluble fiber from defatted RB has low free phenolics and high bound phenolics. They found 17 monomeric phenolic compounds in defatted RB, including gallic acid, syringic acid, vanillin, epicatechin, p-coumaric acid, ferulic acid, sinapic acid, quercitrin, isoquercitrin, caffeic acid methyl, and ferulic acid methyl [59]. Sompong et al. reported the phytochemical content of nine red and three black rice cultivars from Thailand, China, and Sri Lanka. They identified cyanidin-3-glucoside and peonidin-3-glucoside as predominant anthocyanins in black rice. The highest total phenolic content was observed in the red Thai rice variety (Bahng Gawk). Moreover, red rice varieties showed the major free form of ferulic acid, protocatechuic acid, and vanillic acid. In contrast, black rice varieties were rich in protocatechuic acid, then ferulic and vanillic acid [60]. The Hashemi RB extracts are abundant with phenolic contents, including ferulic, gallic, and chlorogenic acids [27]. Thai rice cultivars contain phenolic acids such as caffeic acid, chlorogenic acid, protocatechuic acid, p-hydroxybenzoic acid, and syringic acid, and p-coumaric acid, anthocyanins, tocols and γ-oryzanol. The concentration of phenolic acids differs among rice varieties [61]. The impact of extraction methods on the yield of anthocyanins was studied in nine different Thai rice cultivars, including Hawm nil, Hawm kanya, and Kum (black grains), Sang yod and Red jasmine (red grains), and Hawm ubon, Lao tek, Jasmine rice 105, and Sin lek (white grains). The results showed that the extraction methods affected the phytochemical content of the extracts. Insoluble phenolic compounds were significantly higher than soluble phenolic compounds. Among these rice varieties, black grains showed higher anthocyanins and phenolic compounds than red and white grains [62]. The purple rice bran has a high proportion of anthocyanins [40].
Huang and Lai investigated the profiles of free and bound phenolics and flavonoid compounds in six pigmented rices' outer and inner RB. The 80% ethanol extracts of red rice varieties from Taibalang, Taiwan, and Thailand were reported to have proanthocyanin, anthocyanin, vitamin E, and γ-oryzanol. Proanthocyanins were found in red RB and were absent in black RB. HPLC with photodiode array/electrospray ionization tandem mass spectrometry identified protocatechualdehyde in the bound fraction of red RB. The crude lipid, protein, ash, and total dietary fiber were higher in outer RB than inner RB. Moreover, phenolics and flavonoids in free fractions were higher than inbound fractions. The colored RB contains α, β, γ, and δtocopherol and α, γ, and δ tocotrienol [63].
Flavonoids are another group of secondary metabolites present in rice and are classified as flavones, flavanols, flavanones, flavanonols, and anthocyanins with extraordinary antioxidant capacities [64]. Flavones from the Njavara rice variety had a potential cytotoxicity effect against cancer cells [65]. Sakuranetin is a flavanone type of phytoalexin that works against plant pathogens. Moreover, it acts as a pharmaceutical agent that induces adipogenesis in 3T3-L1 cells, thus regulating glucose homeostasis in animals [66], and has anti-inflammatory [67], antimutagenic [68], antileishmanial, and antitrypanosomal activities [69]. The differences in flavonoid contents in different white, red, and black-pigmented rice varieties were reported. Black RB has a high content of cyanidin-3-glucoside, peonidin-3-glucoside, quercetin, dihydromyricetin, naringin, and taxifolin. Red RB is rich in catechin and epicatechin. The red and black RB extracts exhibited higher antioxidant activity than the white RB extract [70].
The phytochemical composition of RB is influenced by the types of rice cultivars, extraction methods, and milling process. The phytochemical content, extraction, and analysis methods of different rice cultivars are detailed in Table 1. The three-dimensional structure of representative phytochemicals of RB is illustrated in Figure 1.

