Saturated and unsaturated fatty acids independently regulate low density lipoprotein receptor activity and production rate

Influence of dietary cholesterol and fat source on atherosclerosis in the Japanese quail (Coturnix japonica)

The Japanese quail has been used as a model of human atherosclerosis to investigate the mechanisms underlying the development of vascular lesions, i.e. hyperlipoproteinaemia and impaired endogenous antioxidant status. In the present study, Japanese quail were fed on semi-purified diets containing butter, beef tallow or soyabean-oil blends, with either 0.5 or 5 g cholesterol/kg for 9 weeks to examine the effects of dietary fat blends varying in fatty acid composition and cholesterol intake on plasma lipids and aortic atherosclerotic plaque and sterol composition. These findings were related to possible diet-induced changes in antioxidant status of selected tissues. Hypercholesterolaemia was confirmed (P < 0.001) in birds fed on high-cholesterol (HC) diets. Plasma total cholesterol concentration and cholesterol content of lipoprotein fractions in hypercholesterolaemic birds were lower (P < 0.05) in quail fed on the soyabean-oil blend. Plasma triacylglycerol content was increased (P < 0.001) in HC-fed birds. Dietary fat blends did not influence plasma triacylglycerol levels. Tissue antioxidant status (catalase (EC 1.11.1.6), glutathione peroxidase (EC 1.11.1.9), glutathione reductase (EC 1.6.4.1) and superoxide dismutase (EC 1.15.1.1) activities and glutathione content) was generally not greatly affected by dietary fat blend or cholesterol treatment. Birds fed on HC diets exhibited severe (P < 0.001) atherosclerotic plaque in aortas which was not influenced by the source of dietary fat blend. Scanning electron microscopy confirmed results of visual aortic plaque scoring using dissecting light microscopy. Several cholesterol oxides were identified and quantified in aortic plaque from HC-fed birds (5,6 alpha-epoxy-5 alpha-cholesterol, 7(beta-hydroxycholesterol and 7-ketocholesterol) regardless of dietary fat blend. The results indicate that dietary fat blends varying in polyunsaturated:saturated fatty acid ratios only marginally influence the degree of hypercholesterolaemia in atherosclerosis-susceptible quail fed on atherogenic diets only, and are not a factor, compared with sterol feeding, in modulating the degree of atherosclerosis or the aortic oxysterol content in these same birds. Moreover, diet-induced hyperlipoproteinaemia had only a small effect on antioxidant status of selected tissues examined.

Regulation of plasma lipoprotein levels by dietary triglycerides enriched with different fatty acids

Saturated vegetable oils (coconut, palm, and palm kernel oil) containing predominantly saturated fatty acids, lauric (12:0) or myristic (14:0 and palmitic (16:0), raise plasma total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) levels in animals and humans, presumably by decreasing LDL receptor activity and/or increasing LDL-C production rate. Although stearic acid (18:0) is chemically a saturated fatty acid, both human and animal studies suggest it is biologically neutral (neither raising nor lowering) blood cholesterol levels. Although earlier studies indicated that medium chain fatty acids (8:0-10:0) were also thought to be neutral, more recent studies in animals and humans suggest otherwise. Unsaturated vegetable oils such as corn, soybean, olive, and canola oil, by virtue of their predominant levels of either linoleic acid (18:2) or oleic acid (18:1), are hypocholesterolemic, probably as a result of their ability to upregulate LDL receptor activity and/or decrease LDL-C production rate. Whether trans fatty acids such as trans oleate (t18:1), in hydrogenated products such as margarine, are hypercholesterolemic remains controversial. Studies in humans suggest that their cholesterol-raising potential falls between the native nonhydrogenated vegetable oil and the more saturated dairy products such as butter. Assessment of the magnitude of the cholesterolemic response of trans 18:1 is difficult because in most diet studies its addition is often at the expense of cholesterol-lowering unsaturated fatty acids, making an independent evaluation almost impossible.

Saturated fatty acids and LDL receptor modulation in humans and monkeys

It has been known for 40 years that dietary saturated fat (SAT FAT) increases plasma cholesterol, including LDL-C and HDL-C. In humans, where LDL-C is typically > 90 mg/dl this SAT FAT effect largely reflects changes in LDL-C pool size. The original human studies suggested that LDL-C expansion during SAT FAT consumption reflected reduced LDL clearance (LDL receptor activity) in hyperlipemics and increased LDL production rates in normolipemics (LDL-C < 100 mg/dl) . This dual explanation is supported by data from several animal models where specific saturated fatty acids (SFAs) have been the focus. However, the situation is complicated by the fact that polyunsaturated fatty acids (PUFAs) oppose SFAs, i.e. PUFAs decrease LDL-C and increase LDL receptor (LDLr) activity, so the effect of SAT FAT intake may represent the combined influence of increased SFAs and decreased PUFAs. In fact, careful scrutiny of primate data suggests a negligible effect of saturated fat on LDL clearance (and receptor activity) in the absence of dietary cholesterol when PUFA intake is adequate (5-10%en) and the lipoprotein profile is relatively normal (LDL-C < 90 mg/dl), i.e. normolipemic situations at the time of dietary intervention. In such cases increases in LDL-C due to SFAs (particularly 12:0+14:0) appear to reflect LDL overproduction associated with a shift in cholesterol from tissues to the plasma cholesteryl ester (CE) pool (both LDL-C and HDL-C) without altering whole-body cholesterol balance. The reason for this shift, which is accompanied by an increase in the plasma oleic/linoleic CE ratio, is unknown but may reflect a decreased rate of CE hydrolysis by the liver. When individuals or animals are rendered hyperlipemic by other factors (e.g. chronic caloric and dietary cholesterol excesses in humans or by cholesterol feeding in animals) specific SFAs (particularly 16:0) can contribute to decreased LDLr activity initiated by a primary factor, such as dietary cholesterol. However, LDLr down-regulation by dietary cholesterol greatly exceeds any contribution from SFAs.

