Effects of soy protein isolate and soy peptide preload on gastric emptying rate and postprandial glycemic control in healthy humans

概要

Ueoka et al.
Journal of Physiological Anthropology
(2022) 41:25
https://doi.org/10.1186/s40101-022-00299-9

ORIGINAL ARTICLE

Open Access

Effects of soy protein isolate and soy
peptide preload on gastric emptying rate
and postprandial glycemic control in healthy
humans
Hatsumi Ueoka1, Yoshiyuki Fukuba2,3, Masako Yamaoka Endo2, Toshio Kobayashi4, Hironobu Hamada1 and
Hideaki Kashima2*    

Abstract 
Background:  This study aims to compare the effects of soy protein isolate (SPI) and soy peptide (PEP) preload 30 min
before a 75-g oral glucose tolerance test (OGTT) on the gastric emptying rate, plasma insulin, and blood glucose
responses.
Methods:  Nine healthy young subjects were evaluated on four occasions. The participants consumed a 200-ml solution containing either 20 g of SPI or PEP in experiment 1. In experiment 2, 30 min after consuming either 20 g of SPI
or PEP solutions, an OGTT was performed to evaluate the individual glycemic response. The gastric emptying rate was
measured by the 13C-sodium acetate breath test. Blood glucose and plasma insulin were measured before and after
consuming either the SPI or PEP solutions and during the OGTT.
Results:  In experiment 1, plasma insulin levels were higher 30 min after consuming the PEP solution than after
the SPI solution. PEP resulted in a faster gastric emptying rate than SPI. In experiment 2, just before performing the
OGTT, the plasma insulin response was higher for PEP than for SPI. Fifteen minutes after starting the OGTT, the blood
glucose response was lower after consuming PEP than after SPI. The gastric emptying rate tended to be faster after
consuming PEP than after SPI (p = 0.08).
Conclusion:  A PEP preload might be slightly more effective for the suppression of postprandial blood glucose excursion compared with SPI; thus, a PEP preload potentially induces an enhanced insulin response just before the OGTT.
Keywords:  Glucose, Insulin, Soy peptide, Soy protein isolate, Gastric emptying, Oral glucose tolerance test (OGTT)
Background
Controlling postprandial blood glucose levels to remain
within the normal range is important for preventing
complications, such as neuropathy, renal disorders,
and heart failure. The guidelines for the management of
*Correspondence: kashima@pu-hiroshima.ac.jp
2
School of Health Sciences, Prefectural University of Hiroshima, Hiroshima,
Japan
Full list of author information is available at the end of the article

postprandial blood glucose [1] indicate that postprandial
hyperglycemia is a risk factor that accelerates complications of diabetes, and recently, several dietary therapies
for the prevention of postprandial hyperglycemia have
been proposed [1]. The postprandial blood glucose level
is determined by the interactions among insulin, glucagon, incretin hormones, and gastric emptying [2]. It is
well known that postprandial hyperglycemia is induced
by abolition of the initial phase insulin response (IPIR)
following meal ingestion in type 2 diabetic patients [3].

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Ueoka et al. Journal of Physiological Anthropology