Results of In Vitro Studies
Ethanolic extract of glutinous black RB (EEGBRB), rich in phenolics, flavonoids, and anthocyanins, was studied for its lipolytic property using the 3T3-L1 pre-adipocyte model. EEGBRB treatment reduced lipid accumulation and TG levels in 3T3-L1. EEGBRB treatment also induced lipolysis in 3T3-L1. The study showed that EEGBRB could be a therapeutic agent for managing obesity-associated complications [81]. The hydrolyzed de-oiled RB (DORB) was reported for its anti-obesity property. In detail, DORB was hydrolyzed using Alcalase ® , and the fractions were further categorized based on their molecular weight. A < 0.65 kDa fraction exhibited the highest level of lipase inhibitory activity. Mass spectrometry analysis revealed that the fraction has the amino acid composition of FYLGYCDY. Further, molecular docking confirmed that the peptide competitively binds with the porcine pancreatic lipase complex. The results indicated that DORB fractions have potent anti-obesity peptides [82]. The defatted red RB was extracted using different concentrations of ethanol. High total flavonoids were shown in 75 and 95% ethanolic extract of defatted red RB. High phenolic content was observed in 50 and 75% ethanolic extract, and these extracts exhibited the highest lipase inhibitory activity in vitro compared to other extracts [83].
To the best of our knowledge, there are no more in vitro studies on the anti-obesity properties of rice bran. Some studies reported the anti-obesity property of rice extract. For example, different solvent extracts of brown rice were screened for pancreatic lipaseinhibitory activity. The results indicated that hexane extract showed the highest inhibition (13.58 ± 0.86%) at 200 g/mL concentration. The inhibitory effects were affected by the concentration of the extracts in vitro [84]. Recently, Barathikannan et al. (2023) demonstrated the anti-obesity activity of Pediococcus acidilactici MNL5-mediated fermented brown rice. In vitro pancreatic lipase-inhibitory activity of fermented RB was higher (85.5 ± 1.25%) than raw RB (54.4 ± 0.86%). Moreover, the fermentation process improved the antioxidant capacity of RB. The study also reported that fermented brown rice increased the lifespan and reduced lipid accumulation in Caenorhabditis elegans [85].

Results of In Vivo Studies
Several in vivo studies evaluated the anti-obese effects of RB and RB components. Most studies indicated that RB and RB components could ameliorate the obesity and associated co-morbidities. Some of the studies were detailed as follows.
The male Wister rats fed diacylglycerol (DAG)-enriched RB oil and sunflower oil improved the CVD-associated biomarkers. In detail, the supplementation of 20 or 40% of DAG-enriched oils (10% in the diet) for 12 weeks decreased the triacylglycerol (TG), total cholesterol (TC), body fat (BF), C-reactive protein (C-RP), tumor necrosis factor-alpha (TNF-α), and platelet aggregation. Additionally, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) expression were decreased, and 40%DAG-RB supplementation increased fecal cholesterol excretion. The results indicate that regular intake of 40%DAG-RB could reduce the risk of CVD and improve the serum lipid profile [86].
Obese Zucker rats were supplemented with 1 or 5% RB enzymatic extract (RBEE) for 20 weeks. RBEE supplementation significantly reduced the expression of interleukin (IL)-6, IL-1β, iNOS, and TNF-α in visceral abdominal adipose tissue, while increasing the expression of IL-6 and iNOS in visceral epididymal adipose tissue. RBEE supplementation affected the distribution of adipocytes and effectively reduced adipocyte size [87]. RBEE supplementation also reduced vascular hyperactivity, inflammation, and eNOS (endothelial NOS). Moreover, a significant reduction of superoxide production and down-regulation of NADPH oxidase subunits were also observed in the RBEE-supplemented rats [88]. RBEE-supplementation restored the microvascular function and reduced microvascular inflammation [89]. The studies showed that RBEE could ameliorate obesity-associated proinflammatory responses and vascular issues and restore the function of arteries [88][89][90].
Similarly, supplementation of RBEE improved the lipid profile, adipose tissue, and expression of macrophage polarization in high-fat-diet-induced mice [90].
Male albino rats were supplemented with diacylglycerol-rich bran oil (DAG-RBO) in their regular diet for 28 days. The DAG-RBO-supplemented group showed a reduction in plasma TC and non-HDL-C (non-high density lipid cholesterol) and a reduction in TC, TG, and phospholipids in the mesentery and liver. TG and TL (total lipid) levels were also reduced in the erythrocyte membrane and mesentery. A significant reduction in the HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A): mevalonate ratio in the liver was observed in the experimental rats. The study suggested that the supplementation of DAG-RBO improved the lipid profile and reduced cholesterol biosynthesis in rats, indicating that DAG-RBO could control the obesity-associated consequences [91].
