Antidiabetic effects of fermented soybean products on type 2 diabetes
Dae Young Kwon
a, James W. Daily III
b, Hyun Jin Kim
a, Sunmin Park
c,⁎
aEmerging Innovative Technology Research Division, Korean Food Research Institutes, Sungnam
bDaily Manufacturing Inc., Rockwell, NC, USA
cDepartment of Food and Nutrition, Obesity/Diabetes Research Institutes, Hoseo University, Asan Received 30 September 2009; revised 12 November 2009; accepted 15 November 2009
Abstract
Historically, the incidence of type 2 diabetes has been lower in Asian populations compared with those in Western countries. One possible reason for the lower incidence among Asians is that they consume fermented soybean products, which are unique to the traditional Asian diet. Some have hypothesized that dietary phytoestrogens and soy peptides in fermented soybean foods consumed in traditional Asian diets may help prevent and slow the progression of type 2 diabetes. This review evaluates the existing evidence from animal studies and clinical and epidemiologic investigations on fermented soybeans in the prevention and treatment of type 2 diabetes. Nutritional studies performed in animals and intervention studies with humans suggest that the ingestion of soy protein with isoflavones improves glucose control and reduces insulin resistance. Korean fermented soybean products such as doenjang, kochujang, and chungkookjang contain alterations in the structures and content of isoflavonoids and small bioactive peptides, which are produced during fermentation. Several studies revealed improvements in insulin resistance and insulin secretion with the consumption of these fermented products. Therefore, fermented soybean products may help prevent or attenuate the progression of type 2 diabetes. Although the lack of human intervention trials does not permit definitive conclusions, the evidence does suggest that fermented soy products may be better for preventing or delaying the progression of type 2 diabetes compared with nonfermented soybeans.
© 2010 Elsevier Inc. All rights reserved.
Keywords: β-Cell function;β-Cell mass; Fermented soybean products; Insulin resistance; Isoflavonoids; Peptides; Type 2 diabetes Abbreviations: IGF-1, insulin-like growth factor 1; IRS-2, insulin receptor substrate 2; OVX, ovariectomy; PPAR, peroxisome-
proliferator–activated receptors; SERMs, selective estrogen receptor modulators.
1. Introduction
Because type 2 diabetes in Western populations is typically accompanied by obesity and hyperinsulinemia, insulin secretion has received less emphasis than do insulin resistance. However, recent studies have revealed the importance of insulin secretion capacity to prevent type 2 diabetes. Type 2 diabetes is a heterogeneous metabolic
disorder characterized by both insulin deficiency and peripheral insulin resistance[1]. The pancreaticβcell adapts to increased nutrient availability and insulin resistance by increasing its function and mass [2]. These processes are orchestrated by signals derived from nutrient metabolism, hormones, and cytokines. The failure of β-cell adaptation results in type 2 diabetes[3]. Thus,β-cell dysfunction and deficient mass pancreatic mass play crucial roles in the development of insulin resistance and subsequent β-cell compensation failure and diabetes[4].
The primary defect in type 2 diabetes is increasedβ-cell apoptosis, which decreases β-cell mass [5]. Therefore, the progressive nature of type 2 diabetes is due toβ-cell failure.
Nutrition Research 30 (2010) 1–13
www.nrjournal.com
⁎ Corresponding author. Department of Food and Nutrition, Hoseo University, Chungnam-Do 336-795, Korea. Tel.: +82 41 540 5633; fax: +82 41 548 0670.
E-mail address:[email protected](S. Park).
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doi:10.1016/j.nutres.2009.11.004
Attenuating insulin resistance, improving β-cell function, and increasingβ-cell mass are important for preventing and delaying the progression of type 2 diabetes.
Soybeans (Glycine max MERILL) have long been important protein sources, complementing grain proteins, in Asian countries. Asians typically consume 9 to 30 g soybeans per day, with individual and regional variations (Table 1)[6]. In addition to protein, soybeans also contain various nutrients and functional components including isoflavonoids [7]. Because isoflavonoids have structural similarities to endogenous estrogen, they exert estrogenic or antiestrogenic action with weak binding affinity to the estrogen receptors, where they act as agonists or competitive antagonists [8]. Estrogen is reportedly beneficial for preventing and treating type 2 diabetes by attenuating insulin resistance, improving insulin secretion, and increas- ing β-cell mass [9]. Therefore, soy isoflavonoids may improve glucose homeostasis through estrogenic action. In addition, soy proteins may also improve glucose metabo- lism. Hence, soybeans may help prevent type 2 diabetes and delay its progression.
Fermentation is one of the major processes used in the production of food from soybeans. Fermented bean paste is indigenous to the cuisines of East and Southeast Asia.
