## 1 Introduction  
_Morus alba_ is a deciduous tree cultivated in various tropical, subtropical, and temperate countries, including China, Japan, Korea, Thailand, and others. _M. alba_ contains abundant bioactive compounds; for example, mulberry fruit and leaves contain alkaloids, flavones, sterols, volatile oils, and polysaccharides [1]. These natural bioactive compounds exhibit potent physiological functions such as anti-diabetic, anti-oxidative, anti-obesity, and anti-microbial activities [2]. Mulberry is considered as a suitable ingredient of functional foods for its nutrition and health benefits, and mulberry products can widely be found in the market, including mulberry powder, dried fruit, juices, jams, desserts, and candies. In addition, mulberry fruit and leaves are incorporated into a variety of traditional medicines due to their potential of improving diseases [3]. In ancient China, diabetes was called XiaoKe syndrome, with comprehensive symptoms such as polyuria, polydipsia, overeating, weight loss, and fatigue. According to the Compendium of Materia Medica, drinking decoded mulberry leaves to treat diabetes had a long history in ancient China. Recent research also indicates that mulberry fruit and leaves are beneficial for blood sugar levels [4, 5]. Among the bioactive compounds synthesized in mulberry, 1-Deoxynojirimycin (DNJ), a natural alkaloid azasugar and iminopyranose, has been reported as an antihyperglycemic agent [6, 7, 8]. Compared to oligosaccharides, DNJ has a higher affinity for glycosidase so it can inhibit glycosidase from combining with oligosaccharides to produce glucose, thereby achieving blood sugar lowering [9]. In addition, DNJ was reported to be an effective antioxidant. [PERSON] et al. found that in patients with stable angina pectoris, 4-week treatment with DNJ significantly increased the serum superoxide dismutase (SOD) level, which the authors thought was associated with the improvement of stable angina pectoris symptoms [10]. [PERSON] et al. reported that mulberry leaf DNJ treatment improved the antioxidant and anti-inflammatory properties and serum lipid profile in heart disease patients. DNJ reduces carotid intima-media thickness, and the serum levels of DNJ showed a strong relationship with intima-media thickness values [11]. [PERSON] et al. found in their mulberry juice fermentation study that with the increase in DNJ and quercetin contents, the ABTS\({}^{+}\) free-radical-scavenging activity of mulberry beverages was notably enhanced [12]. These previous studies indicated the antioxidant potential of DNJ. In addition to being a cause of diabetes, high blood glucose also can lead to other health problems, such as cell aging and oxidative injuries [13; 14; 15]. [PERSON] et al. reported that mulberry leaf extract, which contains DNJ, flavonoid, and polysaccharide, enhanced insulin sensitivity and alleviated the oxidative damage in a type 2 diabetes rat model by increasing the activities of anti-oxidative enzymes such as SOD, catalase, and glutathione peroxidase [16]. [PERSON] et al. found that in a type 2 diabetes mouse model, DNJ treatment significantly lowered the serum level of glucose and increased the serum SOD activity [17]. These studies proposed the multi-faceted roles of DNJ in its improvement of diabetes. In response to oxidative stress, anti-oxidative signaling pathways will be stimulated to protect cells. Nuclear factor (erythroid-derived 2)-like 2 (NRF2) is a master regulator of anti-oxidative responses in mammalian cells and regulates the expression of series of proteins which function to maintain cellular oxidative status equilibrium. Under normal conditions, NRF2 is sequestered in the cytoplasm and degraded via the ubiquitin-mediated pathway. When cellular oxidative stress increases, NRF2 translocates from the cytoplasm into the nucleus where it transcriptionally activates its target genes.