Desaturation of stearate is insufficient to increase the concentrations of oleate in cultured rat hepatocytes

Desaturation of stearate and palmitate and its effect on cellular accumulation of oleate were determined in primary culture of rat hepatocytes. The rate of oleate synthesis as measured by the formation of monounsaturated fatty acids from stearate was significantly higher than that from palmitate. The rate of [1-(14)C]stearate incorporation into oleate [1208 +/- 195 pmol/(mg protein x 4 h)] was 80% higher than that of [1-(14)C]palmitate [(672 +/- 82 pmol/(mg protein x 4 h)]. Despite the different rates of desaturation, the cellular oleate concentrations did not differ in the cells treated with stearate and palmitate (i.e., 42.5 +/- 4.5 vs. 40.8 +/- 5.2 nmol/mg protein). On the other hand, oleate concentration in the cells incubated with exogenous oleate was 198.1 +/- 9.5 nmol/mg protein. There was a dose-dependent increase in cellular stearate concentration by increasing stearate concentrations from 0.5 mmol/L to 4.0 mmol/L in culture medium. A linear increase in cellular stearate concentration was also achieved by increasing the duration of incubation with 1.0 mmol/L stearate from 2 to 24 h. Despite the marked increases in stearate concentrations under these conditions, oleate concentrations remained unchanged in the cells. These results do not support the contention that the hypocholesterolemic effect of stearate may be mediated by its conversion to oleate, although stearate is a more favorable substrate for desaturation than palmitate.

Effect of dietary fats rich in lauric, myristic, palmitic, oleic or linoleic acid on plasma, hepatic and biliary lipids in cholesterol-fed hamsters

Effects of different dietary fats on plasma, hepatic and biliary lipids were determined in male golden Syrian hamsters (Mesocricetus auratus) fed on purified diets for 7 weeks. Diets were made by blending different fats containing characteristic fatty acids: butter (14:0 + 16:0), palm stearin (16:0), coconut oil (12:0 + 14:0), rapeseed oil (18:1), olive oil (18:1) and sunflowerseed oil (18:2). In all diets except the sunflowerseed oil diet dietary 18:2 was held constant at 2% energy. Total fat supplied 12% of energy and cholesterol was added at 4 g/kg diet. Plasma cholesterol and triacyglycerol concentrations were increased by dietary cholesterol. After 7 weeks, plasma cholesterol concentrations were highest with the palm stearin, coconut oil and olive oil diets (8.9, 8.9 and 9.2 mmol/l) and lowest with the rapeseed oil and sunflowerseed oil diets (6.7 and 5.5 mmol/l) while the butter diet was intermediate (8.5 mmol/l). Hepatic cholesterol concentration was highest in hamsters fed on the olive oil diet and lowest with the palm stearin diet (228 v. 144 mumol/g liver). Biliary lipids, lithogenic index and bile acid profile of the gall-bladder bile did not differ significantly among the six diets. Although the gallstone incidence was generally low in this study, three out of 10 hamsters fed on the palm stearin diet developed cholesterol gallstones. In contrast, no cholesterol gallstones were found with the other diets. Rapeseed and sunflowerseed oils caused the lowest plasma cholesterol and triacyglycerol concentrations whereas olive oil failed to demonstrate a cholesterol-lowering effect compared with diets rich in saturated fatty acids. Since 18:2 was kept constant at 2% of energy in all diets, the different responses to rapeseed and olive oils could possibly be attributed to their different contents of 16:0 (5.6% v. 12.8% respectively). Other possible explanations are discussed.

Nutrient intake patterns, body mass index, and vitamin levels in patients with rheumatoid arthritis

OBJECTIVE

To assess nutrient intakes and vitamin levels in 79 patients with rheumatoid arthritis participating in a trial and to determine whether changes in body mass index were associated with changes in disease activity.

METHODS

This study evaluated baseline vitamin levels, 1-day dietary intakes, and weight every 3 months for 1 year. Linear regression analysis was used to evaluate the relationship of time to body mass index. Analysis of covariance was used to determine if body mass index, time, or treatment had an effect on disease activity.

RESULTS

Deficient vitamin levels and poor nutrient intake patterns were prevalent in the study population. Changes in body mass index over time did not correlate with changes in disease activity.

CONCLUSIONS

Rheumatoid arthritis patients are at high risk of obesity, abnormal vitamin levels, and poor nutrient intakes. Changes in body mass index failed to correlate with changes in disease activity.