Triterpene alcohols, β-sitosterol, and campesterol decreased the diet-induced secretion of the glucose-dependent insulinotropic polypeptide (GIP) in C57BL/6J mice. The supplementation of triterpene alcohol and sterol (derived from RB) (TAS) for 23 weeks in mice showed less weight gain than the control. Fat utilization and fatty acid oxidationrelated gene expression were higher in TAS-fed mice. The fatty acid synthesis-related gene expression was suppressed in the liver of TAS-fed mice. Cycloartenol and 24-methylene cycloartenol treatment reduced the sterol regulatory element-binding protein (SREBP)-1c expression in HepG2 cells. The results indicated that TAS supplementation could prevent diet-induced obesity in vivo [92].
The supplementation of γ-Oryzanol (OZ) and ferulic acid (FA) derived from RB improved the metabolic syndrome-related parameters in high-fat-diet-induced obese Sprague-Dawley rats. In detail, supplementation of 0.05% FA or 0.16% OZ for 13 weeks improved the obesity, insulin resistance, and lipid profile. In particular, OZ supplementation significantly reduced the hepatic TG level, serum C-reactive protein, and Il-6 and increased the adiponectin level. OZ treatment reduced the intracellular accumulation of TG and expression of stearoyl coenzyme-A desaturase-1. The results suggested that OZ exhibited better protective activity against metabolic syndrome than FA [93].
RB unsaponifiable matter (USM) contains tocopherols and tocotrienols (16.35 mg/g), policosanols (55.14 mg/g), phytosterols (364.25 mg/g), and oryzanols (29.68 mg/g). C57BL/6J mice were supplemented with a high-fat diet and USM (10 or 20, or 50 mg/kg body weight/day) for 6 weeks. USM supplementation effectively decreased the body weight gain, food efficiency ratio, and size of the epididymal fat tissue compared to the control. USM supplementation also reduced the serum TG, TC, LDL-C (low-density lipoprotein cholesterol) levels, cardiac risk factor, and atherogenic index compared to the control. The results showed that USM has antilipidemic activity, which might prevent obesity and cardiovascular disease [94].
Administration of RB water extract (RBWE; 2205 mg/kg/day for 4 weeks) protects the rats from high-fat-diet-induced metabolic changes. In detail, RBWE treatment reduced body weight gain, visceral fat tissue weights, TC, and glucose and malondialdehyde levels compared to the control. Additionally, RBWE treatment increased the expression of eNOS and reduced the expression of NF-kB p65 and CD36 without causing histological changes in the aorta in experimental rats. The results indicated that RBWE has vasoprotective effects in the high-fat-diet-induced obese condition in rats [95]. Similarly, high-fat-diet-fed male Sprague-Dawley rats were supplemented with RBWE (2.205 or 4.410 g/kg/day) for 4 weeks. The changes in diet-induced obesity, hyperglycemia, and glucose tolerances were assessed. RBWE supplementation, in both doses, significantly reduced the body weight and weight gain and controlled the fasting blood glucose and insulin levels compared to high-fat-diet-fed rats. The increased expression of insulin receptor substrate-2 (IRS-2), glucose transporter-2 (GLUT-2), and glucokinase (GK), and decreased expression of sterol regulatory element-binding protein-1c (SREBP-1c), were observed in the pancreas of the RBWE-supplemented rats. RBWE supplementation prevents the formation of fat droplets in acinar cells. The results suggested that RBWE could reduce weight gain, prevent fat accumulation, and improve insulin sensitivity in the high-fat-diet-fed rat model [96].
Red RB extract (RRBE; rich in phenolics, flavonoids, anthocyanins, and proanthocyanidins) supplementation (0.5 or 1 g/kg of RRBE for 6 weeks) protected the mice from the highfat-diet-induced obese condition. In detail, RRBE supplementation significantly diminishes diet-induced adipocyte hypertrophy and controls lipid accumulation and inflammation, while the body and adipose tissue weights were unchanged. RRBE supplementation also suppressed the expressions of CCAAT/enhancer binding protein-alpha, sterol regulatory element-binding protein-1c, hormone-sensitive lipase, macrophage marker F4/80, NF-kB p65, monocyte chemoattractant protein-1, TNF-α, and iNOS in adipose tissue of the experimental animal. A reduction in serum level of TNF-α was also observed. The results showed that RRBE might improve obesity-associated adipose tissue dysfunction [97]. Similarly, RRBE supplementation improved the serum insulin level, glucose level, and expression of insulin-degrading enzyme (IDE), insulin receptor substrate (IRS), and glucose transporter (GLUT) in high-fat-diet-induced obese mice. RRBE protects the host from diet-induced damage by improving glucose-insulin homeostasis [98].