Korean soy foods are increasingly present on the worldwide market, and because kochujang and fermented soybean pastes (deonjang and chungkookjang) were registered in CODEX on July 4, 2009, they are now internationally accepted foods. Most fermented soybean pastes are salty and savory and some are spicy. They are often used as condiments to flavor foods such as stir-fries, stews, and soups. Differences in their colors and flavors are due to different production methods such as the conditions of fermentation; the addition of wheat flour, pulverized mantou, rice, or sugar; and the presence of different microflora such as bacteria or molds used in their production, as well as whether the soybeans are roasted (as in chunjang) or aged (as in tauchu) before being ground. The fermentation of these soybean products changes the bioactive components, such as isoflavonoids and peptides, in ways which may alter their efficacy in the treatment of type 2 diabetes. The intake of soybeans and fermented soybeans may be associated with the antecedently lower incidence of type 2 diabetes in Asians [10]. This review is based on the hypothesis that bioactive compounds found in soybean have antidiabetic properties
and that fermentation changes those bioactive compounds in ways that potentiate the antidiabetic properties.
Herein we discuss how fermented soybean products, isoflavones, and soy peptides improve glucose metabolism, insulin resistance, insulin secretion, and β-cell mass. This report reviews epidemiologic and experimental evidence describing the effectiveness and possible mechanisms that support and challenge the hypothesis. Some human studies have been conducted to investigate the antidiabetic effects of soybeans and soy isoflavonoids, but a few studies have investigated fermented soybeans. Thus, this review includes a discussion of soybeans and their compositional changes that occur during fermentation to support the antidiabetic effects and mechanism of action of these fermented soybean products.
2. Fermented soybean products
Various types of fermented soybean foods are consumed in Asian countries such as Korea, China, Japan, Indonesia, and Vietnam. The most common Korean fermented soybean foods are chungkookjang, doenjang, kochujang, and soy sauce. Natto and miso are Japanese versions of chungkook- jang and doenjang, respectively. China also has various fermented soybean products such as doubanjiang, douchi, sweet noodle sauce, tauchu, yellow soybean paste, and dajiang. Doenjang, kochujang, and soy sauce are fermented with varying microorganisms when traditionally made, because fermentation conditions and ambient microbiota are different among regional environments (Fig. 1). To make these products by the traditional method, cooked soybeans are formed into blocks and fermented outdoors for 20 to 60 days by microorganisms naturally present in the environ- ment. The fermented blocks are known as meju and are used to prepare the soy pastes and sauce. Meju can be fermented for less than 5 days by inoculating with microorganisms such as Aspergillussp; however, when fermented by traditional methods meju is typically fermented primarily by Bacilli species during the early stages of fermentation, followed by Aspergillusspecies, which predominate during the remaining fermentation period. Aspergillus oryzae is the major microorganism in the final product of meju when it is made in the traditional way. Chungkookjang is a short-term fermented soybean product similar to Japanese natto, whereas doenjang, kochujang, and soy sauce undergo long- term fermentation as do Chinese tauchu and Japanese miso.
3. Changes in soybean components by fermentation 3.1. Nutritional and functional compounds in soybeans
Soybean products have been designated as one of world's healthiest foods due to being an excellent source of high- quality protein as well as providing various health benefits.
The protein content of soybean is 32% to 42% (depending on the variety and growth conditions) of which approximately
Table 1
Consumption of soybeans and prevalence of cancer in various countries Countries Soybean
consumption (g/d)
Breast cancer prevalence
Prostates cancer prevalence
Japan 29.5 60 35
Korea 19.9 26 5
Hong Kong 10.3 84 29
China 9.3 47 28
United States minor 224 157
Prevalence is defined as the number of diagnosed cancers per 100 000.
80% is composed of 2 storage globulins, 7S globulin (β- conglycinin) and 11S globulin (glycinin), having various functional and physicochemical properties[11-13]. Soybean products are considered a good substitute for animal protein, and their nutritional value is almost equivalent to that of animal protein because soy proteins contain most of the essential amino acids for human nutrition. Soybean products have less saturated fat and various phytochemicals, which possess numerous health benefits [14]. In particular, the association of high-quality protein and phytochemicals, especially isoflavones, is unique among plant-based proteins because isoflavones are not widely distributed in plants other than legumes [15].
Soybeans contain 0.1 to 5 mg total isoflavones per gram, primarily genistein, daidzein, and glycitein [15]. These nonsteroidal compounds, commonly known as soy phytoes- trogens, are naturally present as the β-glucosides genistin, daidzin, and glycitin, representing 50% to 55%, 40% to 45%, and 5% to 10% of the total isoflavone content, respectively [16]. Total isoflavonoid contents are varied among different soy products (Table 2) [14-16]. In addition to high-quality protein and isoflavones, soybeans contain high levels of unsaturated fatty acids, dietary fiber, and minerals.
3.2. Changes in functional components during fermentation The qualitative and quantitative composition of soybean components is dramatically changed by physical and enzymatic processes during the preparation of soy-based foods [11,17-23]. Fermentation is an excellent processing
method for improving nutritional and functional properties of soybeans due to the increased content of small bioactive compounds. The large protein, lipid, and carbohydrate molecules in raw soybean are broken down by enzymatic hydrolysis during fermentation to small molecules such as peptides, amino acids, fatty acids, and sugars, which are responsible for the unique sensory and functional proper- ties of the final products. Short-term fermented soy foods such as chungkookjang and tempeh, which are fermented with Bacillus subtilis and Rhizopus oligosporus, respec- tively, for less than 72 hours have a much greater concentration of large molecules than do long-term fermented foods including doenjang and miso, which are fermented for more than 6 months with Aspergilus and Bacillus species from rice straw and koji, respectively (Table 3) [11,17-23].
