Effects of soy protein isolate hydrolysates on cholecystokinin released by rat intestinal mucosal cells and food intake in rats

Abstract

Soy protein isolate hydrolysates (SPIH) were prepared from soy protein isolate (SPI). Effects of SPIH on a satiety signal cholecystokinin (CCK) and feeding behavior in rats were investigated. SPIH induced more CCK release (164.66 ± 2.40 pg/mL) by rat intestinal mucosal cells than SPI (143.33 ± 3.71 pg/mL). Meal size (MS), intermeal interval (IMI), and satiety ratio (SR = MS/IMI) of rats received different daily doses of SPIH or dietary fiber were detected for 40 days. A 100 mg/kg dose of SPIH resulted in a greater SR than an identical dose of dietary fiber, while a 300 mg/kg dose resulted in a less MS and IMI. A 500 mg/kg dose of SPIH had similar effects to the same dose of dietary fiber on reducing MS, extending IMI, and increasing SR, but resulted in a significantly less body weight at the end of the experiment (318.15 ± 17.83 g) than the dietary fiber group (340.28 ± 6.15 g).

Introduction

With the improvement of living standards and levels, adiposity has become a focus of health issue. It has also been demonstrated that adiposity canimpair immunity, causing increased susceptibility to infectious disease, commensal organisms and allergies, which is the character of tissue inflammation (Goossens et al. 2017; Magnuson et al. 2019). The number of overweight and obese people around the world had increased from 857 million in 1980s to 2.10 billion in 2013. Among all the countries with high proportions of obese people, the United States of America has the greatest obese population accounting for 13% of the total world-wide obese population, followed by China (7.7%) and India (7.2%) (Ng et al. 2014). The "Global Malnutrition Status Report" published by the Global Alliance for Improved Nutrition (GAIN) (International Food Policy Research Institute 2015) showed that the obese population exceeded one-third (2.18 billion) of the total global population (7.28 billion) in 2015. It is well-known that adiposity is highly correlated with cardiovascular diseases, hyperinsulinemia, and abnormal lipid metabolisms (Bonzón-Kulichenko et al. 2018; Kim et al. 2015).

There are two main approaches to lose body weight. One is to accelerate the metabolism of fat and the other is to enhance and continue satiety. Recently, the utilization of dietary fiber was focused on enhance satiety by most of the researches (Magnuson et al. 2019; Na Nakorn et al. 2019). However, excessive amounts of dietary fiber will obstruct the absorption of nutrients such as iron (Feltrin et al. 2009) and amino acid (Wong and Cheung 2003). Also, it has been found that the dietary fiber that escape digestion and absorption in the small intestine can cause delayed gastric emptying and slower transit through the small bowel, resulting in the reduced rate of nutrient absorption (Blackwood et al. 2016). Hence, dietary fiber may not be suitable for long-term consumption.

Protein is considered a macro-nutrient that can also increase satiety, and high protein intake is beneficial for short-term weight loss (Engberink et al. 2015). It has been shown that the intake of soy protein, casein, or whey protein increases satiety and reduces appetite. Hu et al. (2018) researched the effect of macronutrient composition on satiating effects regulation by exposing mice to 29 different diets varying from 8.3 to 80% fat, 10 to 80% carbohydrate, 5 to 30% protein, and 5 to 30% sucrose. The results showed that only the intake of fat increased adiposity, and when meals contained relatively large amounts of protein they felt less hunger and more satiety. Additionally, research results indicated that soy protein has better effects on increasing satiety and reducing body weight than casein and whey protein (Belobrajdic et al. 2004). The experiments on female students indicated that soy milk showed much better effect than milk on losing weight and reducing thickness of sebum. On the treatment of childhood obesity, soy peptides increase the basic metabolic level and promote the reduction of subcutaneous fat more than milk. Therefore, peptides produced during digestion of food proteins have important biological functions, such as activating satiety signals in the gut and suppressing appetite.

Cholecystokinin (CCK) is an important physiological and endocrine factor that regulates appetite and extends gastric emptying time in vivo, while a recent study found that CCK defined a specific population of excitatory neurons in the deeper laminae (Abraira et al. 2017). Pupovac and Anderson (2002) found that soybean protein and casein peptide induced the satiety signal of opioid receptors and CCK-A receptors, which influenced the central nerve system of appetite regulation and brought satiety by activating peripheral opioid receptors and CCK-A receptors. The release of CCK from the central and peripheral nerve system is considered to be helpful to increase the sense of satiety and suppress the appetite (Gutierrez-Mecinas et al. 2019; Babaei et al. 2017).

