Dietary pectin lowers sphingomyelin concentration in VLDL and raises hepatic sphingomyelinase activity in rats

There is evidence that cholesterol and sphingomyelin metabolism are interrelated, and thus the hypothesis tested was that dietary pectin, because it can alter hepatic cholesterol metabolism, would also alter hepatic sphingomyelin metabolism. For that purpose, 4-wk-old female Wistar rats were fed a diet without or with pectin (20 g/100 g) up to 21 d. In accordance with previous work, pectin consumption caused a significant (P < 0.001) reduction in hepatic (65%), whole plasma (37%), and VLDL (80%) cholesterol levels. Pectin also significantly reduced VLDL sphingomyelin concentrations (57%), but raised the amount of sphingomyelin in the high density lipoproteins (HDL)-2 fractions (58%), so that the level of sphingomyelin in whole plasma remained unaffected. Pectin did not affect the sphingomyelin concentration in the liver. Pectin consumption did not affect the hepatic sphingomyelin synthesizing enzymes, serine palmitoyltransferase, phosphatidylcholine:ceramide phosphocholine transferase, or phosphatidylethanolamine:ceramide phosphoethanolamine transferase. In contrast, dietary pectin activated both lysosomal (28%) and plasma membrane (26%) sphingomyelinase and thus may have enhanced sphingomyelin degradation. An attempt was made to describe the effects of dietary pectin on sphingomyelin metabolism in terms of altered fluxes through liver and plasma, with whole liver and whole plasma concentrations of sphingomyelin remaining unaffected.     

VLDL metabolism in rats is affected by the concentration and source of dietary protein

The present study was designed to determine if changes in dietary protein level and source are related to changes in VLDL lipid concentrations and VLDL binding by hepatic membranes and isolated hepatocytes. Male Wistar rats were fed cholesterol-free diets containing 10, 20 or 30 g/100 g casein or highly purified soybean protein for 4 wk. Hepatic, plasma and VLDL lipids, VLDL apo B-100 and VLDL uptake by isolated hepatocytes and VLDL binding to hepatic membrane were determined. Increasing casein or soybean protein level (from 10 to 30 g/100 g) in the diet increased VLDL apo B-100, indicating an increase in the number of VLDL particles. VLDL uptake by isolated hepatocytes and VLDL binding to hepatic membrane increased when the protein level increased from 10 to 20 g/100 g in the diet and decreased with 30 g/100 g protein, regardless of protein type. The dietary protein source did not affect plasma total cholesterol concentrations at any protein level. Feeding 20 g/100 g soybean protein compared with casein lowered plasma triglyceride concentrations and VLDL number as measured by decreased VLDL-protein, -phospholipid, -triglyceride, -cholesterol and -apo B-100. VLDL uptake by isolated hepatocytes and VLDL binding to hepatic membrane were higher in rats fed soybean protein than those fed casein. The higher VLDL uptake could be responsible for the hypotriglyceridemia in rats fed soybean protein.     

Effects of dietary fish protein, soybean protein and casein on cholesterol turnover in rats

The effects of dietary fish, soybean protein and casein on cholesterol turnover were compared in rats. After the injection of [14C]cholesterol into the rats, the specific activities of radioactive cholesterol in feces were followed for 4 weeks. The cholesterol half-lives calculated from the decay curves of the specific activities were 14.7 and 14.6 days in rats fed fish protein and soybean protein, respectively. These were shorter than the half-life (17.4 days) in casein-fed controls. The fish and the soybean protein feedings significantly increased the fecal excretions of cholesterol and coprostanol, respectively, and lowered the plasma cholesterol level, as compared with casein feeding. In addition, both fish and soybean protein feedings also increased the excretion of bile acids. The stimulation of cholesterol metabolism and the increased excretions of cholesterol and its metabolites by feeding fish or soybean protein appear to play important roles in the hypocholesterolemic effects.     