The degradation of lipids and carbohydrates proceeds especially rapidly during the initial stage of fermentation.
Dry matter declines continuously during fermentation time, 70% to 80% of which is due to decreases in crude lipid and carbohydrate, which are used as the major energy sources for the microorganisms[24]; the remaining loss is largely due to fermentative hydrolysis of approximately 25% of the soy protein, thereby producing ammonia. Of this hydrolyzed protein, 65% remains in the fermented products as amino acids and peptides, 25% is assimilated into the mold biomass, and 10% is oxidized [24]. After the initial stage of fermentation, however, soy proteins are rapidly degraded and only 9% to 17% of the crude protein remains at the end stage of long-term fermentation.
Fig. 1. The preparation of fermented Korean foods made from soybeans.
Among the various small metabolites derived from macromolecules, the changes in amino acid and peptide concentrations were especially prominent, although qualita- tive and quantitative changes in individual peptides were not studied. Amino acids increased or remained almost constant with increased fermentation time, but glutamate, the richest amino acid in soybean, was obviously decreased by fermentation, suggesting that microorganisms might use it as a preferred nitrogen source[25].
Isoflavones, which are mostly present as 6-ο-malonylglu- coside and β-glucoside conjugates and associated with proteins in soybean, are also broken down by heat treatment
and fermentation [26,27]. During preparation of fermented soy foods, 6-ο-malonylglucosides, the most prevalent soybean isoflavones, are converted to 6-ο-acetylglucosides or β-glucosides by heating, and β-glucosides are unconju- gated by the action ofβ-glucosidases secreted by fermenta- tion microorganisms [16]. Most isoflavones are not enzymatically hydrolyzed during short-term fermentation, in contrast to long-term fermentation in which 6-ο- malonylglucoside content declines with increasing fermen- tation time with concomitant increases in unconjugated aglycones (genistein and daizein). In chungkookjang, the aglycones are 21 times higher than in soybean [17].
Moreover, the chemical profiles of various minor compo- nents related to health benefits and nutritional quality of products are also affected by fermentation[28,29]. Unlike the production of small molecules by degradation of large ones during fermentation, the content of some compounds is changed by artificial addition during the process.
Adding salt in particular increases the sodium content in soy paste of doenjang and miso, 9-9.7 g/100 g and 3.7 g/
100 g, respectively, whereas potassium content in doenjang, tempeh, and miso is decreased by fermentation. However, chungkookjang and tempeh do not have higher sodium contents because salt is not added [30-32]. Although
Table 2
Isoflavone contents in various soy products
Foods Contents
(mg%)/100 g
Foods Contents
(mg%)/100 g Miso 42.6 ± 9.2 Japanese soybeans 118.5 ± 22.2 Natto 58.9 ± 7.4 Korean soybeans 145.0 ± 10.7 Nonfat soy protein 177. 9 ± 12.6 American soybeans 128.4 ± 11.7 Soymilk in USA 9.6 ± 1.8 Bean sprout 40.7 ± 8.3
Soymilk in Asia 25.2 ± 1.2 Tempe 43.5 ± 8.3
Doenjang 31.5 ± 9.3 Tofu 23.6 ± 6.3
Isolated soy protein 97.4 ± 11.1 Ganjang 1.0
Table 3
Components of soybean and various soy fermented foods
Soybean Doenjang Miso Chungkookjang Tempeh
Reference
[11,15,19,30] [15,16,19,21,32] [18,19,33] [15,20,33] [17,19,30]
Protein (%) 20-42 12 10-17 41 23-55
Lipid (%) 18-22 4-11 3-11 26 14-23
Carbohydrate (%) 35 11 15-38 24 10-30
Isoflavone (mg/100 g) Genistin 36-86 1-2 3-27 87-91 6-19
Daidzin 15-57 0.3-0.6 3-18 79-93 2-3
Glycitin 2-6 0.1 1-2 10-12 0.1-0.4
Malonyl genistin 123-186 0.5-0.8 0.1-3 20-22 39-42
Genstein 0.2-5 0.1 11-47 3-4 7-10
Daidzein 0.3-5 0.1-3.7 9-36 4-7 7-8
Glycitein 0.1-0.6 0.4-0.7 1 11-13 0.5-0.7
Amino acid (g/100 g) Aspartate 1.4-1.7 1.2 1.8-4.6 3.4-3.8 1.5
Glutamate 3.8-4.8 3 4.9-9.9 4.4-5.5 2.3
Serine 1.1-1.4 0.8 2.0-5.7 1.5-1.8 0.8
Glycine 0.7-0.9 0.6 1.4-4.7 1.3-1.5 0.6
Arginine 1.5-1.9 1.2 0.5-3.7 1.9-2.1 0.9
Alanine 0.8-1.0 0.8 – 1.5-1.6 0.6
Proline 1.1-1.3 1.2 2.4-6.4 1.8-2.0 0.6
Histidine 0.7-0.9 0.4 3.0-9.0 0.9 0.4
Valine 0.8-1.0 0.7 2.0-5.0 2.0-3.0 0.6
Methionine 0.2-0.3 0.2 1.8-9.0 0.3-0.4 0.2
Cystein 0.5-0.6 1 0.01-0.03 0.3-0.4 0.2
Isoleucine 1.0-1.3 0.9 3.6-7.3 1.5-1.6 0.7
Leucine 1.6-2.0 0.4 3.2-5.9 2.5-2.7 1.1
Phenylalanine 1.1-1.4 0.7 1.7-3.3 1.7-1.8 0.5
Tyrosine 0.8-1.0 0.9 1.5-2.7 0.9-1.0 0.5
Lysine 1.4-1.7 0.8 2.4-4.5 2.1-2.7 0.8
Threonine 0.8-1.0 0.7 1.4-2.6 1.2-1.4 0.5
Mineral (g/100 g) Na 0.02 9.1-9.7 3.7 0.03 0.006
K 2.3 0.8-1 0.2 2.4 0.4
doenjang and miso contain high contents of salt, they have not increased sodium intakes in Asians because they are used as seasoning in place of salt. Thus, doenjang and miso may not increase salt-related hypertension.
