nutrients Article Effects of Doenjang, a Traditional Korean Soybean Paste, with High-Salt Diet on Blood Pressure in Sprague–Dawley Rats Eun-Gyung Mun, Jung Eun Park and Youn-Soo Cha * Department of Food Science and Human Nutrition, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Korea; [email protected] (E.-G.M.); [email protected] (J.E.P.) * Correspondence: [email protected]; Tel.: +82-63-270-3822 Received: 2 October 2019; Accepted: 7 November 2019; Published: 12 November 2019 Abstract: Fermented foods in Korea contain a lot of salt. Although salt is reported to exacerbate health trouble, fermented foods have beneficial effects. We hypothesized that doenjang could reduce blood pressure in Sprague–Dawley (SD) rats fed a high-salt diet. Eighteen SD rats were divided into three groups: normal-salt (NS) group, high-salt (HS) group, and high-salt with doenjang (HSD) group. The salinity of doenjang and saltwater was adjusted to 8% using Mohr’s method. Blood pressure was significantly reduced in the HSD group compared with the HS group. Water intake and urine excretion volume has significantly increased in the HS group compared with the HSD group. The excreted concentrations of urine sodium, urine potassium, and feces potassium significantly increased in the HSD group compared with the HS and NS groups. Renin level was significantly decreased in the HSD group compared to the other groups. These results indicate that eating traditional salty fermented food is not a direct cause of hypertension, and the intake of doenjang in normal healthy animals improved blood pressure. Keywords: high-salt diet; blood pressure; doenjang; renin; soybean paste 1. Introduction Doenjang, Korean soy paste, is a fermented soybean product in traditional Korean cuisine. Doenjang is the basis of various dishes such as soup, stew, and salad dressing. Doenjang is produced by fermentation with Bacillus subtilis and mold such as Aspergillus, Rhozopus, and Mucor species. Doenjang has a long fermentation period that extends from 2 to 24 months. Due to its fermentation, doenjang contains a high level of bioactive compounds such as isoflavones and saponins [1]. In addition, doenjang is a good source of essential amino acids, minerals, vitamins, and phenolic compounds [2,3]. In previous studies, doenjang showed anti-obesity effects in rat fed with high-fat diet [4], and overweight adults [5–7], anti-diabetic effects in high-fat diet-induced obese mice [8,9], anti-cancer effects in carcinogenic mice [10,11], and anti-inflammation effects in mice [12–14]. The intake of doenjang reduces body fat mass in overweight subjects [5] and decreases visceral or abdominal fat mass in overweight subjects with mutant alleles of PPAR-gamma or uncoupling protein-1 [6,7]. In the long term, the intake of fermented doenjang improved glucose tolerance and inhibited hyperglycemia in high-fat diet-induced obese mice [8,9]. These health benefits may be due to the higher content of aglycone isoflavones and the diversity and abundance of Bacillus probiotic strains [4,9]. Despite the health benefits, doenjang fermentation requires a large amount of salt. Doenjang is traditionally made from fermented meju that has been dipped in a 18%–20% salt solution for more than 30 days [15]. Doenjang is fermented with 4 kg of meju dipped in brine, 10 L of water, and 1.25 kg of salt; the final salinity of traditional doenjang is 12% [2]. Nutrients 2019, 11, 2745; doi:10.3390/nu11112745 www.mdpi.com/journal/nutrients Nutrients 2019, 11, x FOR PEER REVIEW 2 of 12 Nutrients 2019, 11, 2745 2 of 12 than 30 days [15]. Doenjang is fermented with 4 kg of meju dipped in brine, 10 L of water, and 1.25 kg of salt; the final salinity of traditional doenjang is 12% [2]. TheThe mainmain dietarydietary sources sources of of sodium sodium in in Korea Korea are are kimchi, kimchi, salt, salt, soy soy sauce, sauce, and and soybean soybean paste paste [16]. The[16]. World The World Health Health Organization Organization recommends recommends adults eatadul lessts thaneat less 2000 than mg 2000 of sodium mg of or sodium 5 g of salt or 5 every g of day.salt every Elevated day. sodium Elevated levels sodium can raiselevels blood can raise pressure blood and pressure may increase and may the increase risk of heart the risk disease of heart and strokedisease [ 17and]. Thus, stroke reducing [17]. Thus, sodium reducing intake sodium is recommended intake is recommended in the guidelines in the of manyguidelines organizations. of many Theorganizations. optimal level The of optimal sodium level intake of is sodium controversial; intake however,is controversial; the sodium however, reduction the programsodium reduction will help peopleprogram prevent will help cardiovascular people prevent disease cardiovascular in communities disease that in eat communities large amounts that of eat salt large [18 ].amounts However, of althoughsalt [18]. However, there are fewalthough studies ther thate are have few combined