Nishi et al. (2003a) found that soybean protein peptide components infused prior to meal played a more important role in reducing body weight and volume of body fat than soybean protein. It is indicated that peptides liberated by the hydrolysis of proteins rather than amino acids induce satiety. High levels of arginine in protein strongly stimulate the released of CCK in the body, lead to high concentrations of CCK, and provide the body with a sense of fullness. More than 60% of protamine is arginine, which strongly stimulates CCK receptors. The lysine in casein could be converted to arginine to prepare guanidylated casein, which is easily absorbed by intestinal cells and stimulates CCK receptors to produced CCK. The chain length of peptides may be an important factor that determines the release and activity of CCK. The optimal chain length of peptides found in different food proteins varies, depending on the type of food proteins (Nathalie et al. 2000, Nishi et al. 2003a). Bioactive peptides in soybean protein having an amino acid sequence of Val–Arg–Ile–Arg–Leu–Leu–Gln–Arg–Phe–Asn–Lsy–Arg–Ser, corresponding to the 51–63 amino acid fragment of β-glycinin, were suggested to suppress appetite and enhance satiety (Nishi et al. 2003b). The bioactive peptide is known to stimulate intestinal mucosa cells directly to secrete CCK. Based on studies using synthetic peptides, it was found that multiple Arg residues in these bioactive peptides were necessary for both the activity of brush border membranes and the release of CCK (Zhang et al. 2018).

The aims of this study were to analyze the nitrogen release and arginine ratio in SPIH, to evaluate the effect of SPIH on CCK release in rat intestinal mucosa cells and rating three types of intake and behavior: meal size (MS), meal interval (IMI), and satiety (SR), compared with dietary fiber. We wanted to know whether SPIH could enhance satiety, thereby reducing food intake and body weight gains.

Materials and methods

Preparation of SPIH

The hydrolysis of protein was performed as described previously with slight modifications (Hara et al. 1995). Four percent (w/v) soy protein isolate (SPI, Liaoning Buxin City Weiyuan Protein Technology Co., Ltd., Shenyang, China) was dissolved in water and hydrolyzed using 7% (w/v) pepsin (Pig stomach, 3000 IU/g, Beijing Hongrun Baoshun Technology Co. Ltd., Beijing, China) at pH 2.0 and 37 °C for 24 h. The sample was then heated at 120 °C for 10 min to inactive the enzyme, cooled to about 20 °C, and centrifuged at 4000 × g for 20 min. The supernatant was then neutralized, desalted, and lyophilized. Amino acid compositions except for aspartic acid, glutamine, and tryptophan in hydrolysates and original proteins were analyzed using an amino acid analyzer (JLC-500/V, JEOL, Tokyo, Japan).

Sequential in vitro digestion

Sequential in vitro pepsin and trypsin digestion was carried out according to the method of Njintang et al. (2001) with some modifications. One gram of SPI or SPIH were dispersed in 100 mL water, adjusted to pH 1.5 using 0.1 mol/L HCl, and pre-incubated in a water bath at 37 °C for 3–5 min. Then, 10 mg of pepsin powders were added, and the mixture was incubated at 37 °C for 60 min for protein digestion. The reaction was terminated by adjusting to pH 7.0 using 1.0 mol/L NaOH. Neutralized digestion mixtures were mixed with 30 mg of porcine trypsin (Porcine pancreas, 1:250U, Shanghai Yapei Biological Technology Co. Ltd., Shanghai, China) and incubated at 37 °C for 60 min for additional protein digestion. Samples were taken at the end of each stage of digestion and mixed with trichloroacetic acid (TCA) in order to quantify TCA-soluble nitrogen.

Determination of nitrogen release during digestion

TCA-soluble nitrogen fractions of pepsin and trypsin digests were obtained by directly mixing the digests with the same volume of 10% TCA and centrifugation at 8000 × g for 15 min. The nitrogen content was measured by the Kjeldahl method (N × 6.25). The % nitrogen release was defined as follows:

$$\frac{N_{\mathit{tot}}-N_t}{N_{\mathit{tot}}}\times 100\%$$

where N_t represents the weight of TCA-precipitated nitrogen after digestion for t min (mg), and N_tot represents the weight of total nitrogen in undigested SPI samples (mg) (Tang et al. 2006).

