Dietary proteins modulate the effects of fish oil on triglyceridemia in the rat

Sprague-Dawley rats were fed purified diets varying in both protein (20%) and lipid (11%) content for 28 d to verify the independent and interactive effects of dietary proteins and lipids on serum and hepatic lipids, and on tissue lipoprotein lipase (LPL) activity in both fasted and postprandial states. These diets consisted of either casein-menhaden oil, casein-coconut oil, soy protein-menhaden oil (SPMO), soy protein-coconut oil, cod protein-menhaden oil, or cod protein-coconut oil. A randomized 3×2 factorial design was used. A significant protein-lipid interaction was seen on serum triglyceride levels: menhaden oil, compared with coconut oil, induced a decrease in serum triglyceride levels when combined with soy protein but not when combined with cod protein and casein. The lower serum triglyceride concentrations observed in the SPMO-fed rats could be the result of decreased hepatic triglycerides when soy protein was compared with casein and when menhaden oil was compared with coconut oil. Total LPL activity in the heart was higher in menhaden oil-fed rats than in coconut oil-fed rats in the postprandial state. The higher LPL activity in the heart could, however, explain only 10% of the reduction of serum triglycerides, contributing slightly to the lowering effects of SPMO diet on serum triglycerides. Therefore, the present results indicate that dietary proteins can modulate the effects of fish oil on triglyceridemia in the rat, and that could be mainly related to specific alterations in hepatic lipid concentrations.     

Dietary α-linolenic acid lowers postprandial lipid levels with increase of eicosapentaenoic and docosahexaenoic acid contents in rat hepatic membrane

This study was designed to examine the effects of dietary n−3 and n−6 polyunsaturated fatty acids (PUFA) on postprandial lipid levels and fatty acid composition of hepatic membranes. Male Sprague-Dawley rats were trained for a 3−h feeding protocol and fed one of five semipurified diets: one fat-free diet or one of four diets supplemented with 10% (by weight) each of corn oil, beef tallow, perilla oil, and fish oil. Two separate experiments were performed, 4-wk long-term and 4-d short-term feeding models, to compare the effects of feeding periods. Postprandial plasma lipid was affected by dietary fats. Triacylglycerol (TG) and total cholesterol levels were decreased in rats fed perilla oil and fish oil diets compared with corn oil and beef tallow diets. Hepatic TG and total cholesterol levels were also reduced by fish oil and perilla oil diets. Fatty acid composition of hepatic microsomal fraction reflected dietary fatty acids and their metabolic conversion. The major fatty acids of rats fed the beef tallow diet were palmitic, stearic, and oleic. Similarly, linoleic acid (LA) and arachidonic acid in the corn oil group, α-linolenic acid (ALA) and eicosapentaenoic acid (EPA) in the perilla oil group, and palmitic acid and docosahexaenoic acid (DHA) in the fish oil group were detected in high proportions. Both long- and short-term feeding experiments showed similar results. In addition, microsomal DHA content was negatively correlated with plasma lipid levels. Hepatic lipid levels were also negatively correlated with EPA and DHA contents. These results suggest that n−3 ALA has more of a hypolipidemic effect than n−6 LA and that the hypolipidemic effect of n−3 PUFA may be partly related to the increase of EPA and DHA in hepatic membrane.     

Interaction between dietary proteins and lipids in the regulation of serum and liver lipids in the rabbit. Effect of fish protein

