Species-dependent responsiveness of serum cholesterol to dietary proteins

Soy proteins and cardiovascular disease

The soybean diet is the most potent dietary tool for hypercholesterolemia. The United States Food and Drug Administration recently approved the health claim for its role in reducing the risk of coronary disease. The hypocholesterolemic effect is directly correlated to the patient's cholesterolemia, with minimal or no reductions occurring at cholesterol of 6 mmol/L or less, and the most benefit occurring in patients with cholesterol of greater than 7 mmol/L. Hypotheses on the mechanism of action include soy fiber, isoflavones (phytoestrogens), and the protein itself. Although there is no evidence for the effect of fiber, studies with ethanol-extracted soy (devoid of isoflavones) indicated a loss of effect, but the extract itself (isoflavone rich) has no hypocholesterolemic activity. In humans, soy protein activates the low-density lipoprotein (LDL) receptor pathway. Recent data suggest that soy protein subunits, particularly 7S, directly activiate LDL receptors in the human liver, thus providing a novel mechanism of plasma cholesterol reduction different from currently available diets and hypolipidemic drugs.

Comparative effects of HMG-CoA reductase inhibitors on apo B production in the casein-fed rabbit: atorvastatin versus lovastatin

Rabbits fed a diet enriched in casein develop an endogenous hypercholesterolemia (EH) due both to an increased low density lipoprotein (LDL) synthetic rate and decreased LDL receptor activity. Pre-established EH in this model was used to assess the ability and mechanism by which atorvastatin lowers total plasma cholesterol (TPC) compared to the reference agent lovastatin. Rabbits were fed a casein diet for 6 weeks, obtaining average TPC levels above 200 mg/dl. To ensure equivalent mean cholesterol concentrations, animals were randomized into treatment groups based on the 6-week TPC levels, and fed the casein diet alone or in combination with either atorvastatin or lovastatin for an additional 6 weeks. Under these conditions, new steady-state cholesterol values were established. Lipoprotein concentrations and distributions were determined at this point. Compared to pretreatment values, TPC were similar in untreated animals. Atorvastatin, however, significantly reduced TPC by 38%, 45%, and 54% at the 1, 3, and 10 mg/kg doses, respectively. Statistically significant lowering of TPC (35%) by lovastatin was only achieved at the 10 mg/kg dose. To determine the mechanism by which atorvastatin lowered TPC in the EH rabbits, kinetic studies using human [125I]-LDL were performed in a subset of animals maintained on the casein diet alone (n = 5), or those treated with 3 mg/kg of atorvastatin (n = 5) or lovastatin (n = 7). In this set of studies, atorvastatin significantly lowered TPC compared to control and lovastatin-treated rabbits by 57% and 46%, respectively. Lovastatin treatment resulted in a 20% decrease in TPC as compared to untreated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Lowering effect of dietary milk-whey protein v. casein on plasma and liver cholesterol concentrations in rats

The effect of dietary whey protein v. casein on plasma and liver cholesterol concentrations was investigated in female, weanling rats. Balanced, purified diets containing either whey protein or casein, or the amino acid mixtures simulating these proteins, were used. The high-cholesterol diets (10 g cholesterol/kg feed) had either 150 or 300 g protein or amino acids/kg feed. The diets were fed for 3 weeks. At the low dietary protein level, whey protein v. casein did not affect plasma total cholesterol, but lowered the concentration of liver cholesterol. At the high dietary protein level, whey protein significantly lowered plasma and liver cholesterol and also plasma triacylglycerols. The hypocholesterolaemic effect of whey protein was associated with a decrease in very-low-density-lipoprotein cholesterol. At the high dietary protein concentration, whey protein reduced the faecal excretion of bile acids when compared with casein. The effects of intact whey protein v. casein were not reproduced by the amino acid mixtures simulating these proteins. It is suggested tentatively that the cholesterol-lowering effect of whey protein in rats is caused by inhibition of hepatic cholesterol synthesis.

Influence of dietary fish proteins on plasma and liver cholesterol concentrations in rats

The effects of amount and type of dietary fish proteins on plasma and liver cholesterol concentrations were evaluated in female rats. The isonitrogenous diets used contained 10 g cholesterol/kg and were carefully balanced for residual fat, cholesterol, Ca, Mg and P in the protein preparations. Cod meal, soya-bean protein or casein was incorporated into the diets as the only source of dietary protein at three levels: either 24, 48 or 72 g N/kg diet. Extra protein was added to the diet at the expense of the glucose component. In a second experiment soya-bean protein, casein, cod meal, whiting meal or plaice meal was added to the diet at a level of 24 g N/kg. When compared with casein, cod meal and soya-bean protein decreased plasma and liver cholesterol concentrations. A further cholesterol-lowering effect was achieved by increasing the proportion of either soya-bean protein or cod meal in the diet. Substitution of casein for glucose did not influence plasma and liver cholesterol concentrations. Plaice meal in the diet produced lower group mean plasma cholesterol concentrations than did whiting meal. In rats fed on the diet containing plaice meal, liver cholesterol concentrations were significantly lower than those in their counterparts fed on either cod meal or whiting meal. The present study demonstrates that different fish proteins in the diet have different effects on cholesterol metabolism and that the cholesterol-influencing properties of cod meal can be enhanced by the incorporation of higher proportions of this protein in the diet.

