Effects of dietary proteins on absorption and gastrointestinal movement of p-aminobenzoic acid in conscious rats.

We monitored the absorption and movement of dietary soluble components along the gastrointestinal tract of rats by using p-aminobenzoic acid (PABA) as a marker after feeding 8 and 16% casein or soybean protein isolate (SPI) diets containing 1% PABA. The portal concentration of PABA, as an index of absorption, increased rapidly and reached the same high level 10 min after the feeding of all four diets, and the increased level of portal PABA was maintained for 30-80 min in each group. The increased levels of the SPI-fed groups continued longer than those of the casein-fed groups. In contrast, the gastric emptying rate slowed after 20 min in all the groups, and the gastric emptying of PABA for the initial 60 min in the 8% casein group was significantly faster than that in the 8% SPI group. The PABA content of the first small intestinal segment, which may be influenced by small intestinal transit, was higher in the casein group. These results indicate that the absorptive rate of PABA is determined not only by gastric emptying but also by small intestinal transit. The gastric emptying and the content of PABA in the first segment of the small intestine was not correlated in 8% protein groups. This suggests that the effect of SPI on gastrointestinal movement is different from that of casein.     

Absorptive behavior of oligo-L-methionine and dietary proteins in a casein or soybean protein diet: porto-venous differences in amino acid concentrations in unrestrained rats

In a low casein methionine-deficient diet, supplementation with oligo-L-methionine (OM) has a similar improvement on rat growth to supplementation of free L-methionine. Rat growth is little improved, however, when OM is added to a low soybean protein isolate (SPI) diet. To evaluate the mechanisms of the different effects of OM supplementation, we observed intestinal absorption of OM added to either an 8 g/100 g casein diet or an 8 g/100 g SPI diet. Using portal and venous cannulated rats under unrestrained conditions, we evaluated OM absorption by subtracting methionine porto-venous differences after feeding the diets without OM from the values after feeding the diets with 3% added OM. OM absorption was faster after feeding the casein-based diet than after the SPI-based diet during the initial stages of the experiment only, but throughout the experiment there were greater increments of methionine concentrations in both portal and venous blood after the casein-based diet than after the SPI-based diet. We also discovered that amino acids from the dietary casein were absorbed faster than those from SPI, using the porto-venous difference of isoleucine as an indication. The larger increment of methionine concentrations in the blood after feeding of OM with a casein diet and the rapid absorption of amino acids from casein may result in much growth improvement in rats.     

Soy protein isolate and its hydrolysate reduce body fat of dietary obese rats and genetically obese mice (yellow KK)

This study examined in dietary obese and genetically obese rodents the effects of soy protein isolate (SPI) and its hydrolysate (SPI-H) on the rate of body-fat disappearance. Male Sprague-Dawley rats (4-16 wk old) and yellow KK mice (6-10 wk old) were made obese by feeding high-fat diets containing 30% fat. They were then fed energy-restricted, low-fat (5.0%), and high-protein (35% casein, SPI, or SPI-H) diets for 4 wk at 60% of the level of the energy intake of rodents on laboratory chow. The body-fat contents of rats and mice fed a high-fat diet were 27.3 and 33.6 g/100 g body weight, respectively, at the end of the obese period. For rats, the apparent absorbability of dietary energy and fat was significantly lower in the SPI and SPI-H groups than in the casein group, but vice-versa for nitrogen balance. Body-fat content in mice fed SPI and SPI-H diets was significantly lower than in those fed the casein diet. In rats, plasma total cholesterol level was lower with the SPI-H diet, and plasma glucose level was lower with the SPI and SPI-H diets than with the casein diet. These results indicate that SPI and SPI-H are suitable protein sources in energy-restricted diets for the treatment of obesity.     

Effects of feeding rats low protein diets containing casein or soy protein isolate supplemented with methionine or oligo-L-methionine

