Treatment of dietary casein with formaldehyde reduces its hypercholesterolemic effect in rabbits

Rabbits were fed cholesterol-free, semipurified diets containing 42% (wt/wt) casein or 21% casein plus one of the following nitrogen sources: soy isolate, amino acid mixture simulating casein, amino acid mixture simulating soy isolate, formaldehyde-treated casein or formaldehyde-treated soy isolate. Two additional groups of rabbits were fed the 42% casein diet and the diet containing casein plus soy isolate to which 0.4% (wt/wt) pure formaldehyde was added, this amount being identical to the amount of formaldehyde present in the diets with formaldehyde-treated proteins. Growth was somewhat reduced on the three diets containing 42% casein. The diet containing 42% casein to which no formaldehyde had been added induced severe hypercholesterolemia, the level of serum cholesterol after 8 weeks being about 10 mmol/L. The hypercholesterolemia was markedly reduced by the replacement of half of the casein by soy isolate, formaldehyde-treated soy isolate or formaldehyde-treated casein. No significant reduction of the concentration of serum cholesterol was seen when half of the 42% casein was replaced by an amino acid mixture imitating either casein or soy isolate. Formaldehyde per se did not significantly influence the level of serum cholesterol. We conclude that the differential tertiary structure of intact casein and soy isolate is an important factor in determining the cholesterolemic responses in rabbits to these proteins.     

Excessive cholesterolemic response in analbuminemic rats fed a cholesterol-rich diet containing casein.

Female Nagase analbuminemic rats and Sprague-Dawley rats were fed purified diets with or without 1% cholesterol and containing either soybean protein or casein. After consuming the cholesterol-free diets, the analbuminemic rats had significantly higher plasma cholesterol and triglyceride concentrations than the Sprague-Dawley rats. The higher plasma cholesterol levels were essentially in the low density and high density lipoproteins. Based on the fact that the analbuminemic rats excreted more bile acids in feces it is possible that the higher baseline plasma cholesterol concentrations in Nagase analbuminemic rats were partly caused by overproduction of cholesterol. The Nagase analbuminemic rats displayed a greater cholesterolemic response to cholesterol feeding than Sprague-Dawley rats, but only if casein was the protein source in the diet. Casein vs. soybean protein in either cholesterol-free or high cholesterol diets reduced bile acid excretion in Sprague-Dawley but not in Nagase analbuminemic rats. The increased sensitivity to casein plus cholesterol feeding in Nagase analbuminemic rats may be caused by a lack of inhibition of de novo cholesterol synthesis.     

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.     

The hypocholesterolemic effect of dietary soybean protein vs. casein in hamsters fed cholesterol-free or cholesterol-enriched semipurified diets

Golden Syrian hamsters fed a cholesterol-free diet containing 25% casein had higher plasma total triglyceride and cholesterol levels and VLDL + LDL cholesterol levels than animals fed a 25% soybean protein diet. Hamsters fed the cholesterol-free casein diet also had higher HDL cholesterol levels than animals fed the soybean protein diet, but these differences were not statistically significant. Addition of cholesterol to the diets caused even greater mean differences between the animals fed different types of protein, but the increased inter-animal variability of response to the added cholesterol resulted in less statistically significant differences. Although less responsive than the nonhybrid Golden Syrian hamsters, hybrid F1B hamsters showed similar effects of dietary casein vs. soybean protein on plasma lipids. These results indicate that the hamster may be a useful model to examine the effect of different types of protein and the interaction with dietary cholesterol on various plasma lipids and lipoproteins.     

