**2. Soy protein and peptides**

#### **2.1 Soy protein for metabolic syndrome (MetS)**

The concept of MetS has been proposed by several committees. The first formalized concept of MetS was proposed by a consultation group for the definition of diabetes for the World Health Organization (WHO); it was determined to have a high-risk status with multiple risk factors for cardiovascular disease. This group emphasized insulin resistance as the major underlying factor [2]. In 2001, a definition for MetS was devised by the National Cholesterol Education Program (NCEP) Adult Treatment Panel III (ATP III) [3]. The American Heart Association and the National Heart Lung and Blood Institute updated this definition in 2005 [4]. This updated definition is one of the most widely used criteria for MetS. The International Diabetes Foundation (IDF) published new criteria for MetS [5] in 2005. Although it includes the same general criteria as the other definitions, it requires that obesity, but not necessarily insulin resistance, be present. Although visceral obesity is now recognized as an important factor, the IDF definition has been criticized for its emphasis on obesity, rather than insulin resistance, in pathophysiology [6].

In Japan, in 2006, MetS was defined as a multiple risk factor clustering syndrome caused by visceral fat accumulation and insulin resistance that accompanies this accumulation [7]. In the MetS stage, it is advocated that lifestyle intervention to reduce visceral adiposity should be given priority over drug treatment. Subjects with multiple risk factor syndrome were diagnosed with MetS if their visceral fat areas determined by CT scan were over 100 cm<sup>2</sup> .

*Soybean and Other Legume Proteins Exhibit Beneficial Physiological Effects on Metabolic… DOI: http://dx.doi.org/10.5772/intechopen.99955*

The Japanese Committee for the Definition and Diagnosis of MetS aimed to select subjects with multiple risk factors in which lifestyle modification to reduce visceral adiposity has priority over drug treatment [8]. For this purpose, the Japanese government started a new health policy that provides a specific health check-up followed by specific counseling for subjects diagnosed with MetS according to the Japanese criteria from 2008.


*Note:* ↓ *and* ↑ *signs represent decrease and increase, respectively, after supplement of active compounds. Totalcholesterol (Total-C); low-density lipoprotein cholesterol (LDL-C); triglyceride (TG); high-density lipoprotein cholesterol (HDL-C); non-high-density lipoprotein cholesterol (non-HDL-C); apo-lipoprotein-B (Apo-B); apo-lipoprotein-AI (Apo-AI).*

#### **Table 1.**

*Meta-analysis on improving lipid metabolism in soy protein.*

Soy protein exerts not only conventional nutritional value but also beneficial effects on human health. Many randomized controlled trials (RCTs) have assessed the effects of soy products on serum lipids. Systematic reviews and meta-analyses have reported improvements in lipid metabolism (**Table 1**) [9–16].

Soy protein isolate [SPI] is composed of three major components, glycinin [approx. 40%], β-conglycinin [approx. 20%], and lipophilic proteins (approx. 40%) [17]. Glycinin and β-conglycinin are storage proteins in soy, and lipophilic proteins consist primarily of membrane proteins. Among these components, β-conglycinin has the function of lowering serum triglycerides preferentially over serum cholesterol [18]. Digestive decomposition products of β-conglycinin were reported that lowering the activity of fatty acid synthase and increasing the activities of β-oxidation enzymes, and the fecal excretion of TG was high in β-conglycinin-fed mice and rats [19, 20]. Therefore, in the calculation based on the recommendation by the FDA, the same effect can be expected with 5 g of β-conglycinin. In clinical study, daily consumption of 5 g of β-conglycinin per subject significantly lowered serum TG concentrations, and the apo B and VLDL-TG concentrations were significantly decreased [21]. Hence, β-conglycinin consumption may specifically affect TG metabolism. In addition, the intake of 5 g of β-conglycinin per day decreased the body fat ratio and visceral fat [21, 22]. Additionally, serum adiponectin significantly increased with the consumption of β-conglycinin, and serum free fatty acids in the β-conglycinin group were significantly decreased. Tachibana et al. showed that β-conglycinin improves insulin sensitivity in rats [23]. β-conglycinin might be an important food component for the prevention and/or amelioration of visceral fat syndrome, which is also called MetS (**Table 2**) [21, 22, 24–27].