Inhibitory effect of dietary soybean protein vs. casein on magnesium absorption in rats

The effects of casein and soybean protein on magnesium absorption and magnesium concentration in the femur were investigated in rats. Purified diets containing either casein or soybean protein and three concentrations of added magnesium (0.82, 1.64 or 2.46 mmol/100 g diet) were used. The isonitrogenous diets were carefully balanced for the different mineral concentrations in the protein preparations. Absolute and percent magnesium absorption and urinary magnesium excretion were significantly decreased in rats fed soybean protein when compared with casein, irrespective of the dietary concentration of added magnesium. The magnesium content of femur was significantly lower in rats fed soybean protein, but this effect was seen only when the diet contained 0.82 mmol magnesium/100 g diet. The addition of sodium phytate to the casein diets, to a concentration identical to that in the diets containing soybean protein as provided by the soybean protein preparation, produced similar effects on magnesium absorption as the diets containing soybean protein. These results indicate that soybean protein, when compared with casein, decreases magnesium absorption through its phytate component.     

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.     

Biliary steroid excretion in rats fed soybean protein and casein or their amino acid mixtures

Male Wistar rats were fed 1% fat, cholesterol-free semipurified diets containing soybean protein isolate and casein or amino acid mixtures simulating these proteins for 28-30 days. The animals then underwent surgery for biliary diversion, and bile was collected for 2 hours. The rate of bile flow was not influenced by the type of dietary protein. The concentration and output of biliary cholesterol in rats fed soybean protein were significantly higher than in those fed casein, while biliary bile acid excretion was comparable. The group given the soy protein-type amino acid mixture also tended to excrete more biliary cholesterol relative to the casein-type amino acid mixture, although the difference was not significant, while biliary bile acid output was comparable. When 5% fat diets containing 0.5% cholesterol were given to rats, concentrations of both biliary cholesterol and bile acid were elevated significantly on a vegetable protein diet. In these experiments, there was a significant negative correlation between serum cholesterol and biliary cholesterol levels. Stimulation of the biliary excretion of cholesterol seems at least relevant to the cholesterol-lowering action of soybean protein.     

Comparative effect of casein and soybean protein isolate on body fat accumulation in adult rats.

The effect of dietary protein on the body fat accumulation was studied in rats. Adult rats weighing about 300 g were fed 21% protein (casein or soybean protein isolate) and 5% oil diets by pair-feeding for 65 days in Experiment 1. In Experiment 2, only protein and oil contents were changed, 25 and 10%, respectively. Final body weights of the two dietary groups were similar in both experiments, especially in Experiment 2. Total body fat was slightly lower in the soybean protein diet group than in the casein diet group in Experiment 2, only when it was expressed as the percentage against body weight. However, intra-abdominal fat was significantly lower in the soybean protein diet groups than in the casein diet groups in both experiments. Serum lipid levels were greatly lower in the soybean protein diet group than in the casein diet group in Experiment 2 (the data were not available in Experiment 1). The results suggest that dietary soybean protein has the effect to lower the intra-abdominal fat accumulation as compared with casein.     

Dietary casein and soybean protein affect the concentrations of serum cholesterol, triglyceride and free amino acids in rats.

This work was undertaken to investigate the concentrations of free amino acids in blood after food was withheld from growing, male Wistar rats fed cholesterol-free, low fat (1 g corn oil/100 g) diets with casein or soybean protein for 2 wk. A diet containing 22.5 or 23.5 g/100 g of soybean protein was hypocholesterolemic compared with a diet containing 20.0 g casein/100 g diet. A comparison of serum amino acids in soybean protein-fed vs. casein-fed rats showed that, whereas concentrations of many amino acids were lower in the soybean protein-fed rats compared with the casein-fed groups, glycine was the only amino acid having a higher concentration. Further, alanine was significantly lower in the soybean protein-fed rats compared with the casein-fed rats, and the protein-induced differences in glycine and alanine concentrations of unfed rats were reproducible. When diets containing 15.0% casein or 30.0% soybean protein, a casein diet supplemented with glycine and a soybean protein diet supplemented with methionine were compared, the changes in serum glycine and alanine correlated with the changes in serum cholesterol. Concentrations of several amino acids, particularly valine, leucine and tyrosine, also changed when serum cholesterol concentrations varied, but these effects could not be explained by our experiments. The results suggest that a change in serum concentration of glycine and alanine of unfed rats may be related to the change in serum cholesterol concentration.     