Effects of dietary proteins and trypsin inhibitor on growth and lipid metabolism in hamsters

The purpose of this study was to determine trypsin inhibitor (TI) activity in sweetpotato and soy flour diets and their effects on the growth and lipid metabolism of hamsters. Male Golden Syrian hamsters were fed different types of dietary protein containing casein, soyprotein, transgenic sweetpotato plus soy flour (TSPF+SF), nontransgenic sweetpotato (NTSPF) plus soy flour (NTSPF+SF), transgenic sweetpotato (TSPF) and nontransgenic sweetpotato flour for 28 days. The TI activity was highest in TSPF+SF (19.30 TIU/mg) and NTSPF+SF (17.20 TIU/mg) diets that induced growth retardation in animals, lowest in TSPF (5.80 TIU/mg) and NTSPF (5.50 TIU/mg) diets, which did not affect the growth of the animals, and negligible in casein (<1.00 TIU/mg) and soyprotein (2.00 TIU/mg) diets. Plasma total cholesterol (TC), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C) and triglyceride (TG) concentrations were significantly higher in hamsters fed the casein diet compared to those fed the soy protein, TSPF+SF, NTSPF+SF, TSPF and NTSPF diets. A positive correlation was observed between plasma TC concentrations of hamsters and dietary methionine, lysine, leucine content and methionine/glycine ratios. Liver TC and TG concentrations of hamsters fed casein were significantly higher than those of all other diet groups. The supplementation of sweetpotato flour with soy flour increased both protein and TI activity in the diets and the lipid metabolism of hamsters were unaffected by TI activity.     

Use of arginine to reduce the severity of retinoid-induced hypertriglyceridemia

Previous research with an animal model of retinoid-induced hypertriglyceridemia, rats fed a 13-cis retinoic acid-containing diet in which casein was the dietary protein, has demonstrated that replacement of dietary casein with soy protein isolate can reduce the severity of this condition. A depressant action of soy protein vs. that of casein on serum triglyceride concentration has also been demonstrated in rats fed purified diets without supplemental retinoid. Because this action of soy protein appears to be due to its having a higher arginine-to-lysine ratio than casein, a study was done to determine how feeding a casein-containing diet supplemented with sufficient arginine, to give a dietary arginine-to-lysine ratio equivalent to that of soy protein, would affect the development of retinoid-induced hypertriglyceridemia. Groups of five-week-old male Fischer 344 rats (n = 7/group) were fed a control diet containing casein or one of three 13-cis retinoic acid-containing diets in which dietary nitrogen was provided as casein, casein + arginine, or soy protein. Incorporation of dietary 13-cis retinoic acid resulted in hypertriglyceridemia, with serum triglyceride concentrations of 2.00 and 7.23 mmol/l, or 177 and 640 mg/dl, for groups fed the control and casein + 13-cis retinoic acid diets, respectively. For animals fed the 13-cis retinoic acid-containing diets, serum triglyceride levels for the casein + arginine and soy protein diet groups (4.75 and 2.92 mmol/l, or 421 and 258 mg/dl, respectively) were significantly lower than for the casein group (p < 0.05); however, the value was significantly lower for the group fed the soy protein diet than for the group fed the casein + arginine diet. Serum and dietary arginine-to-lysine ratios were highly correlated (r = 0.93, p < 0.0001). Thus, supplementing dietary casein with arginine reduced the severity of retinoid-induced hypertriglyceridemia, but not as effectively as replacing casein with soy protein.     

Essential amino acids in relation to hypercholesterolemia induced in rabbits by dietary casein

Rabbits fed a cholesterol-free, semi-purified diet containing 25% casein amino acids (25% AA) for 2 wk had significantly higher serum total and low density lipoprotein (LDL) cholesterol levels than animals fed the same diet containing 11.2% casein amino acids (11.2% AA). These results were similar to those obtained by feeding diets containing 27% and 12% casein, respectively. When rabbits were fed the 11.2% AA diet supplemented with essential amino acids to the 25% level (11.2% AA + essential AA), their LDL cholesterol level was significantly higher than that in animals fed the 11.2% AA diet supplemented with nonessential amino acids to the 25% level (11.2% AA + nonessential AA). LDL protein and phospholipid levels were significantly higher in rabbits fed the 25% AA diet than in those fed 11.2% AA and tended to be elevated in animals fed 11.2% AA + essential AA compared to those fed 11.2% AA + nonessential AA. Fecal excretion of bile acids and cholesterol was similar with all dietary regimens, and the level of liver lipids showed no correlation with the degree of hypercholesterolemia produced by dietary amino acid mixtures.     