The effects of supplementing 8% casein or 10% soy protein isolate (SPI) diets with graded levels of oligo-L-methionine (a mixture of hexa- and heptapeptides, OM) or L-methionine (Met) were studied in rats to determine the reason for the difference in nutritional quality between proteins and corresponding amino acid mixtures. As the OM concentration of the casein-based diet was increased from 0.02% to 0.6%, maximum weight gain was attained at 0.2%, and the growth-promoting activity of OM was comparable to Met at all the corresponding levels tested. Liver fat began to accumulate when supplemental Met reached a level of 0.08% of the casein diet, but OM addition did not produce a fatty liver at dietary levels of less than 0.3%. When SPI was used as the dietary protein source, the effect of supplemental OM was significantly less than that of Met. Digestibility of OM (assessed by incremental portal plasma Met concentration) was measured 30 min after feeding the casein or SPI diet supplemented with 3% OM using rats fasted for 24 h. Plasma Met concentration was greatly increased in rats fed the casein plus OM diet compared with that of rats fed the SPI + OM diet. Similarly, the 30-min portal Met concentration significantly increased in response to the casein + OM diet compared with the SPI + OM diet regardless of the prefed proteins (25% casein and 25% SPI for 2 wk).(ABSTRACT TRUNCATED AT 250 WORDS)     

Absorption of calcium, magnesium, phosphorus, iron and zinc in growing male rats fed diets containing either phytate-free soybean protein or soybean protein isolate or casein

The effect of dietary phytate-free soybean protein (PFS) on intestinal mineral absorption and retention was examined in growing male rats using a three-day mineral balance technique. The rats were fed diets containing PFS, soybean protein isolate (SPI) or casein at a 20% level for 5 wk. Total calcium (Ca), magnesium (Mg), phosphorus (P), iron (Fe) and zinc (Zn) contents in diets were adjusted to 0.35, 0.05, 0.7, 0.0035 and 0.003%, respectively, by supplementation of the diet with their salts. Mineral absorption and retention ratios in rats fed the PFS diet were significantly higher than those in rats fed either the SPI or casein diet. These results suggest that PFS may be a promising dietary protein source for improving the mineral bioavailability in humans.     

Consumption of soy protein isolate modulates the phosphorylation status of hepatic ATPase/ATP synthase beta protein and increases ATPase activity in rats

ATPase/ATP synthase plays important roles in the regulation of carbohydrate, protein, and lipid metabolism through modulating energy homeostasis. The purpose of this study was to examine the effects of feeding soy proteins and isoflavones (ISF) on the enzymatic activity and protein modification of hepatic mitochondrial ATPase/ATP synthase. In Expt. 1, Sprague-Dawley rats aged 50 d were fed diets containing either 20% casein or 20% alcohol-washed soy protein isolate (SPI) with or without supplemental ISF (770.7 micromol/kg diet) for 70 d. In Expt. 2, weanling Sprague-Dawley rats were fed diets containing 20% casein with or without added ISF (154.1 micromol/kg diet) or 20% SPI for 90 d. Hepatic mitochondrial ATPase activity was significantly higher in the rats fed SPI than in those fed casein. Addition of ISF to SPI eliminated the action of SPI. ATPase/ATP synthase beta protein contents in the liver were unchanged; however, its patterns measured by 2-dimensional Western blot were different among dietary groups. The rats fed SPI or SPI plus ISF had 3 more major protein spots with the same molecular weights (80 kDa and 55 kDa) as those presented in the rats fed casein but with different isoelectric points. Pretreatment of hepatic mitochondrial proteins from the rats fed casein with alkaline phosphatase produced the same ATPase/ATP synthase beta patterns as observed in the SPI-fed rats and significantly elevated the ATPase activity. These results suggest that consumption of soy proteins increases hepatic ATPase activity, which might be a consequence of increased dephosphorylation or decreased phosphorylation of the mitochondrial ATPase/ATP synthase beta protein.     

Effect of dietary protein on the peroxidation of eicosapentaenoic acid in stroke-prone spontaneously hypertensive rats.

In order to investigate the effect of dietary EPA on liver GSH peroxidase (GSH-Px) activity in rats, highly concentrated EPA (78% ethyl ester form) was administrated to SHRSP (Stroke-prone spontaneously hypertensive rat) that were fed a casein, SPI (soybean protein isolate) or SPI diet with methionine for 4 weeks. The content of liver GSH in rats fed SPI was lower than that of rats fed the casein diet. Although no significant difference of liver GSH-Px was observed in rats after EPA supplement, a decrease of liver GSH-Px activity was found in rats fed the SPI diet when compared with rats fed the casein diet. The changes of liver GSH content and GSH-Px activity in rats fed SPI were found to be associated with methionine supplement. Addition of methionine to the SPI diet resulted in an increase of liver GSH content and GSH-Px activity. In addition, liver lipid peroxide concentration was increased in rats fed the SPI diet after EPA treatment. In contrast, EPA administered rats fed the SPI diet containing methionine showed a lower liver lipid peroxide concentration. These results suggest that methionine may play an important role in regulation of the utilization of EPA in SHRSP when fed a SPI diet.     