High-fat/high-cholesterol-diet-fed (HFCD) mice were supplemented with Weissella koreensis DB1-mediated fermented RB (HFCD-FRB) or RB (HFCD-RB) (5% FRB or RB) for 10 weeks. FRB supplementation significantly reduced body weight, serum level TG, TC, non-HDL-C, insulin, glucose and leptin, TG in the liver and adipose, and liver and white fat masses. Serum-level adiponectin and HDL-C were increased in the HFCD-FRB group compared to the HFCD group. The expression of SREBP-1c, fatty acid synthase (FAS), CCAAT-enhancer-binding protein α (C/EBPα), and acetyl CoA carboxylase (ACC) were significantly decreased in the HFCD-FRB group compared to the HFCD group. Significant improvements were also observed in all the parameters in the HFCD-RB group compared to the HFCD group, but the HFCD-FRB group showed a superior result. The study claimed that FRB has anti-obesity properties and could improve lipid metabolism [99].
High-fat-diet-induced obese C57BL/6 male mice were supplemented with RB oil (RBO) or palm oil (PO) (170 g of RBO or PO/Kg of food; no changes in food consumption between groups) for 10 weeks. RBO-supplemented mice reduced epididymal white adipose tissue (EWAT) weights compared to the control. RBO supplementation suppressed the expression of SREBP-1c, PPAR-γ, and M2-macrophage markers (iNOS, COX-2, and f4/80) in mice's EWAT. Additionally, RBO supplementation favorably altered the expression of surface M2 makers (CD206 and CD11c), arginase 1 (arg1), and chitinase-like proteins (ym1) in EWAT. RBO supplementation improved the anti-inflammatory status (increased the IL-10 level and decreased the IL-6 and TNF-α levels) in bone marrow-derived macrophages. The study showed that RBO could improve obesity-induced chronic inflammation by altering the expression of inflammation-associated markers and macrophage polarization [100].
Yang et al. reported that supplementing 4 and 8% of RB in food significantly improved high-energy diet-induced obesity in rats. In detail, RB supplementation significantly reduced the adipocyte size and body weight, hepatic TC and TG, and serum glucose and uric acid levels compared to the control. Moreover, the RB supplementation improved hepatic lipid homeostasis. The study claimed RB could prevent obesity-associated metabolic diseases [101].
The supplementation of solvent (hexane) extract of IR-64 RB (100 or 150, or 200 mg/kg BW of RBE for 6 weeks) to high-fat-diet-induced obese rats significantly reduced the body weight, TG, and malondialdehyde. The beneficial effects have remained between 150 and 200 mg of RBE supplementation. The study claimed that RBE supplementation has anti-obesity activity, but further investigations are needed to confirm the result [102].
High-fat-diet-fed male ICR mice were supplemented with low (220 mg/kg BW/day) and high doses (1100 mg/kg BW/day) of RBWE for 8 weeks, and changes in blood pressure, inflammatory markers, and hepatic steatosis were analyzed. RBWE supplementation significantly reduced diastolic blood pressure, serum, and liver TNF-α and malondialdehyde (MDA) levels, nuclear factor-κB (NF-κB) levels in the liver and heart, lipid accumulation in the liver, myocardial COX-2, and matrix metalloprotease-9 (MMP-9) levels [94]. Moreover, RBWE supplementation significantly reduced the adipose tissue mass and vascular endothelial growth factor (VEGF) and MMP-2 expressions in visceral fat tissue [95]. The study claimed that RBWE could reduce obesity-associated hypertension and adipose tissue mass and improve health status [103,104].
High-fat-diet-fed male Wistar rats were supplemented with anthocyanin-rich black RB extract (BRBE) (100 or 200 mg/kg BW/day) for 8 weeks. BRBE supplementation significantly reduced body weight, visceral fat weight, TC, TG, and plasma glucose levels, and serum creatinine and renal cortical MDA levels in high-fat-diet-fed rats. Additionally, BRBE supplementation attenuates kidney injury by reducing oxidative stress and renal cell damage [105].