Preparation of intestinal mucosal cells

Dispersed intestinal mucosal cells were prepared as previously described (Nishi et al. 1998). Male Wistar rats (n = 3, YiSi Experimental Animal Technology Co., Ltd., Changchun, China) weighing 250–350 g were deprived of food overnight and then sacrificed under sodium pentobarbital anesthesia. Intestinal mucosal cells were taken from 20 cm segments of the proximal small intestine. The cells from three rats were pooled and incubated twice in the 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) buffer at 37 °C for 30 min with slight shaking for cell equilibration. Cells were re-suspended in a 200 times volume of fresh HEPES buffer and incubated at 37 °C with moderate stirring until use.

CKK release by intestinal mucosal cells

The mucosal cell suspension (1 mL) was added to a plastic vial each containing 1 mL of a test sample (100 μg/mL of SPI or SPIH), and was incubated at 37 °C for 30 min. The suspension was centrifuged at 10,000 rpm for 7 s after incubation, and 0.5 mL of supernatant was collected. The supernatant was applied to a C18 sep–pak cartridge pretreated with 10 mL acetonitrile, 10 mL methanol, and 20 mL distilled water and washed with 20 mL distilled water. CCK was eluted by adding 1 mL of 50% acetonitrile and was stored at − 80 °C until assay. The concentration of CCK was measured using enzyme-linked immunosorbent assay (Nishi et al. 2003b).

Food intake and behavioral rating

The baseline of food intake for all rats was established using the following procedures. Wistar rats (n = 35, 6–8 weeks, 200–220 g, male, Quality Inspection Unit: Jinlin province Bureau of Quality and Technology Supervision, Changchun, China) were reared in a temperature-controlled room maintained at 23 ± 2 °C and fed animal fodder (GB14924-2001, YiSi Experimental Animal Technology Co., Ltd., Changchun, China) for 15 days. On the day prior to the experiment, rats were fasted from food (chow), but not water, at 5:00 p.m. At 9:00 a.m. in the following morning, all rats received vehicle injections of 1 mL physiological saline, followed by 50 g of feed and plenty of water.

At 9:00 a.m. on the 16th day, rats received i.g. administrations of SPIH or dietary fiber dissolved in 1 mL physiological saline at one of 3 dosage levels (100, 300, and 500 mg/kg) or 1 mL physiological saline without SPIH nor dietary fiber (blank), followed by 50 g of feed and plenty of water. Well-trained examiners equipped with laptop computers and timers rated the min-to-min behavior of rats for 120 min to determine IMI (Larsen et al. 2010). No feeding activity for more than 5 min was regarded as IMI. At 5:00 p.m., remaining feed was weighed to determine MS or the weight of feed consumed between 9 a.m. and 5 p.m. on the day. The SR was then calculated as MS/IMI. These experiments for determining MS, IMI, and SR continued for 55 days. These rating methods eliminated possible discrepancies, caused by measuring cumulative food intake or predetermined meal size/time, and ensured accurate determinations of MS and IMI and hence precise calculations of SR-a quantitative index enabling to compare satiety effects of SPIH or dietary fiber (Metcalf et al. 2011). Rats were weighed once every three days for the first 54 days and due to the last day of the experiment it took a total period of 55 days.

Determination of height and weight

During the experiment of 55 days, the weight of rats was monitored at 9:00 every 3 days. After mild anesthesia with ether, the rats were placed in a 500 mL beaker and weighed by electronic balance. When anesthesia was used to measure body weight, the distance from the tip of the nose to the anus of rats was measured with a steel ruler, calculate the BMI (Body Mass Index, BMI) as follow:

$$\text{BMI}=\frac{bodyweight}{heigh{t}^2}$$

Statistical analysis

Data from intestinal mucosal cell experiments were analyzed by one-way ANOVA with one variable being procedure treatment. Measurements were triplicated. Results were reported as mean ± standard deviation (SD).

Rat food intake experiment data were analyzed by one-way ANOVA. All rat experiment results were reported as mean ± SD of five replications. Pearson’s correlation coefficient (r) was obtained by correlation analysis and used to determine the relationship between the dosage of SPIH or dietary fiber and MS, IMI, or SR.

One-way ANOVA and correlation analysis were performed using the SPSS 17.0 software (Armonk, NY, USA). Differences were considered significant if P < 0.05.