4. Absorption and bioavailability of soy isoflavones and proteins
Soybeans contain numerous biologically active compo- nents. However, those most studied for antidiabetic proper- ties are the 2 major isoflavones, genistein and daidzein, and soy proteins. The structural changes that occur in soy proteins and isoflavones may be responsible for the confusing and contradictory results of the research into the biologic activities of soy foods, which include many different commonly used fermented and unfermented foods, extracts, and supplements.
4.1. Isoflavones
A study in Japanese men and women by Yang et al[33]
found that soy isoflavone aglycones are better absorbed than their corresponding glucosides. This study found that both genistein and daidzein were much better absorbed, with peak plasma concentrations almost 4 times higher, than were their glucoside counterparts. Peak plasma concentrations also occurred 2 hours earlier for aglycones than for glucosides. It seems reasonable that aglycones in foods would be better absorbed because isoflavones must be in the aglycone form to be absorbed from the gut [34]. However, many other investigators have not reported results consistent with Yang et al[33]. Most of the isoflavones in foods are glycosylated [35]; therefore, isoflavone absorption is delayed until glycosylated isoflavones can be hydrolyzed, but they are ultimately absorbed at the same levels in rats[36]. Likewise, Zubik and Meydani[37]found little difference in either total absorption or rate of absorption in humans.
The lack of consensus on the comparative absorption of isoflavone glucosides and aglycones continues, but a recent study suggested better absorption of daidzein when con- sumed as a glucoside [38]. In that study, the fractional absorption of the daidzein aglycone was 11.6 versus 38.9 for the glucoside. However, simply comparing glucoside with aglycone forms of isoflavones may not be adequate because many other factors such the food matrix, intestinal bacteria, and others may be important determinants of absorption and bioavailability. Studies of the effects of food matrices have also been contradictory[39-43]; however, because there are many different food matrices in typical diets, it is an even more complex evaluation than glucoside versus aglycone comparisons. Despite the lack of consensus in these important areas, some conclusions can be drawn about the absorption and bioavailability of isoflavones. First, it is well established that isoflavones are absorbed as aglycones in various areas of the intestine. Second, it has been repeatedly demonstrated that the glycosylated isoflavones can be
hydrolyzed to the aglycone forms by lactase enzyme in the small intestine and by β-glucosidases associated with gut bacteria, especially Bifobacteriumstrains [44-46], and that intestinal absorption occurs throughout the small intestine but most isoflavone absorption occurs in the large intestine.
Delayed activity of the enzymatic release of aglycone is probably responsible for the late peak appearance of the isoflavones in the blood (4-8 hours after ingestion).
Therefore, it is possible that isoflavonoids in fermented soybeans may have enhanced absorption compared with those in nonfermented soybeans. It is interesting that most of the studies never recovered at least half of the isoflavones in either urine or feces, and it is likely that much of the ingested isoflavones are degraded into other metabolites in the gut.
The possible bioactivities of isoflavone metabolites have not been well studied, with the exception of the daidzein metabolite, equol[47-49].
The complexity of isoflavone metabolism makes it difficult to characterize the absorption and metabolism of these compounds. Several factors contribute to the under- standing the complex biologic processes influencing how the chemical structures are handled. Animal models and humans may be very different in how they metabolize isoflavones.
For example, isoflavone aglycones are absorbed in the stomach of rats[34], something that has not been described in humans. There may also be many variations among humans because it is known that children absorb isoflavones more efficiently than do adults [50,51]. In addition to the effects of age on isoflavone absorption, there may also be effects due to different food matrices and gut bacterial populations. These might explain why some studies have shown better absorption of agyclones[33], but others have not [52]. Populations with very different diets may absorb and metabolize isoflavones differently. Currently, it seems unlikely that fermenting soy foods increases the absorption of isoflavones directly; however, it is possible that fermentation produces isoflavone metabolites with potential biologic activities and that the probiotic effects of fermented foods may result in a gut bacterial population that assists in isoflavone hydrolysis and absorption.