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.

Plasma very low density lipoproteins from male rats fed casein or soybean protein diets: a comparison of fatty acid composition and influence on prostanoid production

In studies with male rats fed for 4 wk semipurified diets containing olive oil and casein or soybean protein, protein-dependent effects were observed in the fatty acid composition of the VLDL lipids, especially with regard to the pattern of polyunsaturated fatty acids. Compared with VLDL from rats fed soybean protein diet (S-VLDL), VLDL from casein-fed rats (C-VLDL) contained a greater level of oleic acid, and a reduced level of linoleic acid and arachidonic acid, in the phosphatidylcholine fraction. The proportion of 5,8,11-eicosatrienoic acid, 20:3 (n-9), varied among the different lipid classes. The highest concentration of this fatty acid (13% by weight of total fatty acids) was observed in the phosphatidylinositol fraction of C-VLDL. The level of linoleic acid was approximately halved in the triacylglycerol and cholesteryl ester fractions of C-VLDL compared with S-VLDL. When mouse peritoneal macrophages were incubated with different concentrations of S-VLDL, a saturable accumulation of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TXB2) was observed in the cell medium. In contrast, very low levels of 6-keto-PGF1 alpha and TXB2 were observed in the cell medium of macrophages incubated with C-VLDL at different lipoprotein concentrations, suggesting that the composition of VLDL may play an important role in relation to cellular prostanoid metabolism.

Cholesterol-lowering activity of various undigested fractions of soybean protein in rats

The undigested high-molecular-weight fraction (HMF) of soybean protein prepared after exhaustive digestion by microbial proteases significantly decreased serum cholesterol levels to approximately 45% (p less than 0.05) of those observed with the parent protein in rats fed diets containing cholesterol (0.5%) and sodium cholate (0.125%). HMF bound conjugated bile salts in vitro and significantly increased fecal excretion of both neutral and acidic steroids by 65-95% and 80-170% more, respectively (p less than 0.05), than did the intact protein. Extraction of HMF with methanol slightly decreased the activity, but the methanol-soluble fraction was not regarded as a principal determinant. Soybean saponin at the dietary level equivalent to that contained in HMF did not effectively lower serum cholesterol. The activity was not necessarily duplicated even when methanol-treated fractions were recombined. Further degradation of the methanol-extracted HMF by various proteases resulted in loss of activity. HMF obtained after pepsin digestion exerted a potential similar to that of HMF prepared after digestion by microbial proteases.

Lack of effect of dietary minerals on liver cholesterol concentrations in rats

Female rats were fed cholesterol-free, purified diets with different concentrations of calcium (0.13-0.75%, w/w), magnesium (0.02 or 0.04%) or phosphorus (0.2-0.8%) as the only dietary variable. After 28 days, no effects of the minerals were found on liver cholesterol concentrations and rates of fecal excretion of bile acids.

Effects of cholesterol-free, semipurified diets containing different levels of casein or soy protein on distribution of cholesterol and protein among serum lipoproteins of rabbits

Rabbits fed cholesterol-free, low-fat, semipurified diets have more cholesterol and protein in serum low density lipoprotein (LDL) relative to high density lipoprotein (HDL) than rabbits fed Chow diet. This difference was accentuated by a casein semipurified diet but was also observed with a soy protein diet even though the latter did not produce an elevation of serum cholesterol. To investigate the reason for these differences, the formulation of the semipurified diets was altered by reducing the level of protein from 27 to 16%, increasing the fat from 1 to 4% and the fiber from 5 to 13%, to correspond more closely to the proportions in Chow. With this formulation, the soy protein diet gave a lipoprotein pattern similar to that of Chow, whereas the casein diet produced a moderately elevated serum cholesterol level with more cholesterol in LDL than in HDL. When the protein in the newly formulated diets was increased back to 27%, the lipoprotein patterns reverted to those obtained with the original formula. In this case, soy protein-fed rabbits had moderately elevated serum cholesterol whereas casein-fed animals showed hypercholesterolemia. These results indicate that the altered lipoprotein pattern observed previously in rabbits fed semipurified diets is related to the high level of protein in those diets.

The hypocholesterolemic action of the undigested fraction of soybean protein in rats

Soybean protein was exhaustively digested with endo- and exo-type microbial proteases and the effect of the digestible low molecular fraction (LMF) and the undigested high molecular fraction (HMF) on the serum cholesterol level was compared to that of the intact protein in rats given a cholesterol-enriched diet. The HMF, peptides relatively abundant in hydrophobic amino acids, was found to be substantially hypocholesterolemic when fed at the nitrogen level equivalent to that of the 20% soybean protein diet, and not only serum but also liver cholesterol levels were similar to those usually encountered in rats given diets free of cholesterol. There was a dose-dependent reduction of serum and liver cholesterol when casein was replaced stepwise with HMF. The cholesterol-lowering action could be attributable to an increased fecal steroid excretion.