Increasing intake of soybean protein or casein, but not cod meal, reduces nephrocalcinosis in female rats.

Female weanling rats were fed diets with soybean protein, casein or cod meal at 171, 342 or 513 mmol nitrogen/100 g for 3 wk. The diets were isonitrogenous and balanced for fat, cholesterol, calcium, magnesium and phosphorus. Cod meal feeding at 171 and 342 mmol nitrogen/100 g diet produced lower kidney calcium concentrations than the feeding of either soybean protein or casein. Increasing protein intakes were associated with reduced kidney calcium concentrations in the rats fed either soybean protein or casein but not in those fed cod meal. The anti-nephrocalcinogenic effect of increasing intakes of soybean protein may relate to the lowering of urinary phosphorus concentration. Increasing intakes of casein probably inhibited nephrocalcinogenesis by lowering urinary pH and raising urinary magnesium concentration. Increasing cod meal concentrations in the diet lowered urinary pH and raised urinary magnesium and calcium concentrations, but the effects on nephrocalcinogenesis of these changes probably counteracted each other.     

大豆蛋白和高钙摄入对高血胆固醇模型大鼠血脂水平的影响

目的探讨膳食大豆蛋白和/或高钙摄入对高血胆固醇模型大鼠血浆胆固醇及有关血脂指标的影响.方法用含5%猪油、1%胆固醇和0.25%胆碱的高脂饲料喂饲健康Wistar雌性大鼠,经14d诱发高血胆固醇以后,测血浆总胆固醇(TC),总甘油三酯(TG),高密度脂蛋白胆固醇(HDL-C)和载脂蛋白B(ApoB100)的浓度.然后,按体重和血浆胆固醇浓度将动物均衡分为4组,以2×2析因设计,分别喂饲含有酪蛋白和大豆分离蛋白及正常钙(0.59%)和高钙水平(1.12%)的纯合成高脂饲料.经21d喂养后,采血,测定指标同前.实验结果用SPSS统计软件进行方差分析.结果经14d诱导高血脂后,实验模型大鼠的TC水平为(4.10±1.11)mmol/L(n=24),而喂饲正常饲料的阴性对照鼠仅为(1.92±0.28)mmol/L(n=8),升高了2.14倍.又经21d喂饲含有处理因素的饲料后,酪蛋白组,酪蛋白加钙组,大豆蛋白组和大豆蛋白加钙组大鼠的TC水平分别为(10.56±2.74,7.99±2.19,5.48±0.51和6.81±0.98)mmol/L;ApoB100为(0.46±0.14,0.31±0.12,0.17±0.07和0.21±0.05)g/L.同酪蛋白组相比,其他3组动物的TC和ApoB100浓度显著降低.而各组间TG和HDL-C的变化均无显著性.结论膳食大豆蛋白和/或高钙摄入均可降低高血胆固醇模型大鼠的TC和ApoB100浓度,但对TG和HDL-C的作用不明显.大豆蛋白是这种作用的主要因素,但同时存在大豆蛋白和高钙摄入的相互作用.膳食大豆蛋白和高钙摄入的降血胆固醇机制还需要进一步探讨和研究.     

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.     

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.     

Effects of dietary casein and soybean protein on triglyceride turnover in rat liver

We previously found that hepatic lipogenic enzyme induction, fatty acid synthesis, and triglyceride level were markedly lower in rats fed soybean protein than in those fed casein (Iritani et al. (1986): J. Nutr., 116: 190). After labeling triglycerides with tritiated water, the effects of dietary protein on the triglyceride degradation have been investigated. After the injection of tritiated water into rats, the radioactivities of fatty acids and triglycerides reached a plateau in 1-2 days and were markedly lower in the soybean protein group than in the casein group. The decreasing rates of triglyceride radioactivities were similar between the casein and soybean protein groups. The enzyme activities in glycerolipid synthesis were similar between the groups. Therefore, the lowering effects of soybean protein on triglyceride levels appear to be ascribed to triglyceride synthesis (due to fatty acid synthesis) rather than to the degradation.     