Purified diets varying in dietary protein, namely casein (CA), soy protein (SP), fish protein (FP), and lipid origin (corn oil (CN), coconut oil (CO)) were fed to rabbits to evaluate the effects of protein and fat source, as well as protein-lipid interactions, on serum total, lipoprotein and hepatic lipid levels. Dietary proteins and lipids exerted a separate effect on serum total cholesterol (C), very low-density lipoprotein cholesterol (VLDL-C), and low-density lipoprotein cholesterol to high density lipoprotein cholesterol (LDL-C/HDL-C) ratio. Hence, CA increased serum cholesterol compared to SP, while coconut oil enhanced serum and VLDL-C, and decreased LDL-C/HDL-C compared to corn oil. Dietary proteins interacted with dietary lipids to modulate HDL-C levels. Thus, FP maintained a high level of HDL-C regardless of lipid origin, compared to CA and SP whose HDL-C levels were decreased by corn oil, compared to coconut oil. A dietary protein-lipid interaction was also observed in the regulation of liver cholesterol levels. Coconut oil, compared to corn oil, decreased liver cholesterol in rabbits fed FP, whereas hepatic cholesterol concentration was unaltered by dietary lipid source in CA- and SP-fed rabbits. These results demonstrate that dietary proteins act synergistically with dietary lipids to regulate cholesterol metabolism in the rabbit. This work was presented in part at the 74th Annual FASEB meeting held in Washington, D.C., April 1–5, 1990.     

Interaction Between Marginal Zinc and High Fat Supply on Lipid Metabolism and Growth of Weanling Rats

The impact of a moderate Zn deficiency on growth and plasma and liver lipids was investigated in two 4-week experiments with male weanling rats fed fat-enriched diets. Semisynthetic, approximately isocaloric diets containing 3% soybean oil were supplemented with either 7 or 100 mg Zn/kg diet and with 22% beef tallow (BT) or sunflower oil (SF). In Experiment 1, which compared the dietary fat level and the fat source in a factorial design of treatments, all diets were fed ad libitum to 6 × 8 animals, whereas intake of the high-Zn BT and SF diets was restricted in Experiment 2 (5 × 6 rats) to the level of intake of the respective low-Zn diets. The low-Zn SF diet consistently depressed food intake and final live weights of the animals to a greater extent than the other low-Zn diets, while intake and growth were comparable among the animals fed the high-Zn diets. The marginal Zn deficit per se did not alter plasma triglyceride and cholesterol concentrations nor hepatic concentrations of triglyceride, cholesterol and phospholipids. The fatty acid pattern of liver phospholipids did not indicate that chain elongation and desaturation of fatty acids was impaired by a lack of zinc. It was concluded that dietary energy and fat intake, and fat source have a greater effect on plasma and liver lipids than a moderate Zn deficiency. Marginally Zn-deficient diets enriched with sunflower oil as a major energy source cause a greater growth retardation than diets rich in carbohydrates or beef tallow.     

Cholesterol transport and uptake in miniature swine fed vegetable and animal fats and proteis. 1. Plasma lipoproteins and LDL clearance

In a 2×2 factorial arrangement, miniature pigs were fed four diets containing vegetable protein/fat (soybean) and animal protein (egg white)/fat (beef tallow) to demonstrate the effects of protein and fat source on total plasma cholesterol, lipoprotein distribution, low density lipoprotein (LDL) composition, and plasma clearance of LDL-cholesterol and protein. Beef tallow consumption resulted in greater plasma cholesterol concentration, decreased LDL-cholesterol concentration, and a lower LDL-cholesterol to LDL-protein ratio than did consumption of soybean oil. High density lipoprotein (HDL)-cholesterol concentration was increased by beef tallow consumption. Cholesterol percentage by weight in LDL was significantly greater in pigs consuming soybean oil than those consuming beef tallow. Percentages by weight of protein, triglyceride and phospholipid in LDL were not significantly different in any group. Dietary protein source had no significant effect on total plasma cholesterol concentration, lipoprotein concentration or LDL composition. Egg white consumption decreased fractional catabolic rate and irreversible loss of LDL-cholesterol and LDL-protein when compared with consumption of soy protein. Dietary fat source had no consistent effect on LDL clearance from plasma. Dietary fat and protein seemed to influence lipoprotein metabolism by different mechanisms. Fat source altered lipoprotein concentration and LDL composition, whereas protein source affected the removal rate of LDL from plasma. Data taken from a dissertation submitted to Iowa State University by L. S. Walsh Hentges as partial fulfillment of the requirements for the Ph.D. degree. A preliminary paper, was presented at the meeting of the American Oil Chemists' Society in Dallas, Texas, May, 1984.     