Whole-body nitrogen and splanchnic amino acid metabolism differ in rats fed mixed diets containing casein or its corresponding amino acid mixture.

Whole-body and splanchnic metabolism of dietary amino acids derived from casein (CAS) or the corresponding crystalline L-amino acid mixture (AA) were compared. Male adult rats were adapted for 9 d to two isoenergetic, isonitrogenous diets (15 g/100 g protein, 5 g/100 g fat) containing either CAS or AA. On d 10, the rats were fed a single mixed meal (3 g dry mass) containing either intrinsically (13)C-labeled goat casein or the amino acid mixture containing [U-(13)C(6)] leucine and [alpha-(15)N] lysine. Rats were killed before and 1, 3, 5 and 7 h after meal ingestion and samples of plasma, stomach wall and contents, small intestine and liver were collected. (13)C and (15)N enrichments of free and protein-bound amino acids in plasma and tissues were analyzed by gas chromatography-combustion isotope ratio mass spectrometry. Urinary nitrogen excretion was higher (P < 0.05) and weight gain lower (P < 0.05) in rats given the AA diet, indicating a lower whole-body net protein synthesis. Free (13)C-leucine from the AA diet appeared in the intestinal mucosa free pool more rapidly (P < 0.05) than the CAS-(13)C-leucine, probably due to the faster transit through the stomach of the AA group. However, the incorporation of dietary leucine into plasma and liver proteins was higher in the CAS group 7 h after the meal (P < 0.05), whereas lysine incorporation into liver protein was higher in the AA group (P < 0.05). We conclude that whole-body protein homeostasis is better supported by dietary casein-bound than crystalline free amino acids, and that protein-bound leucine, but not lysine, is used more efficiently for liver protein synthesis than dietary free leucine.     

Effects of casein or soybean protein on plasma cholesterol level in jejunectomized or ileectomized rats

The contribution of enterohepatic circulation of cholesterol and bile acids in the hypocholesterolemic activity of soybean protein isolate (SPI) was compared with the activity of casein. Intact, sham-operated, jejunectomized or ileectomized adult rats were fed a cholesterol-free, purified diet containing either 20% casein or 20% SPI for 7 or 10 d. For the subsequent 7 d the diets were reversed. In intact rats the plasma cholesterol concentration (p-chol) was significantly higher when the casein diet was fed than when the SPI diet was fed. Within 3 d after the diet crossover, p-chol in each group of intact rats reached rapidly the same level as that in its previous counterpart. These rapid inherent responses of p-chol to the casein and SPI diets remained unchanged even when the jejunum or the ileum was resected, and p-chol in jejunectomized or ileectomized rats was similar to that found in sham-operated rats. The extent of the change in p-chol induced by exchanging dietary proteins was almost the same among intact, sham-operated, jejunectomized and ileectomized rats. These findings indicate that the interruption of enterohepatic circulation of cholesterol and bile acids is not the major factor involved in the differential effects of dietary proteins on p-chol.     

The influence of dietary protein on lipid peroxide formation in old, food restricted rats

The effect of deetary protein on in vivo lipid peroxidation in the liver and kidney of 21 months-old, SD male rats was investigated after feeding them isocaloric diets for 60 days. Rats were divided into four groups as 5% soy protein isolate (SPI), 20% SPI, 5% casein and 20% casein diet groups and they all received 15 g food/day (59 kcal/day or 25% food restriction). The liver and kidney SOD activities, as well as thiobarbituric acid-reactive substances (TBARS), were increased in low protein fed animals. Catalase activities did not have significant differences between groups, but it was 15 times higher in the liver than the kidney. Glutathione peroxidase (GPx) showed increased activities in the liver of animals fed 5% protein diet, but the same was not observed in the kidney. Total glutathione was higher in the organs of animals fed 20% protein diet rather than 5% protein diet for both organs, liver and kidney, and values of the former were much higher than those of the latter. Though enzyme activities were not altered as a function of dietary protein level of quality, there is a strong evidence that TBARS is somehow augmented by feeding a low protein diet.     