Results

Figure 1 shows the nitrogen release of SPI and SPIH digested by pepsin and trypsin in vitro. Nitrogen release from SPIH during pepsin digestion was more rapid and reached 90.37 ± 2.93% in 1 h of digestion, which was almost twice as much as that from SPI (46.48 ± 2.92%). The percent of nitrogen release of SPIH at the end of trypsin digestion reached 96.68 ± 3.18%, which was significantly higher than that of SPI (P < 0.05).

Fig. 1.

Percentage of nitrogen release during pepsin and trypsin digestion of SPI and SPIH in vitro. Error bars indicate SD (n = 3). Different letters indicate significant differences between SPI and SPIH (P < 0.05)

Figure 2 shows the amount of CCK released during the 30 min incubation of rat intestinal mucosal cells after the administration of SPIH or SPI. The amount of CCK released from intestinal mucosal cells under the SPIH treatment was significantly higher than that from the SPI group and control (blank) (P < 0.05). The amount of CCK of the SPI group (143.33 ± 3.71 pg/mL) and SPIH group (164.66 ± 2.40 pg/mL) were about 2.27 times and 2.61 times of that of the control group (63.11 ± 1.68 pg/mL), respectively.

Fig. 2.

CCK release from rat intestinal mucosal cells with100 µg doses of SPI or SPIH in 1 mL incubation media. Error bars indicate SD (n = 3). Different letters indicate significant differences (P < 0.05)

Table 1 shows that the total amino acid content of SPIH is less than that of SPI. The essential amino acid content in SPIH was 22.91% and in SPI was 27.11%(w:w), The content of arginine in SPIH reached 5.99%, which was less than that in SPI (6.80%), but the SPIH in arginine / total amino acid was 0.15% higher than that in SPI (w: w).

Table 1.

Amino acid composition of SPIH and SPI

Amino acid	Content (%)
	SPIH	SPI
Asp	8.02	8.98
Thra	2.53	3.01
Ser	3.69	4.18
Glu	11.98	13.49
Gly	2.77	3.39
Ala	2.68	3.27
Cys-Cys	0.84	1.06
Vala	2.79	3.34
Meta	1.19	1.33
Ilea	2.94	3.64
Leua	5.37	6.34
Tyr	2.93	3.33
Phea	3.60	4.22
Lysa	4.52	5.22
His	1.79	2.17
Arg	5.99	6.80
Pro	3.33	3.50
Essential amino acid	22.91	27.11
Total amino acid	66.96	77.27

^aEssential amino acid

Rats (n = 35) were separated into 7 groups, and were reared for 15 days without SPIH or dietary fiber administrations in order to establish the baseline of body weight and meal size. The average or baseline body weight at the end of the rearing for 15 days is shown in Table 2 as the initial body weight on day 0. Figure 3 also shows changes in the body weight of rats received daily administrations of SPIH or dietary fiber for 55 days. There were no significant differences in the body weight between different groups in the first week. After the first week, the body weight of the blank group continued to increase fairly steadily until about the 50th day and reached 434.14 ± 20.98 g at the end of the test period. At the end of the investigation, the body weight decreased by administering SPIH or dietary fiber daily in dose-dependent manners. There were no significant differences in the final body weight between SPIH and dietary fiber groups at low (100 mg/kg) or moderate dose (300 mg/kg), while the high dose (500 mg/kg) of SPIH resulted in a significantly lower final body weight (318.15 ± 17.83 g) than that of the same dose of dietary fiber (340.28 ± 6.15) (P < 0.05). The body weight of the high dose SPIH group remains to be fairly constant approximately after the first month of investigation. Differences in body weight between the high dose SPIH and dietary fiber groups became significant on the 48th day and continued to increase during the rest of the investigation. Figure 3 shows the body mass index of each group rats during 15 days. There was no significant decrease in body mass index of saline-fed rats compared with other rats.

Table 2.