Dietary sucrose but not starch promotes protein-induced differences in rates of VLDL secretion and plasma lipid concentrations in rats.

Casein instead of soybean protein in a low fat, cholesterol-free diet containing sucrose increases both lipid secretion by rat liver and plasma cholesterol concentration. Male rats were studied to determine whether dietary sucrose is a prerequisite for these protein-induced differences of lipid metabolism in vivo. After 6 wk of consuming either casein plus sucrose, soybean protein isolate plus sucrose, casein plus starch or soybean protein isolate plus starch (carbohydrate, 69 g/100 g diet; protein, 20 g/100 g diet) plasma lipids were measured. VLDL degradation was blocked by Triton WR-1339 and rates of lipid secretion were calculated from the observed linear increase in plasma levels with time. Plasma cholesterol concentrations and rates of secretion of VLDL cholesterol and triglycerides were 27, 47 and 34% higher, respectively, in animals fed casein plus sucrose compared with soybean protein plus sucrose. These differences were clearly diminished or absent when starch was the dietary carbohydrate. The data substantiate the finding that dietary sucrose but not dietary starch promotes a casein-induced increase in hepatic cholesterol secretion and thus contributes to casein-induced hypercholesterolemia in rats. On the contrary, differences of triglyceride secretion were not reflected in different plasma lipid concentrations.     

Influence of background composition of the diet on the lipemic effect of fish oil vs. corn oil in rats

The lipemic effect of dietary fish oil vs. corn oil may be affected by the background composition of the diet. To investigate this theory, rats were fed diets containing either fish oil or corn oil and differing in the amount of fat and cholesterol and in the type of carbohydrate (sucrose vs. glucose) or protein (casein vs. soy protein). The hypotriglyceridemic effect of fish oil vs. corn oil was not influenced by the background composition of the diet. The hypocholesterolemic effect of fish oil was diminished by increasing the amount of coconut fat in the background diet at the expense of an isoenergetic amount of sucrose. With the use of high-cholesterol diets, no cholesterol-lowering effect of fish oil vs. corn oil was observed. This study suggests that the composition of the background diet should be considered when comparing the cholesterolemic effects of fish oil with other oils in various experiments.     

The effect of amino-acid-supplemented wheat gluten on cholesterol metabolism in the rat

The effect of lysine- and threonine-supplemented wheat gluten on cholesterol metabolism was studied using male weanling rats. Animals were fed on cholesterol-free diets containing 100 or 200 g gluten/kg with or without amino acid supplementation, and compared with animals given 50, 100 and 200 g casein/kg diets, for 3 weeks. A hypocholesterolaemic effect observed with the wheat gluten-fed rats, compared with the animals given 100 and 200 g casein/kg diets, was accompanied by increased turnover of cholesterol as expressed by enhanced cholesterol biosynthesis and increased faecal excretion of cholesterol and bile acids. This effect was not abolished by lysine and threonine supplementation. Low levels of blood cholesterol were also observed in the rats fed on the 50 g casein/kg diet. However, a different mechanism, related to impairment of cholesterol transport from the liver, was most likely responsible for the hypocholesterolaemia found in these protein-malnourished animals. The effect on cholesterol metabolism produced by dietary wheat gluten was independent of the low quality of the protein and of its specific deficiency in lysine and threonine.     