Effects of sources of dietary fat and protein on tissue cholesterol

Dietary variables were soy oil, beef tallow, soy protein, and casein. Dietary combinations were soy oil-soy protein, soy-oil casein, beef tallow-soy protein, and beef tallow-casein, and 96 rats were allotted randomly to the four isocaloric diets. [Crystalline cholesterol was added to standardize each diet at 0.2%.] Two randomly selected rats from each dietary group were killed at 0, 7, 9, 11, 13, 15, 18, 22, 30, 36, 42, and 48 days on experiment to determine the effects of sources (plant versus animal) of dietary fat and protein on tissue cholesterol concentrations and on cholesterogenesis in liver and small intestine. Feeding soy oil, a polyunsaturated fat, resulted in lower blood cholesterol concentrations, higher liver cholesterol concentrations, and lower intestinal cholesterogenesis than did feeding beef tallow, a saturated fat. Feeding soy protein, a plant protein, resulted in lower blood and liver cholesterol concentrations and less intestinal cholesterogenesis than did feeding casein, an animal protein. Hepatic cholesterogenesis and intestinal tissue cholesterol levels were not affected significantly by diet. Eight rats killed at day 0 had, on the average, lesser plasma cholesterol concentrations and greater rates of intestinal cholesterogenesis than rats fed experimental diets. Our results demonstrate that the hypocholesterolemic action of soy oil and soy protein fed to rats may be related to decreased intestinal cholesterogenesis. In addition, soy oil, a polyunsaturated fat caused a redistribution of cholesterol from plasma to liver.     

Cholesterol metabolism in mature and immature rats fed animal and plant protein

In two feeding experiments immature (180 g) and mature rats (370 g) were fed a semi-purified diet containing 20% of a protein source (casein, wheat gluten, soybean or potato protein) for 4 wk. Food supply was restricted to 15 g daily. As compared to casein, plant proteins induced significantly lower concentrations of plasma total cholesterol and high density lipoprotein (HDL) cholesterol. The plasma cholesterol increase associated with aging was not prevented by consumption of casein, soybean or potato protein, but wheat gluten seemed to be effective. Lecithin-cholesterol acyltransferase (LCAT) activity was not significantly different in rats of the same age fed different plant proteins, whereas the esterification rate was lower in rats fed casein. With aging the LCAT activity generally decreased. As compared to the casein groups, the rats fed plant proteins showed higher excretion of fecal neutral and acidic steroids. Among the groups fed plant proteins, the fecal output of steroids was variable. Significantly negative correlations were found between fecal total sterol excretion and plasma total cholesterol or HDL cholesterol, respectively. Plant proteins showed a faster migration rate in the stomach, whereas their migration and absorption were slower in the first half of the small intestine. A relation between nonabsorbed nitrogen-containing substances and sterol excretion was hypothesized.     

Intestinal zinc and carboxypeptidase A and B activity in response to consumption of test meals containing various proteins by rats

Rats were fed test meals containing 23% protein as casein (C), soy protein isolate (SPI) or egg white (EW) to assess effects of dietary protein sources on te amount of zinc and carboxypeptidase (CP, CPA, CPB) activity in the small intestinal contents. Groups of six rats each were killed at 1, 2.5 and 5 h postprandial and six rats fasted for 19 h served as a 0 time control. Consumption of a test meal increased the weight of small intestinal contents at 1 and 2.5 h compared with unfed animals and in a similar manner for all three protein sources. However, at various times during the postprandial period differences in the level of zinc and CP activity within the small intestinal contents among the three dietary protein sources were observed. Both the SPI and EW groups had significantly higher levels of CPA and CPB activity at 1 h postprandial than the C group. Only in the SPI group was the intestinal level of zinc increased at 1 and 2.5 h compared with the unfed group. At 1 and 2.5 h postprandial intestinal zinc was significantly higher in the SPI group than in the C and EW groups. The results indicate that the disappearance of zinc from the intestine is delayed in rats fed SPI, probably because of the presence of phytate.     

Dietary soy protein isolate modifies hepatic retinoic acid receptor-beta proteins and inhibits their DNA binding activity in rats