4.2. Soy peptides produced by fermentation
Bioactive oligpeptides from fermented soy is an emerging area of research with great promise. It is well known that the savory flavor of fermented and nonfer- mented soy foods, such as soy sauce and hydrolyzed vegetable protein, is a result of the release of small peptides and amino acids from the fermentative digestion or acidic hydrolysis of soy proteins[53,54]. Soy protein is reported to have numerous beneficial effects in humans, including improvements in body composition and insulin secretion[54], but it is not known if peptides released from digestion of the proteins contribute to the observed effects.
Lunasin, a 43-amino-acid peptide from soy, has been shown to have numerous biologic properties including
anticancer and anti-inflammatory activities, some of which may depend on epigenetic mechanisms by inhibiting the acetylation of histones [55-57].
Fermented soy peptides from miso[58]and doenjang[59]
have been shown to possess angiotensin-converting enzyme inhibitory activity and tripeptides (Val-Pro-Pro and Ile-Pro- Pro) from casein-added miso paste are reported to act as antihypertensive agents in spontaneously hypertensive rats [58]. Peptides from soy are currently the subject of investigation for new drugs and functional food ingredients for gut health and modulating the intestinal absorption of nutrients[60]. Research into bioactive soy peptides is still in its infancy, but holds great promise.
5. Type 2 diabetes and estrogen
5.1. The phathophysiology of type 2 diabetes
Insulin resistance is a characteristic feature of type 2 diabetes mellitus. Insulin resistance is typically accompanied by hyperinsulinemia and islet hyperplasia and hypertrophy to maintain normoglycemia[61-63]. When insulin secretion cannot compensate for insulin resistance, type 2 diabetes develops. Insulin resistance affects β-cell function with biphasic states, initially enhancing insulin output by stimulating islet hyperplasia, but subsequently reversing these compensatory changes when extreme insulin resistance has been sustained for a long period[62,63]. The mechanism by which β-cells become incapable of satisfying insulin demand has never been revealed in humans. One possibility is pancreaticβ-cell exhaustion with decreasedβ-cell mass. A postmortem study indicated that the decompensation for insulin resistance is associated with decreasedβ-cell mass in patients with diabetes [64]. Because obese type 2 diabetic patients have greater insulin secretion capacity with β-cell hypertrophy, it takes time to develop diabetes from glucose intolerance. However, nonobese type 2 diabetes is a rapidly developing insulin deficiency and β-cell insufficiency in which insulin resistance is induced during condition such as aging, inflammation, and obesity. Therefore, diabetes needs to be treated by relieving insulin resistance and potentiating β-cell function and mass.
5.2. Estrogen effects on type 2 diabetes
Although aging plays an important role in insulin resistance and β-cell dysfunction in both men and women [65,66], the prevalence and progression are lower in premenopausal women in comparison to men despite their higher body fat, but their incidence markedly increases with higher fat accumulation after menopause. This phenomenon suggests that estrogen prevents insulin resistance andβ-cell dysfunction. There is substantial evidence that postmeno- pausal estrogen replacement can improve insulin sensitivity [67,68], although the data are not consistent [69,70].
However, rather recent studies have shown that estrogen replacement modulatesβ-cell function and mass[9,71]. The
changes in insulin resistance are related to body fat. Several studies showed that ovariectomy (OVX) did not increase serum leptin levels even with a high-fat diet due to decreased leptin synthesis and also showed an impaired leptin signaling pathway[72,73]. This increase in body weight and body fat is possibly related to impairment of leptin synthesis and signaling pathway. However, leptin cannot explain the weight gains completely because Burguera et al [73]
revealed that leptin treatment was only partially effective in modulating appetite and limiting weight gain in OVX rats.
Increased body fat is a crucial trigger in elevating insulin resistance in estrogen insufficient states [74].
Weight loss itself reverses insulin resistance and insulin hypersecretion by enhancing first-phase insulin secretion [75,76]. However, it is not clear whether weight loss itself or restricted diets modulate insulin secretion capacity and pancreatic β-cell mass. Choi et al [9] showed that weight loss as a result of restricted diets increased first-phase insulin secretion in OVX rats, but decreased second-phase insulin secretion at 120 minutes during a hyperglycemia clamp was not overcome. Estrogen replacement reversed the decrease in second-phase as well as first-phase insulin secretion [65,75]. Estrogen is a key mediator for glucose- stimulated insulin secretion and the expansion and maintenance of β-cell mass in rats and mice [71,76]. The mechanism is not fully understood, but synergistically with glucose, estrogen closes KATP channels through a cyclic guanosine monophosphate–dependent phosphorylation process. As a consequence, a calcium signal stimulates insulin secretion [77]. Glucose-stimulated insulin secretion can be delayed and suppressed in estrogen-insufficient states. Estrogen also increased β-cell mass by potentiating insulin/insulin-like growth factor 1 (IGF-1) signaling [9].
Insulin receptor substrate 2 (IRS2) is a crucial mediator for β-cell growth and survival through its regulation of insulin/
IGF-1 signaling.