In vivo determination of triglyceride secretion using radioactive glycerol in rats fed different dietary saturated fats

Our objective was to determine the relative rates ofin vivo triglyceride (TG) secretion and the composition of very low density lipoproteins (VLDL) in rats fed different dietary saturated fats. Male Sprague-Dawley rats (150–200 g) were fed diets containing 16% corn oil, or 14% butterfat, 14% beef tallow, 14% olive oil, or 14% coconut oil plus 2% corn oil for 5 wk. Changes in plasma TG specific radioactivity were determined in individual, unanesthetized fasted rats after injection of 100 μCi [2-³H]glycerol. Nonlinear regression analysis using a 2-compartment model was used to determine the fractional rate constant for TG turnover in plasma. The plasma TG pool was 33–40% larger with beef tallow than with corn, olive or coconut oil feeding (p<0.05), and 20% larger with beef tallow than with butterfat feeding. The rate of TG secretion into plasma (mg/min/100 g body weight) was 60% higher in animals fed beef tallow than corn or coconut oil (p<0.05) and 26–33% higher in animals fed beef tallow than olive oil or butterfat. Differences in VLDL composition (% wt) were also noted. Our data suggest that greater TG secretion is the primary factor contributing to the larger TG pool with ingestion of beef tallow relative to butterfat, corn or coconut oil. These results suggest that different dietary saturated fats have unique effects on TG metabolism in rats. Presented in part at the 1990 meeting of the Federation of American Societies for Experimental Biology in Washington, D.C. (see ref. 1).     

Fish oil prevents change in arachidonic acid and cholesterol content in rat caused by dietary cholesterol

Rats were fed diets high in either saturated fat (beef tallow) or α-linolenic acid (linseed oil) or eicosapentaenoic and docosahexaenoic acids (fish oil) with or without 2% cholesterol supplementation. Consumption of linseed oil and fish oil diets for 28 days lowered arachidonic acid content of plasma, liver and heart phospholipids. Addition of 2% cholesterol to diets containing beef tallow or linseed oil lowered 20∶4ω6 levels but failed to reduce 20∶4ω6 levels when fed in combination with fish oil. Feeding ω3 fatty acids lowered plasma cholesterol levels. Addition of 2% cholesterol to the beef tallow or linseed oil diet increased plasma cholesterol concentrations but not when fish oil was fed. Feeding the fish oil diet reduced the cholesterol content of liver, whereas feeding the linseed oil diet did not. Dietary cholesterol supplementation elevated the cholesterol concentration in liver in the order: linseed oil > beef tallow > fish oil (8.6-, 5.5-, 2.6-fold, respectively). Feeding fish oil and cholesterol apparently reduced 20∶4ω6 levels in plasma and tissue lipids. Fish oil accentuates the 20∶4ω6 lowering effect of dietary cholesterol and appears to prevent accumulation of cholesterol in plasma and tissue lipids under a high dietary load of cholesterol.     

Dietary saturated fat level alters the competition between α-linolenic and linoleic acid

Male weanling rats were fed semi-synthetic diets high in saturated fat (beef tallow) vs high in linoleic acid (safflower oil) with or without high levels of α-linolenic acid (linseed oil) for a period of 28 days. The effect of feeding these diets on cholesterol content and fatty acid composition of serum and liver lipids was examined. Feeding linseed oil with beef tallow or safflower oil had no significant effect on serum levels of cholesterol. Serum cholesterol concentration was higher in animals fed the safflower oil diet than in animals fed the beef tallow diet without linseed oil. Feeding linseed oil lowered the cholesterol content in liver tissue for all dietary treatments tested. Consumption of linseed oil reduced the arachidonic acid content with concomitant increase in linoleic acid in serum and liver lipid fractions only when fed in combination with beef tallow, but not when fed with safflower oil. Similarly, ω3 fatty acids (18∶3ω3, 20∶5ω3, 22∶5ω3, 22∶6ω3) replaced ω6 fatty acids (20∶4ω6, 22∶4ω6) in serum and liver lipid fractions to a greater extent when linseed oil was fed with beef tallow than with safflower oil. The results suggest that the dietary ratio of linoleic acid to saturated fatty acids or of 18∶3ω3 to 18∶2ω6 may be important to determine the cholesterol and arachidonic acid lowering effect of dietary α-linolenic acid.     