Effects of dietary cholesterol and triglycerides on lipid concentrations in liver, plasma, and bile

Dietary cholesterol (CHL) and triglycerides (TG) can influence plasma, hepatic, and biliary lipid composition, but effects on lipids in these three compartments during the early stages of CHL gallstone formation have not been studied in parallel. We fed prairie dogs diets containing one of four test oils (safflower, coconut, olive, or menhaden) at either 5 or 40% of calories, in the presence of 0 or 0.34% CHL, for 3 wk. In the absence of dietary CHL, increases in dietary TG produced 50-200% increases in the concentrations of biliary CHL and hepatic cholesteryl ester (CE), while the concentrations of hepatic free CHL (FC) as well as plasma FC and CE remained relatively unchanged. Increasing dietary CHL to 0.34% resulted in increases in hepatic FC of approximately 50% for all four fats regardless of whether they were supplied at 5 or 40% of calories. CHL supplementation caused more pronounced increases in biliary CHL (200-400%), hepatic CE (50-200%), plasma FC (up to 100%), and plasma CE (up to 150%), and these increases were exacerbated by concurrent supplementation of dietary fat and CHL (biliary CHL: 300-700%; hepatic CE: 100-250%; plasma FC: up to 165%; plasma CE: 100-350%). These results indicate that enhanced secretion of biliary CHL and, to a lesser extent, increased synthesis of hepatic CE, may be primary mechanisms for maintaining the hepatic FC pool. Furthermore, dietary CHL and high levels of fat intake are independent risk factors for increasing biliary CHL concentrations, and adverse effects on lipid concentrations in plasma and bile tend to be exacerbated by ingestion of diets rich in both fat and CHL.

Effects of dietary fatty acid composition on plasma cholesterol

It should be clear from the preceding sections that the effects of dietary fatty acids on plasma lipids get more complicated the more we try to simplify them! We have presented one argument as to how different fatty acids may interact to impact human plasma lipids. This is by no means an endorsement that ours is the only argument. Nevertheless, a strong case can be made for 14:0 and 18:2 as being the key players in this scenario. The role of palmitic acid seems to be the most controversial. While clearly certain studies do indeed reveal 16:0 to be hypercholesterolemic relative to 18:1, the data from studies suggesting that it behaves similarly to 18:1 are equally compelling. What is certain is that it is erroneous to assume that 16:0 is the major cholesterol-raising SFA simply because it is the most abundant SFA in the diet. Clearly, 18:0 cannot be considered cholesterol-elevating. One is therefore left with the 12-16C SFA. However, 12:0 and 14:0 are only of concern if diets contain palm-kernel, coconut oil or dairy products as major dietary constituents. Accordingly one is left with 16:0 and its response is highly dependent on the metabolic status as well as the age of the subjects being used. While "elderly" hypercholesterolemic humans clearly benefit from decreased 16:0 (and all SFA) consumption, "younger" normocholesterolemic subjects fail to show such clear-cut effects. Additionally, the concomitant levels of dietary cholesterol and 18:2 also have a major bearing on the cholesterolemic response of 16:0 As far as guidelines for the general public are concerned, clearly for people with TC > 225 and LDL-C > 130 mg/dl and/or those who are overweight (i.e. those percieved to be at high risk), the primary emphasis should clearly be on reducing total fat consumption. Decreasing saturated fat consumption will invariably also lower dietary cholesterol consumption. The latter manouver will generally lower TC and LDL-C. Whether the reduction occurs because of the removal of 14:0, or 16:0 and/or dietary cholesterol is a mute point, since most dietary guidelines advocate curtailing intake of animal and dairy products, which will result in reductions of all the SFA. It remains to be established whether life-long adherence to the above dietary guidelines in those subjects with normal cholesterol levels and an absence of the other conventional risk factors for CHD, will result in a subsequent decrease in CHD risk. In the latest NCEP report 39 million Americans were targeted as those who would benefit from reductions in LDL-C, principally by dietary means. This is indeed a very high number. But that leaves almost 220 million Americans! For them the age old recommendation to consume a moderate fat load, maintain ideal body weight and eat a varied and balanced diet would still appear to be the most powerful advice.

Low-fat diets do not lower plasma cholesterol levels in healthy men compared to high-fat diets with similar fatty acid composition at constant caloric intake

In most studies reporting the effects of high-fat (HF) and low-fat (LF) diets on human plasma fatty acids (FA) and lipoprotein levels, the design involved adding to the diet an oil that had an FA composition (FAC) very different from the FAC of the control diet. Thus, it is difficult to determine if simply reducing the fat content of the diet without changing the dietary FAC changes the tissue FAC or alters plasma lipid levels. In this study, we fed diets that contained either 22 or 39% of calories from fat, but had no differences in their FAC, for 50 d to a group (n = 11) of healthy men (20-35 y). Thus, the polyunsaturated/saturated ratios (1.0) of the diets were identical as were the n-3/n-6 ratio and the monounsaturated-to-total fat ratios. The diets contained (wt% of total fat) approximately 28% saturated FA, 33% monounsaturated cis-FA, 6% monounsaturated trans-FA, 22% n-6 polyunsaturated FA, and 7% n-3 polyunsaturated FA, and 4% other minor FA. The diets consisted of natural foods and were formulated to contain 16 en% protein, either 45 or 62 en% carbohydrate (CHO) and at least the recommended daily allowance for all micronutrients. Both diets contained 360 mg of cholesterol per day. All subjects were given the HF diet for 20 d, and then six were placed on the LF and the other five remained on the HF diet for 50 d. The two groups were crossed-over for the remaining 50 d of the study. The subjects' baseline total cholesterol level was 173 mg/dl, after 50 d on the HF diet it was 177 mg/dl and after 50 d on the LF diet, 173 mg/dl. The differences were not significant, and there were no significant changes in either the LDL or HDL cholesterol levels with either diet. Triglyceride levels, and consequently very low density lipoprotein levels, rose significantly on the LF, higher CHO diet compared to the levels found in the subjects on the HF diet (91.5 and 66.4 mg/dl respectively, P < 0.002). The linoleic acid content of the plasma, platelets, and red blood cells was significantly (P < 0.05) reduced in the LF diet compared to HF diet, without any obvious physiological effects. Hence, many earlier observations indicating reductions in plasma lipid levels when people are on LF diets may be due to changes in the FAC of the diet, not the reduction in fat calories.