studies that doenjang have combined and salt, theredoenjang is a lackand ofsalt, research there is on a mechanismslack of research to prevent on mechanisms the side eff ectsto prevent of salt. Wethe hypothesizedside effects of that salt. consumption We hypothesized of traditional that fermentedconsumption foods of wouldtraditional offset fermented the side efffoodsects ofwould table saltoffset intake. the side Therefore, effects theof table antihypertensive salt intake. eTherefore,ffects of doenjang, the antihypertensive a traditional fermentedeffects of doenja soybeanng, paste,a traditional on the regulationfermented of soybean blood pressure paste, on were the investigatedregulation of in blood Sprague–Dawley pressure were (SD) inve rats.stigated in Sprague–Dawley (SD) rats. 2.2. Materials and Methods 2.1.2.1. Preparation of Doenjang DoenjangDoenjang waswas suppliedsupplied byby SunChangJangLyeSunChangJangLye Co.,Co., Ltd.Ltd. (Sunchang-gun,(Sunchang-gun, Korea).Korea). Meju,Meju, soybeansoybean fermentedfermented withwith Aspergillus oryzae and Bacillus subtilis for oneone month,month, andand saltwatersaltwater (26%, w//v)) werewere mixedmixed atat aa 1:31:3 ratioratio andand fermentedfermented forfor 22 months.months. WhenWhen the meju fermentationfermentation is complete,complete, it isis separatedseparated intointo the the liquid liquid phase phase and and the the solid solid phase. phase. The crushedThe crushed solid phasesolid phase was matured was matured for 6 months for 6 (Figuremonths1 ).(Figure Matured 1). doenjangMatured doenjang was stored was by freezestored drying.by freeze Freeze-dried drying. Freeze-dried doenjang and doenjang salt (Samchun and salt Chemical,(Samchun Seoul,Chemical, Korea) Seoul, were Korea) dissolved were in dissolved distilled waterin distilled to a salinity water ofto 8%a salinity [19–21] of using 8% Mohr’s[19–21] methodusing Mohr’s [22]. method [22]. Figure 1. A manufacturing process diagram of doenjang. Meju, block of fermented soybeans; sea salt, Figure 1. A manufacturing process diagram of doenjang. Meju, block of fermented soybeans; sea manufacture of common salt by solar heat. salt, manufacture of common salt by solar heat. 2.2. Animal Study 2.2. Animal Study Male SD rats, aged three weeks, were purchased from Central Lab Animal Inc. (Seoul, Korea). The animalsMale SD were rats, fedaged on three an AIN-76 weeks, diet were (standard purchase diet,d from Research Central Diets, Lab NewAnimal Brunswick, Inc. (Seoul, NY, Korea). USA) forThe one animals week, were and eighteenfed on an rats AIN-76 were diet randomly (standar dividedd diet, intoResearch three Diets, groups New (n = Brunswick,6): normal-salt NY, groupUSA) Nutrients 2019, 11, 2745 3 of 12 (0.3%, NS), high-salt group (8%, HS), and high-salt with doenjang group (HSD). Animals were kept at a temperature of 24 2 C with a humidity of 60 5% and a light/dark cycle of 12:12 h. They were ± ◦ ± given free access to the AIN-76 diet and tap water. The rats were fed orally administered 10 mL/kg body weight (BW) for the eight-week experimental period (Table1). From the fifth to seventh weeks of the experimental period, rats were housed in metabolic cages for 24 h a week. Urine and feces were collected for analysis. The Animal and Use Committee of Chonbuk National University approved the experimental protocol (CBNU 2017-0016). Table 1. Ingredients administered orally during the experimental period. Crud Sodium Potassium Energy Carbohydrate Crud Salt Group 1 Ash (g) Protein (mg/kg (mg/kg (kcal) (g) Fat (g) (%) (g) BW/day) BW/day) NS 0 0 0 0 0 0 0 0 HS 0 0 0 0 0 8.00 314.72 0 HSD 72.75 7.37 4.80 4.24 4.06 8.02 229.29 14.98 Body weight (BW). 1 The normal-salt (NS) group was fed with distilled water. The high-salt (HS) group was fed with saltwater that dissolved NaCl in distilled water. The high-salt with doenjang (HSD) group was fed with doenjang solution that mixed freeze-dried deonjang in distilled water. 2.3. Collection of Serum and Organs After 12 h of overnight fasting, rats were anesthetized with 2 mg/kg BW of alfaxan (Jurox, Australia) and 0.5 mL/kg BW of rompun (Bayer, Seoul, Korea) by intramuscular injection, and blood was collected by orbital vein puncture. Serum was centrifuged at 1100 g for 15 min at 4 C. Liver and kidney were × ◦ harvested, rinsed, and weighed. Both tissues and serum were stored at 80 C until analysis. − ◦ 2.4. Measurement of Blood Pressure Blood pressure was assessed weekly by the tail-cuff method (BP-2000; Visitech Systems, Inc., Apex, NC, USA) after 6 h of oral administration. 2.5. Serum Profile Analysis Serum glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) were analyzed using a commercially available kit (Asan Pharmaceutical Co., Seoul, Korea). Renin, angiotensin II (Ang II), and aldosterone in serum were measured by ELISA using a Rat Renin ELISA Kit (MyBioSource, San Diego, CA, USA), Angiotensin II ELISA Kit, and Aldosterone ELISA Kit (Enzo Life Sciences, Inc., Farmingdale, NY, USA).