Retinoic acid receptors (RAR) belong to the same nuclear receptor superfamily as thyroid hormone receptors (TR) that were previously shown to be modulated by dietary soy protein isolate (SPI). This study has examined the effect of dietary SPI and isoflavones (ISF) on hepatic RAR gene expression and DNA binding activity. In Expt. 1, Sprague-Dawley rats were fed diets containing 20% casein or 20% alcohol-washed SPI in the absence or presence of increasing amounts of ISF (5-1250 mg/kg diet) for 70, 190, or 310 d. In Expt. 2, weanling Sprague-Dawley rats were fed diets containing 20% casein with or without supplemental ISF (50 mg/kg diet) or increasing amounts of alcohol-washed SPI (5, 10, and 20%) for 90 d. Intake of soy proteins significantly elevated hepatic RARbeta2 protein content dose-dependently compared with a casein diet, whereas supplemental ISF had no consistent effect. Neither RARbeta protein in the other tissues measured nor the other RAR (RARalpha and RARgamma) in the liver were affected by dietary SPI, indicating a tissue and isoform-specific effect of SPI. RARbeta2 mRNA abundances were not different between dietary groups except that its expression was markedly suppressed in male rats fed SPI for 310 d. DNA binding activity of nuclear RARbeta was significantly attenuated and the isoelectric points of RARbeta2 were shifted by dietary SPI. Overall, these results show for the first time, to our knowledge, that dietary soy proteins affect hepatic RARbeta2 protein content and RARbeta DNA binding activity, which may contribute to the suppression of retinoid-induced hypertriglyceridemia by SPI as reported.     

The addition of soybean phosphatidylcholine to triglyceride increases suppressive effects on food intake and gastric emptying in rats

The physiologic roles of dietary lecithin have not yet been clearly defined. We examined the effects of soybean lecithin on gastric emptying (Experiments 1 and 2) and food intake (Experiment 3) in rats. Male Wistar rats were fed 2 g of a 20 g lipid/100 g diet containing various levels of lecithin after 24 h of food deprivation; gastric contents were collected 3 h after feeding (Experiment 1). In Experiment 2, the effects of lecithin and a CCK-A receptor antagonist on gastric emptying were examined using a modified phenol red recovery technique. In Experiment 3, their effects on food intake were examined after an intraduodenal infusion of an oil emulsion containing 50 mg soybean oil (SO) or SO partially replaced by lecithin (14-50%). Gastric emptying rates of the lipid and protein in the test diet (Experiment 1) or of phenol red (Experiment 2) were lower in the groups administered lecithin. Food intake for 60 min was lower in rats infused with the oil emulsion containing lecithin (25, 50%) than in rats not administered lecithin. The suppressive effects of lecithin on gastric emptying and food intake were largely reduced by devazepide. These results demonstrate that oil containing lecithin inhibits gastric emptying and food intake, and the effects are associated in part with CCK release.     

The effect of dietary protein source on plasma cholesterol level and fecal steroid excretion in obese mice

Both goldthioglucose (GTG)-treated and the genetically obese (C57BL/6J ob/ob) mice were fed semisynthetic diets containing either soy protein isolate (SPI) or casein as a protein source, or laboratory chow. In GTG-induced obese mice, the plasma cholesterol level correlated positively with their body weight. The level was highest in mice fed high-fat diet, and lowest in ones fed laboratory chow. No difference was observed between SPI and casein groups whether the diet was low-fat or high-fat. Thus, in the GTG-treated mice, SPI did not have a hypocholesterolemic effect while dietary fat had a hypercholesterolemic effect, and laboratory chow contained some component(s) which can lower the plasma cholesterol level. Both neutral and acidic steroid contents in feces of the SPI group were not different from those of the casein group, and both groups of mice excreted a smaller amount of steroids than mice fed laboratory chow. Results of essentially the same tendency were obtained with normal mice regarding the effects of SPI and casein, although the degree of hypercholesterolemia was lower in high-fat-fed normal mice than in similarly fed GTG-treated mice. These results strengthened the inverse correlation between the amount of fecal steroids and the plasma cholesterol level upon feeding various proteins, indicating that the former is one of the important factors that determine the latter. The ob/ob mice showed a marked hypercholesterolemia irrespective of the kind of diet. The amount of fecal steroids was highest in the laboratory chow group and lowest in the casein group. This indicates that some factor(s) other than fecal steroid excretion is dominantly responsible for their hypercholesterolemia.     

The effect of dietary safflower phospholipid on steroids in gastrointestinal tract of rats fed a hypercholesterolemic diet.