MS, IMI, and SR of rats with daily administrations of SPIH or dietary fiber for 40 days

Dose (mg/kg)		MS (g)1	IMI (min)1	SR (10-min/g)1
Base*		30.02 ± 2.31	15.21 ± 3.54	5.02 ± 0.753
Saline#	0	32.89 ± 0.86a,+	16.67 ± 2.55a	5.00 ± 0.65a
SPIH#	100	27.06 ± 3.67A,b	16.80 ± 2.68A,a	6.20 ± 0.20A,b
	300	19.91 ± 1.24A,c	17.82 ± 1.48A,a	8.94 ± 0.21A,c
	500	18.60 ± 0.72A,c	23.47 ± 2.30A,b	12.54 ± 0.90A,d
Dietary fiber#	100	29.88 ± 1.55A,b	15.82 ± 1.92A,a	5.27 ± 0.38B,a
	300	26.54 ± 0.81B,c	21.21 ± 3.11B,b	7.99 ± 0.94A,b
	500	19.46 ± 2.63A,d	25.23 ± 1.92A,c	13.07 ± 0.74A,c

¹Means ± SD (n = 5)

^A,BIndicate significant differences between SPIH and dietary fiber (P < 0.05)

^a,b,c,dIndicate significant differences between the seven groups differing in dose and types (P < 0.05)

^*After 15 days of rearing without the administration of SPIH or dietary fiber (n = 35)

^#After 40 days of SPIH, dietary fiber, or physiological saline administrations

⁺Indicates significant difference between base and Saline (P < 0.05)

Fig. 3.

Body mass index of low /moderate /high dose of SPIH and dietary fiber during 55 days. Error bars indicate SD (n = 3). Different letters indicate significant differences (P < 0.05)

Table 3 shows changes in MS, IMI, and SR of rats that received 0–500 mg/kg administrations of SPIH or dietary fiber. All these data were analyzed by one-way ANOVA and Pearson correlation analysis (n = 5). The MS of the blank group increased significantly from the base group (P < 0.05). However, there were no significant differences in IMI and SR between the blank and base group. MS results of the SPIH and dietary fiber group of any doses were significantly different from the blank group. Although MS results within the SPIH group were not significantly different between doses of 300 mg/kg and 500 mg/kg (P > 0.05), there was a negative correlation (r = -0.87) between MS and the dose of SPIH. The dose of dietary fiber had a strongly negative correlation with MS (r = −0.93). These results illustrated that SPIH and dietary fiber had similar effects on decreases in MS. However, SPIH was found to result in a significantly lower MS than dietary fiber at a dose of 300 mg/kg (P < 0.05). There were no significant differences in IMI between the low and moderate dose (100 and 300 mg/kg) of SPIH, the low dose of dietary fiber, the blank group, and the base group (P > 0.05). The IMI of the high dose (500 mg/kg) of SPIH and moderate and high doses of dietary fiber were significantly larger than any other groups (P < 0.05), indicating that the moderate dose dietary fiber had significant effects on IMI but that the high dose of SPIH was required to achieve similar effects. Dosage levels of SPIH and dietary fiber showed a moderate positive correlation with IMI (r = 0.74 and 0.62, respectively). There were no differences in SR between the low dose of dietary fiber and the blank group (P > 0.05). The SR was significantly increased by any doses of SPIH and moderate and high doses of dietary fiber compared with the blank group (P < 0.05). The dose of SPIH or dietary fiber had a significant positive correlation with SR (r = 0.87 or 0.90, respectively). The SR of the low dose of SPIH was greater than that of the same dose of dietary fiber (P < 0.05). These results demonstrated that SPIH provided more enhanced satiety than dietary fiber at relatively low doses. The high dose of SPIH resulted in similar effects on reducing MS, extending IMI, and increasing SR compared with dietary fiber.

Table 3.

Body weight of rats after receiving different daily doses of SPIH or dietary fiber