Absorption of zinc from dietary casein phosphopeptide complex with zinc in rats given a soybean protein-based diet

The absorption of zinc (Zn) in the form of casein phosphopeptide (CPP) complex was compared with that of its chloride form in rats given a soybean protein-based diet. We prepared 67Zn complex with CPP (67Zn-CPP). Male rats were given a diet containing ZnCl2 in the preliminary period. After overnight food deprivation, they were fed a test meal labeled 67Zn-CPP or 67ZnCl2 (4 g Zn-free diet + 0.12 mg 67Zn) with 0.5 mg Dysprosium (Dy) as a fecal marker. Feces were collected for 5 d and analyzed for 67Zn isotopic enrichment and Dy concentration with an inductively coupled plasma mass spectrometer. Fecal Dy excretion closely paralleled that of 67Zn, and more than 89% of the administered Dy was recovered in the feces collected for 5 d. Moreover, the Zn source did not affect the excretion pattern of 67Zn. Since Dy and 67Zn were almost excreted within 3 d after the administration, the apparent absorption of 67Zn was calculated from the pooled data for 3 d. The source of Zn did not affect the apparent absorption of 67Zn. These results suggested that dietary Zn in the form of CPP complex did not show higher absorbability in rats.     

Comparison of the hypocholesterolemic effects of dietary soybean protein with those of formaldehyde-treated casein in rabbits

Treatment of casein with formaldehyde changes its tertiary structure and decreases its hypercholesterolemic properties in rabbits. To investigate whether formaldehyde-treated casein exerts this hypocholesterolemic effect in the same manner as soybean protein, rabbits were fed high or low cholesterol diets containing soybean protein, casein, formaldehyde-treated casein or a mixture of casein and formaldehyde-treated casein. Formaldehyde-treated casein was hypocholesterolemic when fed in a low, but not in a high, cholesterol diet. The hypocholesterolemic effect of soybean protein was independent of the amount of cholesterol included in the diet. In contrast to rabbits fed soybean protein, steroid absorption in those fed formaldehyde-treated casein did not differ from that in rabbits fed native casein. Furthermore, the absorption of phosphorus and nitrogen was lower in rabbits fed formaldehyde-treated casein than in those fed native casein, whereas the absorption found in rabbits fed soybean protein resembled that of their casein-fed counterparts. The diets containing soybean protein and formaldehyde-treated casein produced a comparable ratio of lysine to arginine in serum. The results presented in this paper indicate that the hypocholesterolemic action of dietary formaldehyde-treated casein does not resemble that of soybean protein.     

Effects of addition of sulfur-containing amino acids and glycine to soybean protein and casein on serum cholesterol levels of rats

The effects of the supplementation of methionine (Met), cystine (Cys), and glycine (Gly) to soybean protein or casein on serum and liver lipid levels were studied in rats. Rats were fed cholesterol-free diets containing 25% soybean protein or casein supplemented with 0.75% Met, 2.5% Gly, or a combination of these two for 4 weeks. The addition of Met to soybean protein caused a significant increase in serum cholesterol and this was slightly ameliorated when Gly was given simultaneously. In rats fed casein diets, serum cholesterol tended to decrease when Gly, or Met and Gly were added. A simultaneous supplementation of Met and Gly to casein resulted in a reduction of hepatic cholesterol. Cystine added at the 0.6% level did not cause demonstrable changes in lipid concentrations except for a drop in serum triglyceride of the casein group. When 2.0% Gly was added to cholesterol-enriched diets containing 20% protein, serum cholesterol decreased significantly only when the protein source was casein and the level attained was comparable to that observed in rats fed soybean protein. Liver cholesterol was also markedly decreased by the addition of Gly to casein. The results suggest a possible role of Gly in the regulation of serum cholesterol levels by dietary protein.     