Multiple strain (Bacillus amyloliquefaciens M4, Bacillus subtilis M5, Bacillus sp.M6, Lactobacillus casei, Bifidobacterium bifidum, and Aspergillus oryzae)-mediated fermented RB was supplemented to high-fat-diet-fed female C57BL/6J mice (0.239% of FRB in the diet per day) for 8 weeks. The changes in the microbiota and metabolites were reported. FRB supplementation reduced weight gain, the abundance of Enterococcus and Peptostreptococcaceae, and fecal succinic acid concentration. Moreover, the levels of xylitol, sorbitol, uracil, glutamic acid, and malic acid were decreased, while the fumaric acid level was increased in peripheral blood. FRB supplementation did not increase the abundance of beneficial microbes, but the abundance of the harmful microbial level was decreased. The study showed that FRB supplementation reduced high-fat-diet-induced obesity by modifying gut microbiota and host metabolism [107].
The supplementation of red RB ethanolic extract (RRBEE; 0.5 or 1 g/kg BW/day) for 12 weeks ameliorates the high-fat-diet-induced obesity-associated complications and insulin resistance in male ICR mice. RRBEE supplementation induced the expression of IRS in the adipose tissue and GLUT in the adipose tissue and muscles. At the same time, RRBEE supplementation reduced the expression of IDE in muscles and pancreatic insulin. Moreover, the size of the pancreatic islet was reduced in RRBEE-supplemented mice [98] (Table 2).  ↓ Diastolic blood pressure ↓ Serum and liver TNF-α and MDA levels ↓ NF-κB levels in the liver and heart ↓ Lipid accumulation in the liver ↓ Myocardial COX-2, and MMP-9 ↓ Adipose tissue mass ↓ VEGF and MMP-2 expressions in visceral fat tissue [103,104] Male Wistar rats Anthocyanin-rich black rice bran extract 100 or 200 mg/kg BW/day for 8 weeks ↓ Body weight and visceral fat weight ↓ Plasma glucose, TC, and TG levels ↓ Serum creatinine and renal cortical MDA levels Attenuates kidney injury [105] Male Wistar rats Rice bran 11% rice bran in the diet for 20 weeks ↓ Body weight, body fat, and adiposity index ↓ IL-6, MDA and TNF-α ↑ SOD and CAT activities in the myocardium ↓ TG, Insulin, HOMA-IR Improved the structural and functional properties of the heart [106] Female C57BL/6J mice Fermented rice bran 0.239% of FRB in the diet for 8 weeks ↓ Weight gain ↓ Abundances of Enterococcus and Peptostreptococcaceae ↓ Fecal succinic acid concentration ↑ Fumaric acid in the blood ↓ Xylitol, sorbitol, uracil, glutamic acid, and malic acid levels in the blood The studies indicated that RB extracts improved obesity by regulating cholesterol profiles, host metabolism, and antioxidant, inflammatory, and immune signaling networks. The possible mechanism behind the anti-obesity activity of RB has been detailed in Section 5.

Clinical Studies
The cholesterol-lowering property of RB extract containing acylated steryl glucoside (RB-ASG) has been reported. In detail, obese Japanese men were supplemented with RB-ASG (30-50 mg/day) for 12 weeks, and their anthropometric parameters, pressure, fat, and cholesterol contents were determined. RB-ASG supplementation significantly reduced the TC, LDL-C, non-HDL-C, LDL/HDL ratio, abdominal circumference, and subcutaneous fat area in obese men. No significant changes were observed in systolic and diastolic blood pressure and cholesterol composition. The study showed that the consumption of RB-ASG could improve dyslipidemia in obese subjects [21].
Chinese adults with borderline hypercholesterolemia were supplemented with 30 g of refined olive oil or blended oil 1 (BO1; 8000:720:300 mg/kg of oil of oryzanol: sesamin: sesamolin) and blended oil 2 (BO2; 4800:300:125 mg/kg of oil of oryzanol: sesamin: sesamolin) for 8 weeks. After 8 weeks of intervention, changes in the lipid profile were assessed. TC, LDL-C, TG, HDL-C, apoB-to-apoA1 ratio, blood pressure, and serum glucose levels were reduced significantly in all groups. The body weight of the subjects was also significantly increased in all groups. All the groups had no changes in apoA1 and HDL-C levels after treatment [22].