Time [day]	Saline(g)	Low dose of SPIH (g)	Moderate dose of SPIH (g)	High dose of SPIH (g)	Low dose of dietary fiber (g)	Moderate dose of dietary fiber (g)	High dose of dietary fiber (g)
0	220.24 ± 2.26	222.52 ± 5.80	224.01 ± 2.39	219.65 ± 3.74	220.51 ± 3.72	220.21 ± 1.79	219.89 ± 1.03
3	228.92 ± 4.44	234.42 ± 8.42	230.52 ± 0.38	230.45 ± 5.72	232.13 ± 8.16	235.26 ± 3.29	230.48 ± 1.43
6	263.72 ± 1.32	265.49 ± 8.44	259.62 ± 3.13	264.94 ± 5.20	258.16 ± 7.66	259.23 ± 4.50	258.47 ± 1.34
9	286.84 ± 7.39	279.36 ± 12.16	274.12 ± 1.94	279.48 ± 10.32	280.57 ± 4.77	280.19 ± 3.58	286.45 ± 1.37
12	295.12 ± 15.66	280.15 ± 16.58a	285.06 ± 0.37	298.15 ± 11.38	293.09 ± 1.60	291.42 ± 1.45	287.54 ± 2.48
15	306.43 ± 6.15	287.15 ± 18.41	284.67 ± 10.07a	300.55 ± 10.84	299.68 ± 3.19	290.27 ± 1.76a	289.54 ± 10.06
18	319.84 ± 6.84	300.09 ± 21.69a	298.98 ± 12.00a	302.45 ± 16.33a	308.57 ± 12.1c	311.67 ± 6.89d	291.54 ± 10.05ac
21	339.19 ± 11.00	308.51 ± 25.48a	306.82 ± 14.45a	305.45 ± 13.80a	312.76 ± 14.38a	317.54 ± 7.42ad	293.54 ± 7.62abc
24	355.94 ± 6.31	315.84 ± 4.01a	312.5 ± 13.03a	307.6 ± 19.09a	320.08 ± 10.09a	324.43 ± 5.29ad	295.54 ± 7.74abc
27	369.33 ± 5.14	318.84 ± 7.83a	316.72 ± 11.65a	308.98 ± 12.67a	326.62 ± 8.10a	334.78 ± 3.74ad	308.72 ± 8.50abc
30	384.97 ± 8.01	331.24 ± 3.65ad	326.86 ± 16.17abd	314.74 ± 10.05abc	339.7 ± 8.02a	337.27 ± 5.46ad	313.98 ± 9.29abc
33	389.16 ± 14.57	334.84 ± 6.12acd	331.81 ± 15.78ad	315.12 ± 13.79abc	346.24 ± 6.66a	348.94 ± 5.33ad	316.61 ± 8.20abc
36	399.83 ± 6.09	337.84 ± 8.77acd	336.76 ± 15.00ad	318.5 ± 17.40abc	351.56 ± 4.94a	350.79 ± 3.40ad	319.24 ± 6.74abc
39	410.51 ± 4.16	351.46 ± 3.89acd	343.3 ± 14.61ad	320.26 ± 14.48abc	365.86 ± 5.09ad	354.49 ± 4.04ad	324.5 ± 4.09abc
42	414.61 ± 3.84	362.58 ± 4.04acd	346.32 ± 14.22abd	320.64 ± 14.43abc	370.45 ± 8.49acd	356.34 ± 5.88abd	327.13 ± 2.84abc
45	419.67 ± 8.65	363.52 ± 5.10acd	349.34 ± 13.83abd	321.02 ± 14.47abc	378.94 ± 9.88acd	358.19 ± 8.88abd	329.76 ± 1.62abc
48	426.5 ± 10.43	377.789 ± 6.23acd	352.36 ± 13.22abd	318.4 ± 15.39abc	385.48 ± 10.22acd	360.04 ± 9.96abd	332.39 ± 2.49abc
51	432.526 ± 8.16	387.436 ± 7.41acd	355.38 ± 13.00abd	320.78 ± 8.56abc	391.02 ± 7.93acd	361.45 ± 6.90abd	335.02 ± 6.63abc
54	434.552 ± 8.78	393.83 ± 8.62acd	359.41 ± 12.44abd	320.16 ± 8.44abc	398.56 ± 8.13acd	364.64 ± 7.16abd	338.65 ± 6.85abc
55	434.14 ± 9.39	394.705 ± 9.82acd	361.432 ± 11.88abd	318.145 ± 8.33abc	400.18 ± 8.32acd	365.15 ± 7.41abd	340.28 ± 7.07abc

The results are presented as means ± SD (n = 5)

Statistically significant differences: ^aversus saline; ^bversus low dose; ^cversus moderate dose; ^dversus high dose,(P < 0.05)