Growth and lipid metabolism responses in rats fed different dietary fat sources

An experiment with male albino rats was conducted to examine the effect of dietary fat sources on growth performance and apparent digestibility of total fat and fatty acids. The effect on plasma and organ cholesterol concentrations as well as lipoproteins cholesterol and fatty acid profile of hepatic phospholipids was also examined. Forty 3 week old rats were fed purified diets containing 0.041% cholesterol and 7% fat (wt/wt) from soy oil (SO), or cow ghee (GH), or 25% fish oil (FO) plus soy oil (SF) or 25% fish oil plus 75% ghee (GF), or 50% soy oil plus 50% ghee (SG) for a period of 35 days. Final body weight, weight gain, organ weights and feed efficiency ratio showed no significant differences (P < 0.05) among the dietary treatments. The apparent digestibility of total dietary fat and saturated fatty acids in the animals fed diet containing cow ghee only or in combination with fish oil or soy oil was significantly lower than the other dietary groups. Consumption of ghee with combination of fish oil or soy oil lead to significant improvement in the fat apparent digestibility of dietary fat. Total cholesterol and triglyceride concentrations of plasma and organs as well as lipoprotein cholesterol levels were higher in animals fed diets containing ghee. For rats fed diet containing cow ghee in combination with fish oil or soy oil, the cholesterol content of total plasma, lipoprotein fractions (VLDL, LDL and HDL) and organs was decreased significantly. Furthermore, dietary fats altered the fatty acids composition of hepatic phosphlipid fatty acids composition. Feeding dietary fish oil reduced arachidonic acid (20:4) and increased linoleic acid (18:2) and eicosapentaenoic acid (20:5) contents. The reduction in the arachidonic acid was being more pronounced in animals fed dietary ghee with combination of fish oil.     

Growth and lipid metabolism responses in rats fed different dietary fat sources

An experiment with male albino rats was conducted to examine the effect of dietary fat sources on growth performance and apparent digestibility of total fat and fatty acids. The effect on plasma and organ cholesterol concentrations as well as lipoproteins cholesterol and fatty acid profile of hepatic phospholipids was also examined. Forty 3 week old rats were fed purified diets containing 0.041% cholesterol and 7% fat (wt/wt) from soy oil (SO), or cow ghee (GH), or 25% fish oil (FO) plus soy oil (SF) or 25% fish oil plus 75% ghee (GF), or 50% soy oil plus 50% ghee (SG) for a period of 35 days. Final body weight, weight gain, organ weights and feed efficiency ratio showed no significant differences (P < 0.05) among the dietary treatments. The apparent digestibility of total dietary fat and saturated fatty acids in the animals fed diet containing cow ghee only or in combination with fish oil or soy oil was significantly lower than the other dietary groups. Consumption of ghee with combination of fish oil or soy oil lead to significant improvement in the fat apparent digestibility of dietary fat. Total cholesterol and triglyceride concentrations of plasma and organs as well as lipoprotein cholesterol levels were higher in animals fed diets containing ghee. For rats fed diet containing cow ghee in combination with fish oil or soy oil, the cholesterol content of total plasma, lipoprotein fractions (VLDL, LDL and HDL) and organs was decreased significantly. Furthermore, dietary fats altered the fatty acids composition of hepatic phospholipid fatty acids composition. Feeding dietary fish oil reduced arachidonic acid (20:4) and increased linoleic acid (18:2) and eicosapentaenoic acid (20:5) contents. The reduction in the arachidonic acid was being more pronounced in animals fed dietary ghee with combination of fish oil.     