Sterol regulation of fatty acid synthase promoter. Coordinate feedback regulation of two major lipid pathways

The gene encoding fatty acid synthase, the essential multi-functional enzyme of fatty acid biosynthesis, is shown to be regulated by cellular sterol levels similar to genes that encode important proteins of cholesterol metabolism. We show that expression of the endogenous FAS gene is repressed when regulatory sterols are included in the culture medium of HepG2 cells and that the FAS promoter is subject to similar regulation when fused to the luciferase reporter gene. Mutational studies demonstrate that sterol regulation is mediated by binding sites for the sterol regulatory element-binding protein (SREBP) and transcription factor Sp1, making it mechanistically similar to sterol regulation of the low density lipoprotein receptor gene. It is also demonstrated that SREBP and Sp1 synergistically activate the FAS promoter in Drosophila tissue culture cells, which lack endogenous Sp1. These experiments provide key molecular evidence that directly links the metabolism of fatty acids and cholesterol together.

Low density lipoprotein binding to monolayer cultures of hepatocytes isolated from hamsters fed different dietary fatty acids

(i) The aim of these studies was to investigate the effect of dietary cholesterol and dietary fat on the expression of the hepatic low density lipoprotein (LDL) receptor. (ii) Low density lipoprotein (LDL) binding to monolayer cultures of hepatocytes isolated from hamsters fed different amounts of cholesterol and fats was measured. (iii) Hepatocytes isolated from hamsters fed a low cholesterol diet (< 0.008% w/w) specifically bound LDL and this binding increased with time in culture. By comparison, LDL binding to hepatocytes isolated from hamsters fed 0.12% (w/w) cholesterol was lower and there was no time dependent increase. Intermediate levels of binding were seen for hepatocytes from hamsters fed 0.06% (w/w) cholesterol. (iv) In hamsters fed 10% (w/w) fat in the presence of 0.06% (w/w) cholesterol, significantly more LDL receptors were expressed by hepatocytes from animals fed safflower oil compared to those isolated from lard or olive oil fed animals. (v) The reduced LDL cholesterol concentrations frequently observed for hamsters fed polyunsaturated fatty acids thus reflects an increase in the number of LDL receptors expressed in the liver. However, other explanations are needed for the effects of monounsaturated fatty acids.

Studies on the mechanism of the ursodeoxycholic acid-induced increase in hepatic low-density lipoprotein binding

Previously, we have shown, in golden Syrian hamsters, that chronic feeding of ursodeoxycholic acid (UDCA), in contrast to that of its 7 alpha-epimer, chenodeoxycholic acid (CDCA), produced a significant increment in hepatic low-density lipoprotein (LDL) uptake, despite similar suppression of bile acid synthesis by both bile acids. Evidence for a direct effect of this bile acid on hepatic LDL metabolism was shown in vitro, with isolated hamster hepatocytes, suggesting that this effect was unique to UDCA and specific for receptor-mediated LDL catabolism. The aim of the present study was to define the cellular mechanism(s) associated with this phenomenon, using male golden Syrian hamsters. Regardless of chronic exposure of the liver to either UDCA or CDCA, acute incubation with UDCA consistently resulted in an increase of LDL binding to isolated hepatocytes by 15 to 40%. Furthermore, chronic treatment with either UDCA or CDCA did not result in alterations in lipoprotein particle composition. Likewise, incubation of hepatocytes with UDCA was not associated with a change of the membrane lipid composition. In isolated liver membrane fractions, UDCA increased both the maximum number of LDL binding sites and the affinity constant for LDL by around 35%, suggesting an interaction of UDCA with the LDL receptor, at the plasma membrane level, independent of an effect on receptor cycling. The results of the studies support a role for UDCA in the recruitment of cryptic LDL receptors from a cellular membrane pool, possibly due to the unique localization of UDCA in the plasma membrane lipid bilayer.

Dietary fats rich in saturated fatty acids (12:0, 14:0, and 16:0) enhance gallstone formation relative to monounsaturated fat (18:1) in cholesterol-fed hamsters