Discussion

In our study, the content of arginine in SPIH was 5.99% (w/w) of total amino acids (Table 1). In spite that the content of arginine in SPIH was less than that in SPI (6.80%), SPIH was digested more quickly (Fig. 1) than SPI and stimulated the release of CCK, which would inhibit appetite to a greater degree (Fig. 2). Hira et al. (2003) found that the peptides containing 1.9% (w/w) arginine stimulated the release of CCK and controlled the hypertrophy of the pancrea. Kagebayashi et al. (2012) suggested that 2.57% (w/v) arginine could slow down intestinal drainage after gavage and reduce MS. Therefore, arginine content in SPIH was 5.99%, which would more effectlystimulated CCK release in our research. Nishi et al. (1998) found that the arginine content in soy protein hydrolysates was 6.86% (w/w) of total protein, higher than that of other types of protein hydrolysates (e.g.: casein: 4.46%, protein: 6.27%, wheat gluten: 4.01%). Soybean protein hydrolysate exerted the lagrest effect on the stimulation of CCK release and satiety enhancement, which was consistent with the highest l-arginine content. This indicated that arginine content in SPIH was second only to SPI and higher than that of other proteolytes.

The blank group received the daily administration of saline instead of SPIH or SPI also showed relatively small amounts of CCK released from intestinal mucosal cells (Fig. 2). Sufficient amounts of CCK inhibit the secretion of gastric acid, delay gastric emptying time, stimulate the inner side of the hypothalamus, and generate intensive satiety signals (Nassel and Williams 2014). It has been found that not only the content of arginine, but also the chain length of peptides played a determining role on the stimulation of CCK release. Nishi et al. (2003b) found that in β-conglycinin shorter chain peptides stimulated CCK release more readily. Nishi et al. (2003a) also found that some peptide chain structures in SPIH have strong binding force with small intestinal mucosa cells, which can be easilyabsorbed by small intestinal mucosa cells and stimulate CCK receptor to produce CCK, increasing the satiety of rats. In this study, nitrogen release of SPIH was significantly faster than that of SPI (Fig. 1). This may imply that the production of short chain peptides from SPIH in the digestive system was faster than that from SPI and stimulate more release of CCK. The satiety effects were demonstrated by the reduced body weight gains and improved eating behavior of rats administered with SPIH.

Dietary fiber has been recognized as an important food component that provides high levels of satiety (Slavin and Green 2007; Jarrar et al. 2019). Dietary fiber may extend gastric emptying time and provide satiety for a longer period of time. Satiety could help to reduce MS in the subsequent eating episode, and also help to increase IMI (Allirot et al. 2013). Therefore, one who desires to lose weight may choose to consume more dietary fiber to reduce food intake. Indeed, many research studies focused on the effect of dietary fiber on food intake reduction and body weight loss, and verified the hypothesis that the consumption of diet rich in dietary fiber might lead to body weight loss (Hjorth et al. 2018). In this study, the moderate dose (300 mg/kg) of dietary fiber resulted in a higher MS and IMI than the SPIH group at the same dose level, while both groups showed similar SR and body weights.

Effects of SPIH and dietary fiber on satiety and body weight were found to be similar, while the mechanism of the modulation of fat metabolism by SPIH and dietary fiber were different. Dorien et al. (2012) found that hydrolysates prepared from soy and milk proteins stimulated the release of CCK and caused the loss of body weight, which were consistent with the present studies. SPIH may stimulate the release of hormones containing CCK, thereby regulating body weight (Nobile et al. 2016). Ng et al. (2014) investigated the effect of black soy peptide supplementation on body weight and body fat in overweight/obese human subjects and concluded that the supplementation of black soy peptides was beneficial in reducing body weight, body mass index, and body fat mass in overweight/obese subjects. The total amino acid content in the black soy peptide supplement was 6.79%, 9.07% less than that in the control with soy peptide supplement, while the content of arginine in the black soy peptide supplement (17.3%) was highe rthan that in the control (12.8%). Tang et al. (2009) also studies the application of protein hydrolysate on young men’s product, which obviously promote the muscle synthesis and reduce the thickness of sebum.

Conclusion

Compared with SPH, SPIH had higher digestibility, nitrogen release and high arginine ratio. The high content arginine in SPIH combined action with stronger nitrogen release ability, which could enhance the satiety sense of rats through strongly stimulating their intestinal mucosal cells to produce CCK, and reduce their body weight gains. These results suggest that SPIH can be used as the replacement of dietary fiber for people in order to obtain increased levels of satiety and lose body weight. Considering the fact that dietary fiber may inhibit the absorption of certain nutrients such as vitamins, amino acid, and divalent cations and that SPIH is rich in essential amino acids, SPIH may be more suitable for long-term consumption to control body weight.

Compliance with ethical standards

Conflict of interest

The authors declare that they have non-financial interests.