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 importance of the non-protein components of the diet in the plasma cholesterol response of rabbits to casein. Zinc and copper

To characterize further the effect of protein-mineral interaction on plasma cholesterol, four groups of rabbits were placed on cholesterol-free, semi-purified diets for 12 weeks. The diets were similar in proximate composition with either casein or soya-bean protein isolate as the protein source (250 g/kg). The dietary zinc content of the casein diets was increased to 22 mg/kg to equal that of the soya-bean diets. The copper content was set at 2 and 6 mg/kg. There were no differences in weight gain; all animals grew at a similar rate. When the dietary Zn content was equalized, the plasma and liver cholesterol responses were similar regardless of the source of protein. Differences in liver Cu and Zn were observed. As dietary Cu increased, Cu decreased and Zn increased in the liver. Hepatic Zn was also higher in soya-bean-fed than in casein-fed animals. No effects on plasma Cu and Zn were seen. The addition of Zn to the casein diet resulted in a similar plasma cholesterol to that of animals fed on the soya-bean diet, suggesting that casein-induced hypercholesterolaemia is due to a marginal Zn intake.     

Soy protein enhances the cholesterol-lowering effect of plant sterol esters in cholesterol-fed hamsters

This study aimed to investigate whether the combination of plant sterol esters (PSE) with soy protein or soy isoflavones may have extra cholesterol-lowering effects. Male hamsters (n=20/group) were fed diets containing (g/100 g diet) (A) 20 casein (control), (B) 0.24 PSE, (C) 20 intact soy protein (replacing casein), (D) 0.02 soy isoflavones, (E) 0.24 PSE plus 20 soy protein (replacing casein), or (F) 0.24 PSE plus 0.02 soy isoflavones, for 5 wk. All diets contained 0.08 g cholesterol/100 g diet. Compared with the control diet, the PSE and soy protein diets significantly lowered the plasma total cholesterol concentration by 13% (P<0.05) and 9% (P<0.05), respectively, whereas the isoflavone diet (D) had no effect. The combination of PSE and soy protein (diet E) decreased plasma total cholesterol by 26% (P<0.05). The decrease in plasma cholesterol concentration was mainly in the non-HDL fraction. In addition, the combination of PSE and soy protein significantly decreased plasma triacylglycerol concentration (37%, P<0.05) and reduced cholesterol accumulation in the liver. The abundance of hepatic LDL-receptors was not influenced by any of the test diets. PSE selectively increased fecal excretion of neutral sterols by 190% (P<0.05), whereas soy protein increased fecal excretion of neutral sterols and bile acids by 66% (P<0.05) and 130% (P<0.05), respectively. The combination of PSE and soy protein increased the fecal excretion of neutral sterols and bile acids compared with PSE and soy protein alone. In conclusion, the combination of PSE and soy protein more dramatically lowers plasma lipids than the individual ingredients.     

Effect of soy protein and casein intake on intestinal absorption and lymphatic transport of cholesterol and oleic acid

Rats were fed for 4 wk on defined diets containing either casein or soy as the protein source, or diets in which the lysine/arginine ratios were modified by addition of arginine to casein, and lysine to the soy diet. During this period, weight gains and food intakes were comparable in the four dietary groups. Animals were subjected to cannulation of the left thoracic lymphatic duct, and after an overnight fast, were given a single intragastric dose of a lipid emulsion containing oleic acid and cholesterol. The overall 24-h recoveries of cholesterol and fatty acid in lymph were similar in the four groups, as were the distribution of lipids among the major lipid fractions and lipoprotein classes of thoracic duct lymph. However, analysis of timed lymph collections indicated that absorption of lipids was more rapid in casein-fed rats than in those fed soy protein. Furthermore, addition of arginine to the casein diet resulted in a slowed rate of lipid absorption, and addition of lysine to the soy diet markedly increased the rate of lipid absorption.     