Separate effects of dietary protein and fat on serum cholesterol levels: another view of amino acid content of proteins

Casein or soy protein with vegetable or animal fat were used to determine the dietary protein or fat effects and their possible interaction on serum cholesterol levels. Young, male New Zealand white rabbits with a mean weight of 2.1 kg were divided into groups of six and fed one of four different diets containing 20% of the calories as protein, 30% as fat (according to dietary guidelines for the United States) and 50% as carbohydrate. The diets contained casein or soy (lysine/arginine ratio = 2.2 or 0.9, respectively) as the protein sources with fat from either almond oil or butter. There was no significant difference in weight gain among the diet groups. Total serum cholesterol level was highest among animals fed the diet containing butter with casein (177 +/- 25 mg/dl) or soy protein (189 +/- 50 mg/dl), it was intermediate in animals fed the vegetable oil with casein (121 +/- 14 mg/dl), and lowest in the soy protein with vegetable oil group (58 +/- 12 mg/dl). There was a significant difference in serum cholesterol levels due to the protein effect when vegetable oil was used (p less than 0.05) but not with butter. There was also a significant fat effect on serum cholesterol when the diet contained soy protein (p less than 0.005) but not when the protein was casein. No significant interaction was observed between the dietary fat and protein sources on serum cholesterol levels, which suggests that dietary protein and fat independently affect the levels of serum cholesterol. Thus, dietary protein has a significant effect on serum cholesterol levels and may be a factor in the low levels of serum cholesterol observed among vegetarians and in humans of Third World countries where the diets is primarily of vegetable origin.     

Gender and dietary fat affect α-tocopherol status in F344/N rats

For four weeks, groups of eight male and eight female F344/N rats were fed diets containing 15.5, 20, 30 or 40% of energy (en%) as fat. The fat was composed of corn oil and beef tallow with 9 en% from linoleate in all diets. Females had greater mean hepatic α-tocopherol levels, whereas males had greater plasma α-tocopherol and cholesterol concentrations. In males, the plasma ratio of α-tocopherol/cholesterol was significantly greater than in females (P<0.05). Plasma α-tocopherol increased with increasing en% fat (r=0.51,P<0.001) in both sexes, but dietary fat did not alter hepatic α-tocopherol levels. These results suggest that plasma α-tocopherol may serve as a biomarker of total dietary fat intake and that in F344/N rats gender differences affect α-tocopherol and cholesterol status.     

Response of free and esterified plasma cholesterol levels in the mongolian gerbil to the fatty acid composition of dietary lipid

The present study was undertaken to investigate the potential suitability of the Mongolian gerbil as a useful animal model to study the effects of dietary fats on plasma cholesterol levels. Semipurified diets containing either 20% lard, 20% safflower oil, or 19.5% beef tallow +0.5% safflower oil were equalized to contain 0.01% cholesterol and 0.05% plant sterol and were fed for a four week experimental period. The proportions of total calories contributed by fat, protein and carbohydrate (starch/sucrose ratio of 2∶1) were 40, 14 and 46%, respectively, so as to approach the distribution of calories within the average North American diet. Free, esterified, and total plasma cholesterol levels of male gerbils were determined weekly by gas liquid chromatography after drawing blood via a serial sampling technique. After 1, 2, 3, and 4 weeks of feeding the experimental diets, total cholesterol levels were lowest in the safflower oil fed animals; the corresponding values were 19–64% greater in gerbils fed lard and 68–91% greater in those consuming the beef tallow diet. Cholesterol in the free form generally responded more dramatically to the type of dietary lipid than did cholesterol in the ester form. Irrespective of the type of dietary lipid or the length of the feeding trial, 18–23% of the total plasma cholesterol was in the free form and 77–82% was present as the ester. In view of the similarity to the human of the relative proportions of free versus esterified cholesterol, the type of cholesteryl esters, and their response to dietary manipulation, the gerbil appears to be a useful animal model for studying the regulatory effect of dietary lipid on plasma cholesterol levels. Presented in part at the A.O.C.S. Annual Meeting, San Francisco, CA, May 1979.     