Insulin receptor substrate 2 knockout mice revealed that insulin secretion cannot compensate for insulin resistance when there is decreased β-cell cell mass, and as a consequence, they develop severe diabetes [62,78]. It is important to increase islet mass during a high-insulin- resistant state to prevent progression to diabetes. Induction of IRS2 expression via the activation of cAMP–response element-binding protein in Min6 cells and islets [78,79]
accounted for the fact that the upstream region of IRS2 gene coding contains a cAMP response element. When elevated levels of IRS2 potentiated an insulin/IGF-1 signaling cascade, it increased duodenal homeobox factor 1 expression in β cells. The increased duodenal homeobox factor 1 expression in islets was consistent with the proliferation ofβ cells, resulting in increased mass [9,64]. A possible mechanism for enhancement of β-cell function and mass by estrogen and phytoestrogens in islets is shown inFig. 2.
Based on these findings, estrogen is a key mediator for maintaining glucose homeostasis in females through im- proving insulin sensitivity and β-cell function and mass.
Therefore, phytoestrogens such as isoflavonoids may improve glucose homeostasis through improving estrogen signaling pathways.
6. Effects of soybeans in glucose metabolism
Soybean isoflavonoid and protein consumption alleviate some of the symptoms associated with type 2 diabetes, such as insulin resistance and glycemic control by some yet- unrecognized mechanism[7,80,81]. However, not all studies have reported the same results [82,83]. Isoflavonoids may improve antidiabetic actions through estrogen receptors. The estrogen receptorαis emerging as a key molecule involved in glucose and lipid metabolism. Isoflavonoids are phytoes- trogens that naturally occur with soy proteins and are structurally and functionally similar to estradiol [84].
Estrogen receptorsα andβare both present inβcells. The role of estrogen receptor β is still unknown, but estrogen receptorαplays an important role in the regulation of insulin biosynthesis, insulin secretion, and β-cell survival. Isofla- vones such as genistein and daidzein bind weakly to estrogen receptorαand more strongly to estrogen receptorβ, and they may possess organ-specific estrogenic and antiestrogenic effects [85,86]. Because some agents are antiestrogenic in breast and endometrial cells but have estrogenic actions in other tissues such as adipose tissues and muscles, these agents are classified as selective estrogen receptor modula- tors (SERMs). These SERMs have recently been studied for their metabolic and therapeutic properties and are known to improve energy and lipid metabolism in an estrogen-like fashion in rodent models. The SERM, Acolbifene (EM-652), a pure antiestrogen in human breast and uterine cancer cells,
is an effective agent for preventing diet- and OVX-induced obesity[87].
Animal and human studies have been conducted to investigate antidiabetic effects of soybeans and their action (Table 4) [48,88-94]. The effects of soybeans, including isoflavonoids and soy proteins, on glucose metabolism are inconsistent, and the mechanisms have not been exten- sively studied. The consumption of isolated isoflavonoids (114 mg/d) for 3 months did not affect insulin sensitivity as assessed by an oral 2-hour glucose tolerance test (75 g) in a crossover study of postmenopausal women, even though serum ghrelin levels were decreased by the isoflavonoid treatment, indicating some changes in appetite [95]. In addition, insulin secretion, visceral fat, total body fat, and lean body mass were not different among postmenopausal women who consumed soy protein for 3 months compared with those that consumed casein protein [82]. However, some studies showed positive effects. For example, 46 postmenopausal women taking isolated isoflavone extracts had significantly increased high-density lipoprotein choles- terol and a decrease in apolipoprotein B, the primary apolipoprotein in low-density lipoprotein particles [96,97].
Unlike human studies, recent experiments have shown that isoflavonoids in soybeans enhance insulin secretion and insulin sensitivity in experimental animals such as KKAy diabetic rats, streptozotocin-induced diabetic rats, and CD1 mice[89,93,94,98], and that soy protein attenuates insulin resistance in male Sprague-Dawley rats[99]. However, the effect of isoflavonoids and soy protein remains unclear, although several studies have revealed mechanisms by which soy isoflavones may impact glucose metabolism.
The consumption of soy protein isolates activates peroxisome-proliferator–activated receptors (PPARs) and
Fig. 2. A possible mechanism for the enhancement ofβ-cell function and the survival of these cells. The dashed lines represent the possible mechanism for enhanced glucose-stimulated insulin secretion andβ-cell mass by enhancing proliferation and decreasing apoptosis.
liver X receptor signaling and inhibits sterol regulatory element-binding protein-1c signaling, contributing to insulin sensitization and improved lipid homeostasis in rats after consumption of diets high in fat and cholesterol in experimental animals [7,79,99]. In addition, soy-fed CD-1 mice exhibited enhanced insulin sensitivity, especially in white adipose tissue, due to potentiation of phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxyl- ase and upregulation of the expression of genes implicated in peroxisomal fatty acid oxidation and mitochondrial biogen- esis and in skeletal muscles by increasing glucose uptake [94]. However, these rodents had reduced serum insulin levels and pancreatic insulin content. In contrast, Veloso et al [100] demonstrated that a soybean diet enhanced the secretory pattern of β cells, at least in part, by activating the cAMP/protein kinase A signaling cascade. Thus, the effect and mechanism of soybean-based diets on insulin secretion remains controversial.