To test the possibility that dietary palmitic acid (16:0) may be lithogenic, different fats were blended to exchange 18:1 in olive oil with either 16:0 in palm stearin, 12:0 + 14:0 in coconut oil, or 14:0 + 16:0 in butterfat. Dietary 18:2 was held constant at 1.2% energy (en) (with extra safflower oil as needed) in these four purified diets containing low fat (11% of total energy) and 0.4% cholesterol. A fifth, high-fat diet provided 40% of the total energy as the 16:0-rich blend. All hamsters fed the low-fat 16:0-rich blend for six weeks developed cholesterol gallstones (8/8). Although the gallstone incidence was lower for the 12:0 + 14:0-rich diet (5/8), the severity of stone formation in affected hamsters was equal to that in the low-fat, 16:0-rich group. Mucin accumulation in gallbladder bile was often associated with cholesterol gallstones in diets containing 16:0, but was minimal in 18:1-rich and 12:0 + 14:0-rich groups. Neither the lithogenic index (all > 1.0), plasma lipids, nor liver cholesterol was a selective predictor of stone formation. The high-fat, 16:0-rich diet actually decreased cholesterol stone incidence (3/8) and severity, but yielded a high incidence of pigment stones (5/8). Thus, saturated fat and 16:0 per se were not responsible for the exaggerated lithogenesis. Because the antilithogenic 18:1-rich diet also normalized the 18:2 intake (1.2% en) relative to previous butter diets (0.3% en), the potential importance of essential fatty acids (EFA) deficiency in the model was tested in a second study by feeding graded amounts of 18:2 (0.3, 0.6, 0.9, and 1.2% en) as safflower oil in four low-fat, butter-rich diets (11% en as fat) without alleviating gallstone incidence or severity. These studies indicate that substitution of 18:1 for saturated fatty acids in low-fat diets reduces gallstone formation without affecting the lithogenic index. Furthermore, intake of 18:2 at or below the EFA requirement does not appear to be a major factor in this model.

Oleate and other long chain fatty acids stimulate low density lipoprotein receptor activity by enhancing acyl coenzyme A:cholesterol acyltransferase activity and altering intracellular regulatory cholesterol pools in cultured cells

Modification of dietary fatty acid composition results in changes in plasma cholesterol levels in man. We examined the effect of in vitro fatty acid supplementation on low density lipoprotein (LDL) receptor activity in cultured cells and questioned whether changes were related to fatty acid-induced alterations in acyl-CoA: cholesterol acyltransferase (ACAT) activity. Preincubation of cultured cells (i.e. human skin fibroblasts, J774 macrophages, and HepG2 cells) with oleic acid (oleic acid:bovine serum albumin molar ratio 2:1) at 37 degrees C for longer than 2 h resulted in a 1.2- to 1.5-fold increase in LDL cell binding at 4 degrees C and LDL cell degradation at 37 degrees C. Scatchard analysis showed that oleic acid increased LDL receptor number but not LDL affinity (Kd). Fatty acid supplementation of J774 macrophages increased both LDL receptor activity and cholesteryl ester accumulation. The ACAT inhibitor, 58-035, eliminated both effects, and increased ACAT activity preceded stimulation of LDL receptor activity by 1-2 h. Supplementation of macrophages with triolein emulsion particles also increased LDL cell binding and degradation, and addition of cholesterol to the emulsions abolished this effect. Among fatty acids tested, oleate (18:1), arachidonate (20:4), and eicosapentanoate (20:5) demonstrated the greatest effects. We hypothesize that certain fatty acids delivered to cells either in free form, or as triglyceride, first increase cellular ACAT activity, which then causes a decrease in an intracellular free cholesterol pool, signaling a need for increased LDL receptor activity. This mechanism may play a role in the effect of certain dietary fatty acids on LDL metabolism in vivo.

Comparison of hypocholesterolemic effects induced by dietary linoleic acid and oleic acid in hamsters

We investigated the differences between the hypocholesterolemic effects induced by dietary linoleic acid and those induced by oleic acid in hamsters. Addition of 5% linoleic acid or oleic acid to a 0.1% cholesterol-supplemented diet diminished the increases in plasma total and low density lipoprotein (LDL) cholesterol induced by cholesterol alone. Linoleic acid decreased high density lipoprotein (HDL) cholesterol in comparison with cholesterol alone, whereas oleic acid did not. As compared with a standard diet or a cholesterol-supplemented diet, linoleic acid and oleic acid each prevented hepatic LDL receptor suppression, although linoleic acid was more effective. Oleic acid prevented the increase in plasma cholesteryl ester transfer protein (CETP) activity induced by dietary cholesterol, whereas linoleic acid did not. Neither linoleic acid nor oleic acid altered hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity. Only oleic acid increased hepatic cholesterol 7 alpha-hydroxylase activity. These results suggest that dietary linoleic and oleic acids diminish the cholesterol-induced increases in plasma total and LDL-cholesterol by preventing hepatic LDL receptor suppression, and in the case of oleic acid by also preventing the increase in the plasma CETP activity. These effects on cholesterol 7 alpha-hydroxylase activity may influence bile lipid metabolism.

Mechanisms of LDL-cholesterol lowering action of psyllium hydrophillic mucilloid in the hamster