(2022) 41:25

Gastric emptying plays a major role in the digestion
of nutrients and is an important factor for determining postprandial glycemic responses [2]. As a strategy
to accelerate the IPIR immediately after meal ingestion,
protein ingestion before the main meal, called protein
preloading (PPL), has recently been reported [4–9].
Whey protein derived from milk protein consumption
(50–55 g) 30 min prior to consuming a carbohydrate-rich
meal reduced the postprandial hyperglycemic response
compared with no prior whey protein consumption in
patients with type 2 diabetes [6, 8]. Akhavan et  al. [4]
reported that even relatively small amounts of whey protein (10–40 g) were still effective to reduce the postprandial glycemic response in healthy subjects. This positive
effect of whey protein preloading on the postprandial
glycemic response was due to an enhanced IPIR and
slower gastric emptying with increased glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and cholecystokinin (CCK) secretion
preceding the main meal [4, 6, 8]. Recently, two studies reported consistent results that consumption of real
food (milk) before main meals suppress the subsequent
blood glucose response [10, 11]. These results are beneficial for North American and European people who frequently consume milk at particular times. By contrast, in
Asian countries, soy milk has been popular since ancient
times. Asians typically consume 9 to 30  g soybeans and
soy products per day, which is higher than Americans
[12]. In Japan, soybean is ingested on a daily basis in the
form of processed foods such as tofu, miso, and natto. In
addition, soy products can be safely consumed by people
with lactose intolerance [13]. It is, therefore, meaningful to investigate the effect of PPL with different types of
proteins.
To our knowledge, there is less evidence on other types
of PPL (e.g., soy protein) than whey PPL. Two previous
studies reported that PPL with soy protein (0.5 g/kg) [9]
and soy protein isolate (SPI) (40  g) [7] can also reduce
the postprandial blood glucose response compared with
control conditions without PPL. However, to practically
use PPL with soy protein, protein intake might need to be
further reduced because of overall energy intake and an
increase in the risk of type 2 diabetes [14, 15].
To mitigate concerns about the high-dose consumption
of soy protein, soy peptide (PEP) might be a good protein
source because hydrolyzed soy protein displays greater
insulin secretion than intact soy protein [16]. Therefore,
we hypothesized that PPL with PEP could more effectively reduce the postprandial blood glucose response
compared with intact soy protein via exaggerated insulin
response at the initial phase of the OGTT (i.e., IPIR). To
test this hypothesis, we investigated the influence of preloading PEP and protein before the OGTT on subsequent

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glycemic control in healthy subjects. Moreover, to provide a further understanding of differences in the physiological response between intact soy protein and PEP, we
investigated the following two factors. First, we examined
whether a greater insulin response can be induced by PEP
consumption than by intact soy protein consumption
according to a previous study [16]. Second, we compared
the gastric emptying rate between soy protein and PEP.
The incretin effect is greater when the gastric emptying
rate is faster [2]. These findings are expected to serve as a
basis for the mechanism of postprandial glycemic control
with PPL.

Methods
Subjects

Nine healthy young Japanese subjects (seven females and
two males; age, 22 ± 3 years; height, 162 ± 5 cm; weight,
56 ± 6 kg; mean ± SD) participated in this study. The subjects were normotensive, did not smoke or take any medications, and had no history of autonomic dysfunction or
cardiovascular disease. This study was approved by the
Ethics Committee of the Prefectural University of Hiroshima (No. HH14004), Japan, and each subject provided
written informed consent to participate prior to the commencement of the study. The subjects arrived at 08:30 in
the laboratory after fasting for 10 h overnight and having
abstained from strenuous exercise, alcohol, and caffeine
for at least 1  day. The subjects were seated in a chair in
a quiet room, where the temperature and humidity were
maintained at 22 ± 1 °C and 30% ± 4%, respectively.
Experimental design

This study consisted of two experiments, and the subjects underwent a total of four protocols. Experiment was
conducted with a single-blinded, randomized, crossover
design. Female subjects were scheduled for assessment
during their late follicular phase because the menstrual
cycle affects the gastric emptying rate and blood glucose,
insulin, and GLP-1 concentrations [17, 18]. Male subjects
participated a maximum of once per week.
In experiment 1, after collecting baseline blood samples
and breath samples, the subjects were instructed to consume a 200-ml drink containing either 20 g of SPI powder (HD-101R, Fuji Oil Co., Ltd., Osaka, Japan) or 20  g
of PEP powder (HI-NUTE AM (Fuji Oil Co., Ltd., Osaka,
Japan) within 1 min; the subjects were then monitored for
120 min. In experiment 2, after collecting baseline blood
samples and breath samples, the subjects were instructed
to consume a 200-ml preloading drink containing either
20 g of SPI or PEP (i.e., the same drink as experiment 1).
Thirty minutes later, they consumed a 225-mL drink containing 75 g of glucose (Trelan-G; Ajinomoto Pharma Co.,
Ltd., Tokyo, Japan) (i.e., OGTT) and subsequently rested

 Ueoka et al. Journal of Physiological Anthropology

(2022) 41:25

for 120  min. Three grams of the artificial sweetener Pal
Sweet (Ajinomoto Co., Ltd., Tokyo, Japan) was added
to all preloading drinks for palatability. ...

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