Consumption of olive oil has opposite effects on plasma total cholesterol and sphingomyelin concentrations in rats

The hypothesis that olive-oil consumption alters plasma sphingomyelin concentrations and hepatic sphingomyelin metabolism was tested. Rats were fed on purified, high-cholesterol diets with either coconut fat or olive-oil (180 g/kg). In accordance with previous work, olive-oil v. coconut-fat consumption significantly elevated hepatic and total plasma cholesterol concentrations. During the course of the experiment, the concentration of plasma sphingomyelin rose in the coconut-fat group and remained constant in the olive-oil group. When compared with the coconut-fat-fed group, the plasma sphingomyelin levels were significantly lower in the olive-oil-fed group after 14 and 21 d of treatment. Dietary olive oil raised the amounts of cholesterol and sphingomyelin in the VLDL density region, and this change was associated with a reduction in the cholesterol and sphingomyelin contents of the LDL and HDL density ranges. Olive-oil consumption reduced the activity of serine palmitoyltransferase, while the activities of phosphatidylcholine:ceramide cholinephosphotransferase and phosphatidylethanolamine:ceramide ethanolaminephosphotransferase were left unchanged. Dietary olive oil also enhanced the activity of acidic sphingomyelinase, but not that of neutral sphingomyelinase. The present data indicate that dietary olive oil v. coconut fat has opposite effects on total plasma cholesterol and sphingomyelin concentrations. The lower plasma sphingomyelin levels observed in olive-oil-fed, as compared with coconut-fat-fed rats, may be explained by a simultaneous elevation and reduction in sphingomyelin catabolism and synthesis respectively, as based on the measured enzyme activities.     

Secretion of alpha-tocopherol in VLDL is decreased by dietary protein insufficiency in young growing rats

The concentrations of alpha-tocopherol in plasma and most peripheral tissues were shown previously to be low in young growing rats fed a low protein diet. To examine the secretion rates of VLDL alpha-tocopherol and triglycerides, and lipoprotein lipase activity, weanling rats were fed a low protein (LP, 8 g/100 g lactalbumin) or a normal protein (NP, 20 g/100 g lactalbumin) diet for 6 wk. The absolute secretion rate of VLDL triglyceride (micromol/h) of the LP group was not significantly different from that of the NP group (P: > 0.05), but was significantly higher (P: < 0.05) when expressed relative to body weight [micromol/(h. kg)]. The secretion rates of VLDL alpha-tocopherol were significantly lower (P: < 0.05) in the LP group than in the NP group. The activities of hepatic lipase, lipoprotein lipase and total heparin-releasable lipase in plasma of the LP group were only 50-60% those of the NP group (P: < 0.05). The results demonstrated that the secretion rate of VLDL alpha-tocopherol and activities of lipases in postheparin plasma were significantly lower in rats fed a low protein diet. Thus, the redistribution of alpha-tocopherol from liver to peripheral tissues appears to have been impaired by dietary protein insufficiency.     

Effects of soya-bean protein and casein on serum cholesterol levels in rats

1. The effect of the soya-bean protein isolate and casein, both given 200 g/kg diet for 3-4 weeks, on serum cholesterol was compared in male rats. 2. Soya-bean protein exerted a hypocholesteramic effect only in a cholesterol-free low-fat (10 g maize oil/kg) diet, when the lowering action appeared independent of the strain of the rat or the feeding pattern. The results obtained with diets containing cholesterol or higher levels of fats or both showed no definite pattern of response. 3. Although the decrease in serum cholesterol appeared greater in alpha-lipoproteins than in beta-lipoproteins, the proportion of the former to total cholesterol remained almost unchanged. The concentration of serum apo A-I was significantly lower in rats given the vegetable protein. 4. Rats given soya-bean protein excreted significantly more neutral sterols. 5. The serum amino acid pattern did not reflect the difference in dietary protein. Addition of cholesterol to the diets modified the serum aminogarm, the decrease in threonine being most marked in both protein groups. 6. This study shows that the hypocholesteraemic action of soya-bean protein is easily modified by the type of diet.