Psyllium hydrophillic mucilloid (psyllium) is a soluble fiber that significantly lowers plasma low-density lipoprotein (LDL)-cholesterol levels in humans and experimental animals. These studies were designed to determine whether this action is the result of a reduction in LDL-cholesterol production, an increase in receptor-mediated LDL clearance by the tissues, or a combination of these mechanisms. Adult male Golden Syrian hamsters were fed ad libitum for 30 days a cereal-based diet containing added cholesterol (0.1%) and hydrogenated coconut oil (10%), as well as either microcrystalline cellulose (Avicel) (7.5%) or psyllium (7.5%). In contrast to their Avicel-fed controls, the hamsters given psyllium had markedly lower plasma total (122.1 +/- 4.1 vs. 399.4 +/- 39.4 mg/dl) and LDL-cholesterol (46.0 +/- 2.2 vs. 143.5 +/- 12.0 mg/dl) levels. Psyllium feeding also prevented both the dramatic increase in hepatic total cholesterol levels (2.6 +/- 0.1 vs. 16.6 +/- 1.1 mg/g), and the suppression of hepatic cholesterol synthesis (165.1 +/- 27.1 vs. 26.1 +/- 1.2 nmol/h per g) that occurred in the animals given Avicel. Compared to their controls, the psyllium-fed animals also manifested a 44% lower rate of LDL-cholesterol production (167.6 +/- 8.1 vs. 300.2 +/- 16.0 micrograms/h per 100 g bw), and a 2.2-fold higher rate of hepatic LDL clearance (50.1 +/- 2.3 vs. 22.6 +/- 2.1 microliters/h per g). When expressed as a percentage of corresponding values obtained for hamsters fed the basal diet without any additions, the relative rate of LDL-cholesterol production was 175 +/- 10% and 99 +/- 4% for the Avicel- and psyllium-fed groups, respectively. It was similarly determined that the level of whole animal relative LDL receptor activity was marginally higher in the hamsters given psyllium (55.9 +/- 1.4%) than in those fed Avicel (47.5 +/- 3.3%). Thus, it was concluded that while the LDL-cholesterol lowering action of psyllium in the hamster is mediated through two mechanisms, the major effect is exerted at the level of LDL-cholesterol production.

Dietary n - 3 polyunsaturated fatty acids modify Syrian hamster platelet and macrophage phospholipid fatty acyl composition and eicosanoid synthesis: a controlled study

The aim of this study was to determine the effects of varying intakes of dietary n - 3 polyunsaturated fatty acids (PUFA) on the fatty acyl composition and arachidonic acid metabolite synthesis of platelets and macrophages in Syrian hamsters consuming diets that were strictly controlled for n - 6 PUFA content. Animals consumed highly controlled diets which were not supplemented with n - 3 PUFA (control) or supplemented with 0.4%, 0.8% or 2% (w/w) n - 3 fatty acids. The content of n - 3 PUFA in cellular phospholipids increased progressively with the intake of n - 3 PUFA, while n - 6 PUFA, including arachidonic acid, decreased despite the constant intake of 18:2(n - 6); this latter effect was more substantial in macrophages than in platelets. The synthesis by stimulated macrophages of prostaglandin E2, 6-keto-prostaglandin F1 alpha, thromboxane B2 and 11- and 15-hydroxyeicosatetraenoic acids decreased with the intake of 0.8% n - 3 PUFA to 30-50% of the control values. Little effect of diets on platelet aggregation and eicosanoid synthesis was observed reflecting the limited effect on platelet arachidonic acid content. The synthesis of 12-hydroxyeicosapentaenoic acid by stimulated platelets increased with n - 3 PUFA consumption in a dose-dependent fashion. Circulating triacylglycerols and HDL-cholesterol were decreased only in animals consuming 2% n - 3 PUFA. The strict control of n - 6 PUFA intake allows the determination of the effects of n - 3 PUFA intake on the measured parameters without confounding effects of other dietary lipids.

ApoB metabolism in familial hypercholesterolemia. Inconsistencies with the LDL receptor paradigm

The biology of the low-density lipoprotein (LDL) receptor has been examined in detail, and a paradigm for LDL metabolism has evolved from comparative studies of cholesterol metabolism in a variety of cells cultured from normal individuals and subjects with familial hypercholesterolemia (FH). Cultured cells from patients with homozygous FH lack a functional LDL receptor and show diminished LDL clearance, induction of the enzyme hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase, increased cholesterol synthesis, decreased cholesterol ester production, and depleted cholesterol ester stores. The observed decrease in the fractional catabolic rate (FCR) of LDL is attributed to the mutated LDL receptor gene. However, in the experimental animal model of this disease, the Watanabe heritable hyperlipidemic (WHHL) rabbit, cholesterol ester stores are increased, while hepatic cholesterol synthesis is decreased. Furthermore, in humans HMG-CoA reductase is suppressed, and the LDL apolipoprotein (apo) B production rate is increased in patients with FH. These findings raise questions about the adequacy of the paradigm in understanding hepatic cholesterol metabolism in vivo. In humans, apoB metabolism is believed to be principally determined by the liver, where apoB is both synthesized and catabolized. Assuming the neutral lipid content of the liver is the major determinant of apoB metabolism, we postulated that the changes in apoB metabolism in FH are predictable when based on the assumption of an increase in hepatic cholesterol and cholesterol ester content, as observed both in the WHHL rabbit and in humans. We examined this hypothesis in vivo in patients with heterozygous FH by using tracer kinetic methodology and have used similar data from normal and hypertriglyceridemic (HTG) subjects as controls. Whereas normal and HTG subjects secrete apoB primarily as large, triglyceride-enriched very-low-density lipoprotein (VLDL), heterozygous FH patients have an absolute decrease in apoB production and secrete almost 40% of apoB as smaller intermediate-density lipoprotein (IDL)/LDL. In normal humans, about half of secreted apoB is catabolized rather than being converted to LDL. In HTG subjects two thirds of apoB follows this same route, by which VLDL remnants remaining after triglyceride hydrolysis are largely returned to the liver. In contrast, in FH subjects secreted apoB is fully converted to LDL. Thus, although total apoB secretion is reduced in FH subjects, total LDL production is greater than in either normal or HTG subjects. Under basal conditions the elevated LDL in heterozygous FH is due to both decreased LDL receptor-mediated catabolism and increased LDL production. However, the number of LDL receptors actually expressed is suppressed below the number of potentially functional receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of dietary fat selection on plasma cholesterol synthesis in older, moderately hypercholesterolemic humans