Overweight and obese adults on a 25% calorie-restricted diet were supplemented with pigmented RB (PRB) or PRB with plant sterols (PRB + PS) (30 g per day) for 8 weeks. After 8 weeks of supplementation, all the participants lost weight, and the weight loss did not significantly differ between the PRB and PRB + PS groups. The PRB + PS group showed significant levels of reduction in TC and LDL-C. PRB or PRB + PS supplementation significantly reduced the blood pressure, serum level of F2-isoprostanes, and leptin compared to baseline. The study claimed that the supplementation of PRB or PRB + PS, along with a calorie-restricted diet, could aid in reducing the risk of cardiovascular disease [23].
Similarly, overweight and obese adults on energy-restriction diets were supplemented with RB (70 g of RB/day) or rice husk (RH) (25 g of RH/day) for 12 weeks. The changes in the inflammatory markers and anthropometric changes were determined. All groups significantly reduced BMI, weight, and waist circumference. There were no significant changes in hs-CRP and IL-6 levels between RB-and RH-supplemented groups. However, significant differences were observed in serum hs-CRP and IL-6 levels in both RB-and RH-treated groups compared to their respective baseline values. The study revealed that supplementing RB or RH and an energy-restriction diet plan could improve the studied inflammatory markers in overweight and obese adults [24] (Table 3). The clinical studies showed that the RB extracts and calorie-restricted diet synergistically reduce obesity. RB with ASG and plant sterols showed better results, which could reduce the risk of cardiovascular disease and dyslipidemia. More clinical studies are needed to define the duration, dose, and combination of other natural supplements and RB supplementation to prevent and manage obesity.

Influence of Rice Bran Supplementation on Host Microbiome
The impact of RB supplementation on the microbiome of overweight and obese subjects has not been reported in detail. Some recent studies reported RB's influences on the obese host microbiome [107][108][109][110][111].
Bacillus amyloliquefaciens M4, B. subtilis M5, Bacillus sp. M6, Lactobacillus casei, Bifidobacterium bifidum, and Aspergillus oryzae-mediated fermented RB supplementation improved the microbiome and metabolism in high-fat-induced obese C57BL/6J mice. In detail, mice were fed a high-fat diet and 0.239% fermented RB for 8 weeks. The microbiome analysis showed that fermented RB supplementation did not increase the beneficial microbes, whereas it suppressed the unclassified family Peptostreptococcaceae and Enterococcus. Moreover, fermented RB supplementation significantly increased the fecal succinic acid concentration, correlated with the unclassified family Peptostreptococcaceae, Turicibacter, and Enterococcus abundances. Blood glutamic acid, malic acid, uracil, xylitol, and sorbitol levels were decreased, while the fumaric acid level was increased in the fermented RBsupplemented mice. The author claimed that fermented RB supplementation could affect the host metabolism and gut microbiome in high-fat-induced obese C57BL/6J mice [107].
Rb (20% in diet) and Saccharomyces cerevisiae Misaki-1 and Lactobacillus plantarum Sanriku-SU8-mediated fermented RB (FRB; 20% in diet) were supplemented to high sucrose and no-fiber-fed ICR mice for 2 weeks. The FRB supplementation decreased the TG and TC values. The RB-supplemented mice showed higher fecal frequency. Regarding the microbiome, the α-diversity of the microbiome was increased in RB-and FRB-supplemented groups. The abundance of Bacteroidetes and Firmicutes was increased in RB-and FRBsupplemented groups, respectively. FRB supplementation also reduced the microbes associated with diabetes and gut toxicity. The study claimed that FRB improved the gut microbiome and lipid profile of the high sucrose and no-fiber-fed ICR mice [108].
The influence of RB oil (RBO) on isoflavonoids and intestinal microbiota was studied in mice. Mice were supplemented with daidzein (0.05%) and RBO (10%) in the diet for 30 days. The RBO-supplemented group showed less urine daidzein and dihydrodaidzein, and a higher equol/daidzein ratio and fecal bile acids than the control group. RBO supplementation significantly increased the abundance of Lactobacillales, and a positive correlation was observed between fecal bile acids and Lactobacillales. The study stated that dietary RBO influences daidzein metabolism by modifying the intestinal microbiome [109].