7. Effects of fermented soybean products on glucose metabolism
7.1. Effects of short-term fermented soybean paste on glucose metabolism
Several animal studies and a few human studies have evaluated the effects of fermented soybeans on glucose metabolism (Table 5) [101-108]. After fermentation, iso- flavonoid glycones are changed into isoflavonoid aglycones, which seem to have greater activity than do isoflavonoid glycones. Kawakami et al[103]demonstrated that the level of serum total isoflavones in the isoflavone aglycone–rich diet was significantly higher than that of the isoflavone glycoside–rich diet in male Sprague-Dawley rats. Moreover, an isoflavone aglycone–rich diet reduced liver and serum total cholesterol levels, and liver triglyceride levels in rats
fed cholesterol. Thus, fermented soybean products may be more effective for controlling glucose metabolism due to increased isoflavonoid aglycones.
Kwon et al [104,105] showed that chungkookjang improved glucose homeostasis by enhancing hepatic insulin sensitivity and insulinotropic actions in 90%
pancreatectomized rats, a type 2 diabetic animal model.
In addition, 0.3 g water extract of touchi, fermented soybean, decreased fasting and postprandial blood glucose levels and HbA1C in KKAy diabetic animals and mild type 2 diabetic patients [106,107]. In comparison to cooked soybeans, chungkookjang decreased hepatic glu- cose output in a hyperinsulinemic state by potentiating insulin signaling via induction of IRS2 in diabetic rats, indicating the attenuation of hepatic insulin resistance. In addition, chungkookjang enhanced glucose-stimulated insulin secretion in a hyperglycemic clamp study in diabetic rats and increased pancreatic β-cell mass via increased proliferation and decreased apoptosis. This may be associated with activating estrogen receptors in β cells (Fig. 2). Kim et al [108] also showed similar results in C57BL/KsJ-db/db mice. Chungkookjang supplementation induced a significant decrease in blood glucose and glycosylated hemoglobin levels and improved insulin tolerance compared with the diabetic control group via increasing serum insulin and pancreatic insulin contents.
Therefore, chungkookjang delayed diabetic symptoms in type 2 diabetic rats, and this was related to increased isoflavonoid aglycones such as daidzein and genistein and small peptides.
Although no studies have evaluated the effects of specific smaller peptides made from soybean fermentation on glucose regulation, these peptides may have significant effects on glucose metabolism because the changes in isoflavonoids cannot explain the improvement of glucose homeostasis. Only daidzein-rich fractions of chungkookjang
Table 4
Comparison of antidiabetic outcomes after consumption of nonfermented soybeans
Model Bioactive components and dose used Duration End point Reference
Adult males and postmenopausal females
40 g soy protein isolates containing 88 mg isoflavonoid aglycones
57 d Lower serum LDL, LDL/HDL, and apolipoprotein B/apolipoprotein A-I
[88]
100 mg/kg STZ diabetic rats (type 1 diabetes)
20% high-isoflavone soy protein (HIS), 20% low-isoflavone soy protein (LIS)
8 wk Lower serum glucose levels and increased insulin secretion by HIS
[93]
KKAy diabetic mice 50% high-content isoflavone soy protein 9 wk Reduce fasting plasma glucose and insulin;
increase insulin sensitivity
[89]
CD1 male and female mice and offsprings
High soy-containing diet 4 mo Reduced serum insulin levels and pancreatic insulin content; improved insulin sensitivity
[94]
Type 2 diabetic patients with nephropathy
0.28 g soy protein/kg body weight 4 y Reduced serum total cholesterol, LDL, triglyceride, and C-reactive protein
[90]
Postmenopausal women Soy protein plus 132 mg isoflavones 4 wk Reduced total and LDL cholesterol and triglycerides;
no effect of isoflavones alone
[83]
Obese Zucker rat 20% soy protein 120 d Improved renal function and proteinuria; reduced tubular dilation, glomerulosclerosis, intersticial fibrosis, and extracapilar proliferation
[91]
Male patients with type 2 diabetes and nephropathy
Isolated soy protein 8 wk Decreased urinary albumin excretion and increased HDL [92]
LDL indicates low-density lipoprotein; HDL, low-density lipoprotein; STZ, streptozotocin.
elevated PPAR-γ activation by approximately 50% of the rosiglitazone activity, a member of the nuclear hormone receptor superfamily and a central regulator of insulin and glucose metabolism[109]. In addition, the smaller peptides (less than 3 kD) in chungkookjang increased PPAR-γ activity by approximately 59% of rosiglitazone, which indicates that it works as a mild PPAR-γagonist from our preliminary study. However, there was no activation of PPAR-γby larger peptides (more than 3 kD). In addition, certain types of peptides isolated from the breakdown of soybeans or black beans by microbes have shown antihy- pertensive and anti-inflammatory properties, but no specific peptide has revealed antidiabetic actions [57,58]. Thus, further studies are needed to determine the antidiabetic effects of specific peptides.