To study factors controlling plasma cholesterol levels, the effect of dietary fat type on cholesterol synthesis was examined in 15 hypercholesterolemic subjects (low-density lipoprotein [LDL] cholesterol > 130 mg.dL-1) consuming over a period of 32 days (1) a baseline diet (36% kcal as fat: 15% saturated, 15% monounsaturated, and 6% polyunsaturated fat; 180 mg cholesterol.1000 kcal-1) and diets meeting National Cholesterol Education Program step 2 criteria (30% kcal as fat, < or = 7% saturated fat, 80 to 85 mg cholesterol/Mcal), where two thirds of the fat was either (2) olive, (3) corn, or (4) canola oil. Plasma total, LDL, and high-density lipoprotein (HDL) cholesterol and triglyceride levels were determined at the end of each period. Cholesterol fractional synthesis rate (FSR) was also measured as the deuterium (D) incorporation into plasma total cholesterol relative to body D2O level (1.2 g D2O.kg-1 estimated body water) over 24 hours. Absolute synthesis rates (ASRs) were determined as the product of FSR and rapid turnover cholesterol pool size. Plasma total and LDL cholesterol levels declined significantly (P < .005) on all plant-oil diets compared with the baseline diet; however, triglyceride levels were not different. FSRs were higher (P < .05) for the corn oil (0.0665 +/- 0.0097 pool.d-1) compared with baseline (0.0412 +/- 0.0060 pool.d-1) and olive oil (0.0409 +/- 0.0052 pool.d-1) but not canola oil (0.0492 +/- 0.0072 pool.d-1) diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Impact of myristic acid versus palmitic acid on serum lipid and lipoprotein levels in healthy women and men

The cholesterol-raising effect of dietary saturated fatty acids is largely accounted for by lauric, myristic, and palmitic acids. Dairy fat is a major source of myristic acid, and palm oil is especially rich in palmitic acid. Myristic acid is suspected of being much more cholesterolemic than palmitic acid, but direct comparisons have been lacking. We therefore fed 36 women and 23 men three diets that differed from each other in palmitic, oleic, and myristic acid content by about 10% of total energy. We used palm oil, high-oleic acid sunflower oil, and a specially produced high-myristic acid fat to achieve these differences. Each diet was consumed for 3 weeks in random order. Mean serum cholesterol was 4.53 mmol/L on the high-oleic acid diet, 4.96 mmol/L on the palmitic acid diet, and 5.19 mmol/L on the myristic acid diet (P < .0001 for all comparisons). Myristic acid raised low-density lipoprotein (LDL) cholesterol by 0.11 mmol/L, high-density lipoprotein (HDL) cholesterol by 0.12 mmol/L, and apolipoprotein (apo) A-I by 7.2 mg/dL relative to palmitic acid; increases relative to oleic acid were 0.50 mmol/L for LDL cholesterol, 0.15 mmol/L for HDL cholesterol, 6.0 mg/dL for apoB, and 8.9 mg/dL for apoA-I (P < .01 for all comparisons). The HDL cholesterol and apoA-I levels on the palmitic and oleic acid diets were the same. None of the responses differed significantly between woman and men. Myristic acid and palmitic acid both caused high LDL cholesterol and apoB levels and low HDL to LDL ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of different dietary fats and cholesterol on serum lipoprotein concentrations in hamsters

(i) We have studied the effect of dietary cholesterol and fat on lipoprotein concentrations in the male Golden Syrian hamster. (ii) On a low fat diet, dietary cholesterol increased the cholesterol concentration in all the major serum lipoprotein fractions. It also increased the storage of cholesterol ester in the liver. (iii) In the absence of added dietary cholesterol, additional dietary fat had little influence on serum or hepatic cholesterol concentrations, and this is irrespective of the nature of the dietary fat. (iv) In the presence of 0.12% (w/w) cholesterol, lard (rich in saturated fatty acids) increased serum VLDL cholesterol and triacylglycerol concentrations. By contrast, olive oil (rich in oleic acid) had no effect on VLDL lipid concentrations and sunflower oil, rich linoleic acid, reduced them. (v) Lard also increased serum LDL cholesterol concentrations in cholesterol-fed animals. Olive oil reduced LDL cholesterol concentrations and sunflower oil had no effect. (vi) In cholesterol-fed animals, lard had no effect on the hepatic cholesterol ester concentration, while both olive and sunflower oil increased it. This increase was significantly higher in olive oil-fed hamsters compared to those fed sunflower oil. (vii) Thus, in this species, the primary effects of dietary fat on lipoprotein metabolism appear to represent a modulation of the effects of dietary cholesterol. In cholesterol-fed hamsters we confirm the hypercholesterolaemic effects of saturated fatty acids and highlight important differences in the effects of mono- and poly-unsaturated fatty acids on lipoprotein metabolism.