The supplementation of RRB (Raw RB), RRBS (RRB stored for 3 months), IRRB (Infrared radiation-stabilized RB), and IRRBS (IRRB stored for 3 months) (300 mg/kg BW/day for 39 days) improved the obesity-associated parameters and gut microbiota in high-fatdiet-fed C57BL/6 mice. The RB supplementation reduced the body weight gain, serum TC, TG, LDL-C, alanine aminotransferase, aspartate aminotransferase levels, and TNF-α, IL-6, and INF-γ. The level of HDL-C/TC increased in RB-supplemented groups. The RB supplementation decreased the expressions of uncoupling protein 1 (UCP1), positive regulatory domain containing 16 (PRDM16), and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and increased the expressions of transcription factor 21 (TCF21) and homeobox protein Hox-C8 (HOXC8). The RB supplementation attenuated liver fat accumulation and enhanced white adipose browning, except for RRBS. The microbiome analysis showed that RB supplementation improved the abundance of Bacteroidetes and the Bacteroidetes/Firmicutes ratio. Moreover, the abundance of Desulfovibrio was decreased, while Akkermansia and Lachnospiraceae abundance was increased in RB-supplemented groups. These changes were not observed in the RRBS-supplemented group. The results indicated that RRB, IRRB, and IRRBS potently improved the lipid profile, inflammatory status, and microbiome of the high-fat-diet-fed obese mice model [111].
The studies revealed that RB supplementation aids in normalizing the host microbiome by reducing the abundance of pathogens and supporting the growth of beneficiary microbes. RB derivatives could act as prebiotics that could promote healthy normal microbial flora. The healthy microbial composition provides bioactive metabolites, further improving the host's health.
Several studies reported RB or RB derivatives-mediated changes in the host microbiome (Table 4), other than in obese conditions [112][113][114][115][116][117][118][119][120]. For example, arabinoxylan (AX) is found in RB, and the prebiotic property of AX has been studied in vitro. The in vitro fecal fermentation study with AX showed that it modified the fecal microbiota of obese and non-obese subjects. Collinsella, Blautia, and Bifidobacterium were increased; Sutterella, Bilophila, and Parabacteroides were decreased. Moreover, AX significantly increased the total and individual short-chain fatty acid levels [121].

Mechanisms Associated with the Anti-Obesity Property of Rice Bran
Studies in human and rodent models have suggested that dietary RB improves the cholesterol profile, antioxidant and inflammatory status, blood and vascular parameters, adipose, liver, and pancreas parameters, host metabolism, and microbiome, thereby delivering health benefits to the host.
The anti-obesity property of RB was not attributed to a single biological event, which is the result of several bioprocesses. The complete molecular mechanism associated with the beneficial effects of dietary RB has yet to be elucidated.
Obesity alters adipocytes and several other cellular mechanisms, which causes an increase in systemic oxidative stress [120] and chronic inflammation [122]. Dietary RB improved the antioxidant system of the host by significantly affecting the levels and expressions of iNOS, superoxide, NADPH oxidase, and SOD and CAT activities [86,90,108].
RB intervention significantly improved adipogenesis, reduced adipocyte size and adipose tissue mass, reduced adipocyte hypertrophy and lipid accumulation, and expressions of VEGF and MMP-2 in visceral fat tissue [89,93,97,103,104].
Regarding the microbiome, dietary RB and fermented RB significantly improved the beneficial microbial load (Bacteroidetes and Firmicutes) and α-diversity of microbiota and reduced the pathogenic bacterial load (e.g., Shigella sp.) in obese experimental models [109,110,112]. The possible comprehensive mechanisms relating to the anti-obesity property of RB have been illustrated (Figure 2).

Conclusions
The RB composition varies on several factors, including cultivars and geographic and weather conditions during the cultivation. Preclinical studies showed that the RB extracts, RB fractions, γ-oryzanol, and fermented RB supplementation improved vascular function, liver, and pancreas states, reduced adipocyte hypertrophy and lipid accumulation, improved the host antioxidant and inflammatory system, and host metabolism, and increased fecal cholesterol excretion. Specifically, dietary RB and its fractions improved the host microbiota and aided in restoring eubiosis. Because of the above-mentioned impact on the host, dietary RB collectively acts as an anti-obesity therapeutic agent. However, the therapeutic nature of RB needed to be developed appropriately. Further studies on dosage, duration of the supplementation, therapeutic strategies, and pharmacological and pharmacokinetic properties of dietary RB are mandatory. Developing RB-based functional products and their daily utilization may help prevent and manage obesity.