Serum lipoproteins of rabbits fed semi-purified diets varying in protein and carbohydrate source

The effect of dietary protein and carbohydrate type on the composition and properties of serum lipoproteins in rabbits was studied. The animals were pair-fed either casein-sucrose and soy protein-sucrose or casein-dextrose and soy protein-dextrose-containing low fat, low cholesterol, semi-purified diets for 84 days. Main effects due to protein and carbohydrate type were observed to be altered lipoprotein class compositions. These effects were due primarily to the presence of casein which resulted in cholesterol-rich, triglyceride-poor lipoproteins among all isolated classes. Increases in serum low and high density lipoprotein cholesterol and protein concentrations, as well as lipoprotein total mass, were also observed with casein feeding. Carbohydrate effects due to dietary sucrose were elevated high density lipoprotein phospholipid and protein concentrations, independent of protein source. When casein was included, sucrose-containing diets resulted in increased intermediate density lipoprotein cholesterol; when soy protein was used, low density lipoprotein cholesterol elevations were observed. Dietary interactions were also found especially between casein and sucrose, resulting in increased intermediate lipoprotein mass and total lipoprotein concentrations. Comparisons of lipoprotein class particle size among rabbits indicated similar particle diameters among animals in all dietary groups. Evidence that casein feeding caused increased lipoprotein particle number in addition to compositional differences was obtained. It was concluded that lipoprotein structure within a given class is maintained in spite of compositional variations due to dietary protein or carbohydrate source.     

Dietary protein modulates the effect of CLA on lipid metabolism in rats

The effect of the interaction of CLA and type of dietary protein on lipid metabolism was studied in male rats by feeding diets containing casein (CAS) or soy protein (SOY) as dietary protein and either linoleic acid (LA, a control FA) or graded levels of CLA at 0,0.1, 0.5, and 1.0% for 28 d. CLA reduced the weight of perirenal adipose tissue in a dose-dependent manner, but the magnitude of the reduction was greater when rats were fed SOY. Feeding SOY resulted in a significant reduction of the concentrations of serum total and HDL cholesterol, TG, glucose, and insulin irrespective of dietary CLA. The concentration of serum leptin tended to be lower on the SOY diet free of CLA than in the corresponding CAS diet, but it fell with an increasing dietary level of CLA in the CAS groups. In contrast, serum leptin tended to increase when CLA was added to SOY diets. The concentration of serum adiponectin was higher in the CAS than in the SOY groups, and it tended to increase in response to dietary CLA levels in the CAS-fed rats, whereas CLA showed no effect in SOY-fed rats. The activity of liver mitochondrial carnitine palmitoyltransferase was higher in the SOY than in the CAS groups, but it tended to increase with an increasing dietary level of CLA in both protein groups. Although the body fat-reducing activity of CLA was more effective when the protein source was SOY, rats fed CAS appeared to be more susceptible to CLA than in those fed SOY with respect to cytokines examined. These results suggest that the type of dietary protein may modify the antiobesity activity of CLA.     