Fat and protein from olive oil-fried sardines interact to normalize serum lipoproteins and reduce liver lipids in hypercholesterolemic rats

Fatty fish consumption has been recommended due to its high (n-3) PUFA content. However, an effect of its protein on serum lipids and lipoproteins has also been suggested. The present study was designed to determine the acceptability of diets containing sardines fried in olive oil or the fat extracted from those sardines and the normalization of serum lipids and lipoproteins, the hepatic lipid profile, and the fatty acid composition of the liver of growing Wistar rats fed these diets after dietary loading of cholesterol. Hypercholesterolemia was induced for 3 wk by feeding rats a casein/olive oil/cholesterol diet. Rats were then switched for 2 wk to cholesterol-free purified diets containing casein plus olive oil (CO), sardines fried in olive oil (S) and casein plus the fat extracted from sardines fried in olive oil (CSF). The S and CSF diets were well accepted by the rats. Withdrawal of dietary cholesterol markedly reduced (P < 0.05) serum cholesterol level in all of the groups, but the S group had the greatest decrease and the CO group the smallest decrease. The S group had a more normal lipoprotein profile, in which HDL was the major lipid carrier, whereas rats of the CO group still had beta-VLDL particles. CSF-fed rats had an intermediate profile. Liver fat and total, free and esterified cholesterol levels were lower (P < 0,05) in the S group than in the other two groups. The S and CSF diets increased (P < 0.05) the (n-3) PUFA content in hepatic triacylglyceride, cholesterol ester and phospholipid fractions. The liver fatty acid profile of the S group was more normalized than those of the other two groups. These findings suggest that the inclusion in the diet of whole sardines fried in olive oil normalizes cholesterol metabolism in hypercholesterolemic rats more quickly than consumption of their extracted fat.     

Cholesterol-lowering in hamsters fed rice bran at various levels, defatted rice bran and rice bran oil.

This study was conducted to determine the relative cholesterol-lowering effects of several levels of full-fat rice bran in hamsters. In addition, the separate effects of defatted rice bran and/or crude rice bran oil were investigated at levels equivalent to those present in 43.7% full-fat rice bran. Diets containing 10.9, 21.8, 32.8 or 43.7% full-fat rice bran, 35% defatted rice bran and/or 9% rice bran oil were fed to 4-wk-old male hamsters. All diets contained 10% total dietary fiber, 9% fat and 3.2% nitrogen; hypercholesterolemic diets contained 0.3% cholesterol; two diets were cholesterol-free, i.e., 10% cellulose and 43.7% full-fat rice bran. After 21 d, plasma and liver cholesterol, plasma triglycerides and liver weights were significantly greater in hamsters fed the 10% cellulose diet with 0.3% cholesterol compared with those fed the cholesterol-free cellulose diet. In animals fed cholesterol-free diets, plasma cholesterol values were significantly lower in those fed the 43.7% full-fat rice bran diet than in those fed the cellulose diet. In animals fed cholesterol-containing diets, plasma and liver cholesterol were significantly lower in animals fed the 43.7% full-fat rice bran diet than in those fed the cellulose diet. Plasma cholesterol reductions were significantly correlated to the level of rice bran in the diet. In cholesterol-fed hamsters, total liver cholesterol content was significantly lower in those fed the defatted rice bran diet with rice bran oil compared with those fed the cellulose diet. Full-fat rice bran was the only treatment that significantly lowered both plasma and liver cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of cholesterol supplementation on plasma and liver cholesteryl ester fatty acids in rats fed casein or soy protein

The effect of supplementation with cholesterol on plasma and liver cholesteryl ester fatty acids was examined in rats fed diets containing different protein sources. Weanling male rats were fed a semi-purified diet containing 20% casein or 20% soy protein for seven weeks and then 1% (by weight) of cholesterol was added for three more weeks. Rats fed soy protein grew consistently slower than those fed casein. The plasma and liver cholesterol contents were not significantly different between the two protein groups in the unsupplemented rats; however, they were significantly increased in cholesterol-fed animals, particularly in casein-fed rats. Analysis of the cholesteryl ester fatty acid composition in plasma and liver demonstrated that cholesterol-feeding significantly increased the levels of saturated fatty acids, monounsaturated fatty acids and linoleic acid in casein-fed rats as compared to those fed soy protein. The level of cholesteryl arachidonate in liver, which was low initially, increased slightly in the liver of casein-fed rats but did not significantly change in the plasma of either protein group.     

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.