Black soybean improves the vascular function through an increase in nitric oxide and a decrease in oxidative stress in healthy women

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Highlights

  • Black soybean improves vascular function in healthy women.

  • Black soybean increases nitric oxide concentration and decreases oxidative stress markers.

  • Black soybean polyphenols are involved in the improvement of vascular function.

Abstract

Vascular dysfunction and injurious stimuli such as oxidative stress is closely related to the risk of cardiovascular diseases (CVD). Dietary polyphenols is reported to exert the beneficial effects on reducing the risk of CVD. Black soybean is rich in polyphenols, including isoflavones, anthocyanidins and flavan-3-ols, and its prevention effects on CVD risk were reported in the animal experiments. In this study, we investigated the effect of black soybean consumption on the vascular function and oxidative stress associating with the polyphenol concentrations in healthy women. Lowered vascular age was observed in 33 out of 44 volunteers who completed the 8-week trial. It was observed that improvement of the vascular stiffness, increasing in the urinary NO2 and NO3 level, and decreasing in the oxidative stress markers, 8-hydroxy-2′-deoxyguanosine, hexanoyl-lysine and myeloperoxidase. In addition, concentration of 12 polyphenols in black soybean increased in the plasma and urine. Increased concentration of polyphenols would be involved in the decreased oxidative stress. Thus, black soybean consumption improved the vascular function through an increase in nitric oxide and a decrease in oxidative stress accompanied by increasing the polyphenol concentrations in healthy women.

Introduction

Vascular function is closely related to the risk of cardiovascular diseases (CVD) [1]. Aging process is related to the vascular dysfunction as the dominant risk factor [2,3]. It causes functional and structural changes of the vascular wall. An increase in the vascular stiffness is the major symptom, and it compromises vascular adaption to blood flow and pressure changes. Oxidative stress is another important trigger of vascular dysfunction, because it is widely accepted that oxidative stress is closely related to the aging process [4], indicating that decline in the vascular function is an inevitable result of aging. Alteration of nitric oxide (NO) is a key molecular event for the underlying mechanism of vascular function [5]: Releasing of NO from endothelial cells decreases the intracellular concentration of calcium, causes relaxation of vascular smooth muscle as a potential vasodilator. However, aging and oxidative stress quenches the production of NO and hampers the NO-mediated responses and eventually leads to the vascular dysfunction [6].

Black soybean (Glycine max) is originated from Asia and has been consumed as a health food and folk medicine for these centuries [7]. Grain of black soybean is rich in proteins, lipids, minerals and isoflavones as the same as that of yellow soybean. However, it has black pigments in the seed coat consisting of polyphenols such as anthocyanidins and flavan-3-ols [8,9]. These polyphenols contribute to the total antioxidant capacity of whole black soybean [10]. Polyphenols in the seed coat distinguish black soybean from the yellow one, and black soybean shows higher antioxidant capacity than other legumes including yellow soybean [11]. Previous study demonstrated that consumption of black soybean (35% of experiment diet) for 10 weeks in ovariectomized rats inhibited oxidative stress by increasing antioxidant activity and improving lipid profiles, resulting in the risk factors associated with CVD were greatly improved [12]. Results from another study demonstrated that an oral administration of black soybean extract at 50 and 100 mg/kg body weight to rats for 14 days reduced the risk of CVD by improving blood circulation through inhibiting platelet aggregation and thrombus formation [13]. For human trial, it was reported that supplementation of black soybean test extract (2.5 g/day) for 8 weeks improved visceral fat accumulation and plasma lipid profiles in overweight Korean adults [14]. However, there is no human trial on the beneficial effects of black soybean consumption on reducing the risk of CVD. Moreover, these previous studies did not measure the physiological concentration of each polyphenol after the consumption of test materials and analyze the correlation between the observed effects and polyphenol concentrations in the body, although polyphenols are considered to responsible for these beneficial effects. Therefore, in this study, we investigated that the effect of black soybean consumption on the vascular function and oxidative stress associating with polyphenol concentrations in healthy women by an open-label study.

Section snippets

Design of human study

This study was approved and conducted by the Institutional Review Board of Fujicco Co., Ltd. (Trial registration: #5702) in accordance with the Declaration of Helsinki. To perform the study, informed consent was obtained from all volunteers. Volunteers were excluded if they fulfilled any of the following criteria: (1) having a clinical history of severe gastrointestinal disease, liver disease, kidney disease or heart disease; (2) currently undergoing treatment for metabolic syndrome or its

General characteristics of volunteers

Before the trial, an average vascular age of volunteers was 39.2 ± 10.2 years old with the average BMI of 20.8 ± 0.42 kg/m2. The vascular age was significantly higher than the chronological age which was 33.5 ± 10.0 years old (p < 0.01). At the end of the trial, 3 volunteers dropped out for non-health related reasons and 44 volunteers completed the 8-week trial with no report on poor health or any abnormality. No significant changes in anthropometric and hematologic parameters were observed

Discussion

Vascular function is closely related to the high risk of CVD [[1], [2], [3], [4]]. Recently, many researchers are focusing on the effect of dietary intake of polyphenols, especially flavonoids on the prevention of CVD [19,20]. Black soybeans are rich in flavonoids and its amelioration and preventive effects on CVD was reported in the animal experiments [12,13,21,22]. In our best knowledge, this is the first report showing the improvement of vascular function after consumption of black soybeans

Conclusions

The intake of 30 g/day of roasted black soybeans for 8 weeks significantly improved the vascular function and reduced oxidative stress. The intake of black soybeans also increased the physiological concentration of polyphenols in the plasma and urine, which contributed to the improvement of vascular function and reduction of oxidative stress leading to lowering the risk of CVD in human.

Declaration of competing interest

Fujicco Co. Ltd. partly funded the investigations described in the present manuscript. FN, SS, TT are employees of Fujicco Co. Ltd. They contributed to the study design, data collection and analysis.

Acknowledgement

This work was supported by Cross-ministerial Strategic Innovation Promotion Program by Cabinet Office, Government of Japan, and Japan Society for the Promotion of Science KAKENHI Grant-in-Aid for Scientific Research (A) Grant Number 17H00818 (H.A.).

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  • 1

    These authors contributed equally to this work.

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