7.2. Effects of long-term fermented soybean paste on glucose metabolism
Unlike chungkookjang, long-term fermented soybeans with added salt, such as doenjang, have not been demonstrated to affect glucose homeostasis. However, doenjang has been shown to have greater antimutagenic and anticancer activities than other fermented soybean foods such as chungkookjang (Korean-style natto) and miso (Japanese-style doenjang) and nonfermented soybeans.
Anticancer and antimetastatic properties of doenjang were enhanced as aging time progressed [110]. However, kochujang containing meju and red pepper may affect energy, lipid, and glucose metabolism. Red pepper, a major component of kochujang, and its active principle capsaicin are known to enhance energy and lipid metabolism, possibly by increasing catecholamine secretion from the adrenal medulla through the activation of the sympathetic nervous system[111,112]. The decreased numbers of adipocytes may improve glucose tolerance by the enhancement of insulin sensitivity. Although capsaicin content in red pepper was
reduced by approximately 50% during the fermentation process, kochujang has been shown to reduce body weight, visceral fat, and serum leptin levels without modulating energy intake in diabetic rats[102]. It also improves glucose tolerance by enhancing insulin sensitivity. The improvement in hepatic insulin sensitivity lowered hepatic glucose output and triglyceride accumulation and increased glycogen storage. The possible mechanism is the potentiated phos- phorylation of signal transducer and activator of transcrip- tion-3 → AMP-activated protein kinase → acetyl CoA carboxylase and the reduced phosphoenol pyruvate carboxy kinase expression [113]. However, the putative effects of capsaicin on insulin secretion capacity remain controversial [111,113]. Few studies have evaluated the effects of kochujang on glucose-stimulated insulin secretion, although Kwon et al [102] showed that kochujang supplementation does not modify glucose-stimulated insulin secretion in diabetic rats. Thus, kochujang may have greater efficacy for preventing and delaying diabetic progression than capsaicin, but further study is needed.
8. Summary and conclusion
We presented limited but supporting evidence that soy foods (soy protein and phytoestrogens) are beneficial for decreasing the risk of onset and progression of insulin resistance and type 2 diabetes and that the effectiveness is enhanced by fermentation. However, available data from human studies on soybeans do not offer complete evidence, and further research is required before a firm conclusion can be made about the benefits if soy and phytoestrogens in glucose metabolism. Furthermore, no human intervention studies have been performed to directly investigate the effect of fermented soybeans on diabetes. Phytoestrogens and proteins in soybeans seem to have beneficial actions both on glucose metabolisms, and additional micronutrients such as
Table 5
Comparison of antidiabetic outcomes after consumption of fermented soybean products
Model Dose used Duration End point Reference
Healthy subjects Combination of 50 g natto, 60 g Japanese yams, 40 g okras
1 d Decreased peak glucose and insulin concentrations and the incremental areas under the curve for glucose and insulin more than 0-120 min during OGTT
[101]
90% pancreatectomized rats 5% kochujang powder 8 wk Improved glucose tolerance by enhancing hepatic insulin sensitivity
[102]
C57BL/KsJ-db/db mice 5% chungkookjang 8 wk Reduced blood glucose and glycosylated hemoglobin;
improved insulin tolerance; increased serum and pancreatic insulin levels
[106]
90% pancreatectomized rats 40% chungkookjang 8 wk Improved insulin sensitivity and insulin signaling in the liver; enhanced glucose-stimulated insulin secretion insulin/IGF-1 signaling in islets
[104,105]
Male KKAy mice 0.4% water-soluble touchi-extract 60 d Reduced fasting and postprandial blood glucose levels;
decreased index of liver function such as GOT and GPT [107]
Mild type 2 diabetic humans 0.3 g water-soluble touchi-extract in every meal
3 mo Decreased fasting blood glucose and HbA1C [108]
OGTT indicates oral glucose tolerance test; GOT, glutamic-oxaloacetic transaminase; GPT, glutamic-pyruvic transaminase.
saponins, phytosterols, trypsin inhibitors, as well as the amino acid and protein composition may have additive or synergistic effects. Fermentation of soybeans leads to structural changes in proteins and phytoestrogen, which may contribute to more beneficial effects on glucose metabolism. However, individual isoflavonoids and peptides in soybeans and fermented soybeans have not been studied for determining antidiabetic actions. As with any study related to functional foods, investigating soybeans and fermented soy products have not used standardized formula- tions, doses, routes of exposure, durations of exposure, and subsequent analyses to evaluate antidiabetic effects and mechanisms of action. Additional studies both in humans and animals are required to identify which components and constituents have beneficial roles in glucose metabolism.
Although only a few studies have been done in fermented soybeans, they seem to have better antidiabetic actions than do unfermented soybeans. In addition, it remains unclear if soy phytoestrogens mediate this effect through estrogenic pathways or if the effect is mediated by nonestrogenic activity of isoflavones, isoflavone metabolites, soy peptides, or other bioactive soy compounds. The benefits and mechanism of action of fermented soy foods for preventing or treating diabetes are an area of much needed research that would include experimental animal models and clinical human studies.
Acknowledgment
This work was supported by a grant from the Korea Science and Engineering Foundation in Korea (M10510120001- 05N1012-00111). The authors have no conflict of interest for the information presented in this review.
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