The effect of dietary safflower phospholipid (Saf-PL) on the postprandial changes of steroids in the small intestinal and cecal contents was examined in rats fed a hypercholesterolemic diet. The triglyceride mixture (SP-Oil) containing a comparable amount of linoleic acid to Saf-PL was used as a reference fat source. Saf-PL suppressed the elevation of plasma cholesterol levels at all times after meal intake, when compared to SP-Oil. The reduction of plasma cholesterol in rats fed the Saf-PL diet was exclusively observed both in chylomicron plus very low density lipoprotein (VLDL) and low density lipoprotein (LDL) fractions. The rate of gastric emptying was not modified by the Saf-PL diet. The level of neutral steroids in the small intestinal contents was almost comparable in both groups, but in the cecal contents and feces it was significantly higher in rats fed the Saf-PL diet. On the other hand, the level of acidic steroids in the small intestinal contents tended to be higher in rats fed the Saf-PL diet than in those fed the SP-Oil diet, whereas in the cecal contents and feces it was comparable in the two diets. These results suggest that Saf-PL causes the accumulation of neutral steroids in the cecum due to the rapid transit through the small intestine.     

The effect of short-term dietary supplementation with glucose on gastric emptying in humans

In order to test whether gastric motility can adapt to changes in nutrient load, gastric emptying of hyperosmotic glucose and protein drinks was measured by applied potential tomography in two groups of ten volunteers following dietary supplementation with 400 g glucose/d for 3 d. The half emptying time for the glucose test meal was significantly faster after the standard diet had been supplemented with glucose compared with the standard diet alone (median and range, 20.7 (4.6-36.8) v. 29.1 (19.8-38.4) min; P less than 0.05), while the emptying of the protein drink (Oxo; Brooke Bond Ltd) was unchanged (median and range, 18.0 (12.5-23.6) v. 16.1 (9.6-22.7) min). These results suggest that rapid and specific adaptation of the small intestinal regulatory mechanisms for gastric emptying of nutrient solutions can occur in response to increases in dietary load. This adaptation may be explained by desensitization of nutrient receptors or by a reduction in the area of receptor field exposed to nutrients caused by increased absorption of glucose in the upper small intestine.     

Regulation of rat liver branched-chain alpha-keto acid dehydrogenase activity by meal frequency and dietary protein

Effects of feeding frequency on liver branched-chain alpha-keto acid dehydrogenase (BCKAD) activity are unknown. In the present study, rats were trained to consume their daily allotment of food in 6 h (meal-feeding). Rats were fed diets containing 0, 9, 25 or 50% casein and after 10 d were killed before or 3 h after the meal. The enzyme in rats fed diets containing 0, 9 and 25% casein was activated three- to sixfold after meal consumption. Previous studies showed that the liver enzyme is essentially fully activated in post-absorptive rats fed an adequate protein diet ad libitum. Meal-feeding an adequate protein (25% casein) diet resulted in a marked decrease in the postabsorptive percentage of active complex compared to ad libitum feeding of the same diet (29 +/- 6% vs. 93 +/- 6% active). Administration of alpha-ketoisocaproate (200 mumol/100 g body weight, an inhibitor of BCKAD kinase) reversed the meal-feeding-induced inactivation of the complex within 10 min. We conclude that the frequency of food intake, in addition to the level of dietary protein, influences the proportion of liver BCKAD in the active state. Inactivation of hepatic BCKAD in rats trained to feed once a day may be an adaptive mechanism that results in increased efficiency of branched-chain amino acid utilization between meals.     

Effect of different dietary protein composition on skeletal muscle atrophy by suspension hypokinesia/hypodynamia in rats

Under microgravity conditions similar to those in space, it is known that various nutritional and physiological changes in the body are induced. Especially in the aspect of nutrition, muscle atrophy is a characteristic phenomenon accompanying weightlessness. This study was conducted to investigate the ameliorated effect of muscle atrophy caused by suspension hypokinesia by using the soy protein isolate (SPI) as the protein source in comparison with casein. Male Wistar strain rats (8 wk old) were divided into two groups, each suspended with a suspension harness, and fed on a 20% SPI diet or a 20% casein diet for 10 d. The body weights of the suspended rats fed casein or SPI decreased similarly. The weight of the gastrocnemius and soleus muscle were decreased by suspension hypokinesia; however, the degree of the decrease of the muscle weights, especially soleus muscles, of rats fed the SPI diet was smaller than that of rats fed the casein diet. Serum Ntau-methylhistidine concentration was significantly lower in rats fed the SPI diet than in rats fed the casein diet. Similarly, the activities of muscle protein-degrading enzymes such as calpain and proteasome were significantly lower in rats fed the SPI diet than in rats fed the casein diet. Cathepsin B+L activities were not affected by the SPI or the casein diet. Therefore it is suggested that SPI caused a reduction of the proteolysis of myofibrillar protein in skeletal muscles through a reduction of calpain and proteasome activities, in consequence to ameliorate the muscle atrophy.