Differential effects of dietary linoleic and α-linolenic acid on lipid metabolism in rat tissues

Comparative effects of feeding dietary linoleic (safflower oil) and α-linolenic (linseed oil) acids on the cholesterol content and fatty acid composition of plasma, liver, heart and epididymal fat pads of rats were examined. Animals fed hydrogenated beef tallow were used as isocaloric controls. Plasma cholesterol concentration was lower and the cholesterol level in liver increased in animals fed the safflower oil diet. Feeding the linseed oil diet was more effective in lowering plasma cholesterol content and did not result in cholesterol accumulation in the liver. The cholesterol concentration in heart and the epididymal fat pad was not affected by the type of dietary fatty acid fed. Arachidonic acid content of plasma lipids was significantly elevated in animals fed the safflower oil diet and remained unchanged by feeding the linseed oil diet, when compared with the isocaloric control animals fed hydrogenated beef tallow. Arachidonic acid content of liver and heart lipids was lower in animals fed diets containing safflower oil or linseed oil. Replacement of 50% of the safflower oil in the diet with linseed oil increased α-linolenic, docosapentaenoic and docosahexaenoic acids in plasma, liver, heart and epididymal fat pad lipids. These results suggest that dietary 18∶2ω6 shifts cholesterol from plasma to liver pools followed by redistribution of 20∶4ω6 from tissue to plasma pools. This redistribution pattern was not apparent when 18∶3ω3 was included in the diet.     

The combined effects of dietary proteins and fish oil on cholesterol metabolism in rats of different ages

Male Sprague-Dawley rats at the ages of four weeks and nine months were fed purified diets containing 20% proteins either as casein (CAS), milk whey protein (WHY), or soybean protein (SOY) with 5% sardine oil for four weeks. The hypocholesterolemic effect of SOY was not statistically evident as compared to milk proteins at both ages, although serum cholesterol tended to be low in the SOY groups. A significant agedependent increase in serum cholesterol was observed in all dietary groups. Liver cholesterol concentrations were comparable in young rats, whereas in adults they were significantly lower in the SOY than in the CAS or WHY groups. At both ages, the activity of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase tended to be higher in the SOY than in the other groups. Fecal steroid excretion was significantly higher in rats fed SOY than those fed either CAS or WHY, especially in adult rats. Significant age- and dietary protein-effects were observed in fatty acid profiles of liver microsomal phospholipids. Thus, the effects of dietary proteins on various lipid parameters were essentially maintained even when fish oil served as the source of dietary fat.     

Effects of dietary casein and soy protein on metabolism of radiolabelled low density apolipoprotein B in rabbits

Rabbits fed semipurified diets containing casein have elevated plasma cholesterol levels compared to those fed soy protein. As part of continuing studies on the mechanism of casein-induced hypercholesterolemia, two groups of six rabbits were fed these diets for 14 to 16 weeks. Animals fed the casein diet were found to have significantly higher plasma concentrations of protein, cholesterol, triacylglycerol, phospholipid and apolipoprotein B (apo B) associated with low density lipoprotein (LDL) than those fed the soy protein diet. Kinetic studies showed that the fractional catabolic rate of LDL-apo B was significantly lower in animals fed casein than in those fed soy protein regardless of whether the tracer LDL was obtained from donors fed casein or soy protein. The production rate of LDL-apo B was higher in casein-fed animals but this was not statistically significant. These results show that the efficiency of removal of LDL is significantly reduced in animals fed casein compared to those fed soy protein, and that the source of LDL did not affect the efficiency of its subsequent removal. The acumulation of LDL in casein-fed animals is consistent with down-regulation of the LDL receptor.     

Fatty acid oxidation in relation to cholesterol biosynthesis in rats

Groups of male and female rats were fed diets containing (calorie basis) 2% corn oil (low-fat, LF), 42% corn oil (CO) or 2% corn oil plus 40% beef tallow (BT) for 2 weeks. Then rats of each sex and diet group were given an intraperitoneal injection of¹⁴C-acetate,- stearate- oleate or linoleate. Acetate incorporation into cholesterol and rate of oxidation of each fatty acid were determined. Specific activity of cholesterol was higher in females than males, higher with 40% lipid in the diet than with 2% corn oil and higher for CO than BT. Linoleate was oxidized more rapidly than oleate which exceeded stearate. An index of dietary lipid oxidation was computed based on fatty acid oxidation rate, per cent of each fatty acid in the diet and per cent of lipid calories in the diet. Serum cholesterol-¹⁴C was found to be proportional to dietary lipid oxidation index.     

The influence of protein and carbohydrate type on serum and liver lipids and lipoprotein cholesterol in rabbits

The non-lipid portions of semi-synthetic diets appear to be important determinants of hypercholesterolemia and atherosclerosis in the rabbit. Serum and liver lipid concentrations were determined in rabbits which had been pair-fed various protein (casein or soy protein isolate) and carbohydrate (sucrose or dextrose) sources as part of low fat, low cholesterol, semi-synthetic diets. It was verified that caseincontaining diets render rabbits hypercholesterolemic, while soy protein caused a degree of hypocholesterolemia. Additionally, sucrose, when fed in conjunction with casein, appears to augment this hypercholesterolemic effect. The distribution of total cholesterol among lipoprotein subclasses was increased in both the intermediate density lipoprotein (IDL) (1.006–1.019 g/ml) and low density lipoprotein (LDL) (1.019–1.063 g/ml) fractions and decreased in the high density lipoprotein (HDL) (1.063–1.21 g/ml) fraction when casein is fed. Soy protein feeding caused relatively more cholesterol to appear only in the IDL fraction when compared with commercial chow fed rabbits. Reasons for these differences may involve the saturation or suppression of endogenous lipoprotein hepatic receptors.     

Serum lipids in spontaneously hypertensive rats and sprague-dawley rats fed menhaden oil

Dietary n-3 fatty acids, abundant in fish oil, exert a variety of effects that attenuate cardiovascular disease. In this study, we assessed the effect of fish oil (menhaden oil) on the serum lipid profile in hypertensive and normotensive rats. Spontaneously hypertensive rats (SHR) or Sprague-Dawley rats (SD) were fed either standard powdered diet (L-485), or L-485+5% menhaden oil (MO) or L-485+5% corn oil (CO) from weaning through eight months of age. Systolic blood pressure (BP) was periodically determined on SHR. Serum lipid profiles were performed at eight months on sample taken from the exposed hearts of anesthetized, fasted rats. SHR, compared with SD (diets combined) had significantly lower triaclyglycerols (TG), higher cholesterol (CHOL), higher high density lipoprotein cholesterol (HDL CHOL), higher low density lipoprotein cholesterol (LDL CHOL), and a higher LDL:HDL ratio. Comparison among diets (strains combined) revealed that rats fed MO had the lowest values for TG, CHOL, LDL and LDL:HDL; HDL did no vary with diet. SHR were less responsive to diet-induced changes than were SD; no decrease in TG, LDL or LDL:HDL was observed in SHR, nor was degree of hypertension altered in SHR by the MO or CO diet. In summary, MO is more effective than CO in shifting the lipid profile of rats toward one that is less atherogenic. However, the SD rat is more susceptible to diet-induced lipid modification than is the SHR.     

Dietary protein effects on gallstone formation

Semipurified lithogenic diets for hamsters contain casein as the protein source. Since substitution of soy protein isolate for casein reduces serum cholesterol concentrations in several species, we studied replacement of casein by soy protein for effects on gallstone formation. Feeding soy protein consistently resulted in a significantly reduced incidence of gallstones. Switching to a soy-based diet after induction of gallstones resulted in dissolution of a significant percentage of the stones. Partial substitution of soy for casein gave results intermediate between 100% casein and 100% soy. The lysine/arginine ratio of the proteins may be responsible for the observed differences in cholelithiasis. The reduction in lithogenicity associated with feeding soy protein appears to be mediated primarily through decreased secretion of cholesterol into bile.