Effects of high monounsaturated and polyunsaturated fat diets on plasma lipoproteins and lipid peroxidation in type 2 diabetes mellitus

Increased free radical peroxidation of lipoproteins may contribute to the excess atherosclerosis in Type 2 diabetes mellitus and may be aggravated by increasing polyunsaturate intake. Increasing intake of monounsaturates might have similar hypolipidaemic effects while avoiding this problem. Therefore the effects of high monounsaturated and polyunsaturated fat diets on plasma lipoproteins and lipid peroxidation were compared, against each other and a relatively high saturated fat baseline diet, in 13 men with Type 2 diabetes and 12 healthy controls, randomized in crossover fashion to each diet. There were no differences in plasma lipoproteins between the monounsaturated and polyunsaturated fat diets in the diabetic and control groups separately, but when both groups were combined, high density lipoprotein cholesterol was higher on the monounsaturated fat diet (p = 0.04). Plasma lipid peroxidation was similar on the monounsaturated and polyunsaturated fat diets in both groups, but all indices of plasma lipid peroxidation in the diabetic group and lipid peroxides in the controls were significantly lower on these diets compared to the baseline diet. Both high monounsaturated and polyunsaturated fat diets increase hepatic metabolism of low density lipoprotein and shorten its circulating half-life and both may reduce lipid peroxidation, compared to high saturated fat diets, by this mechanism. On high polyunsaturated fat diets, this effect may offset any increased susceptibility of polyunsaturate enriched low density lipoprotein to peroxidation.

Dietary palmitic acid raises plasma LDL cholesterol relative to oleic acid only at a high intake of cholesterol

Using a crossover design, the effects of exchanging up to 10% dietary energy (%en) between oleic (18:1) and palmitic acid (16:0) on plasma lipoprotein metabolism was investigated in 12 normocholesterolemic cebus monkeys, both in the absence and presence of dietary cholesterol (0.3%, w/w). In all the purified diets, which contained 33%en as fat blends, myristic acid (14:0) and linoleic acid (18:2) were held constant at 0.3%en and 3.7%en, respectively. Cholesterol-free diets containing either high 18:1 (19%en), roughly equivalent levels of 16:0 and 18:1 (12 and 15%en, respectively), or a high level of 16:0 (18%en), generated similar values for total plasma cholesterol (TC), HDL-C and LDL-C. Plasma triacylglycerol concentrations (TG) were significantly higher when monkeys were fed the 16: 0-rich diet than when fed the 18: 1-rich diet (75 +/- 6 vs. 52 +/- 8 mg/dl; P < 0.05). LDL and HDL kinetic parameters (assessed after simultaneous injection of homologous 131I-LDL and 125I-HDL) revealed no significant differences between the 18: 1-rich or 16: 0-rich diets. By contrast, with added dietary cholesterol (0.78 mg/kcal) the 16: 0-rich diet resulted in significantly higher TC (318 +/- 20 vs. 299 +/- 20 mg/dl; P < 0.05) and LDL-C (136 +/- 10 vs. 117 +/- 10 mg/dl; P < 0.05) in comparison to the 18: 1-rich diet. HDL-C was unaffected (159 +/- 8 vs. 156 +/- 5 mg/dl), but plasma TG concentrations also tended to be higher (70 +/- 8 vs. 60 +/- 6 mg/dl, P < 0.08). Kinetic studies revealed that the higher LDL-C concentration was associated with an elevated pool size of LDL apo B (40 +/- 2 vs. 34 +/- 2 mg/kg body weight; P < 0.005), the latter attributed to decreased FCR (1.06 +/- 0.07 vs. 1.27 +/- 0.12 pools/day; P < 0.04) with no effect on the transport rate of LDL apo B (41 +/- 2 vs. 42 +/- 3 mg/kg body weight per day). HDL kinetic parameters were comparable during the 16: 0 and 18: 1 dietary periods, but dietary cholesterol caused an increase in apo A-I pool size and transport rate without impacting FCR. In this study a palmitic acid-rich diet failed to alter plasma or LDL-C when compared to an oleic acid-rich diet, unless the diet also contained cholesterol. In the latter case, 16: 0 increased LDL-C, which reflected a decrease in the efficiency of LDL apo B removal.

Dietary saturated fatty acids increase cholesterol synthesis and fecal steroid excretion in healthy men and women

In a strictly controlled 6-week trial with 47 healthy volunteers we have determined the effect of replacement of polyunsaturated by saturated fatty acids on the fecal steroid excretion and on the rate of whole body cholesterol synthesis, as measured both by the sterol balance method and by the concentration of the cholesterol precursor lathosterol in serum. Subjects were fed mixed natural diets, of which the total fat content was kept constant at 45% energy. Consumption of polyunsaturated fatty acids, mainly linoleic acid, was 21% energy for the first 3-week period (P:S ratio 1.9), and 5% of energy (P:S ratio 0.2) for the next 3-week period, or vice versa. Cholesterol intake as determined by analysis of duplicate diets was 41 mg MJ-1 (about 500 mg day-1) during both periods. Feces were collected for 5 days at the end of both periods. The steroid composition of the feces was not affected by the change of diets. The fecal excretion of neutral steroids was significantly higher on the low P:S high-saturated-fat (2.25 +/- 0.68 mmol day-1) than on the high P:S high-linoleic-acid diet (2.00 +/- 0.69 mmol day-1; P < 0.01). The excretion of bile acids was similar (0.77 +/- 0.40 and 0.79 +/- 0.41 mmol day-1, respectively). The cholesterol balance and the rate of cholesterol synthesis were higher during the low P:S (1.86 +/- 0.83 mmol day-1) than during the high P:S period (1.55 +/- 0.85 mmol day-1; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Composition of molecular species of triacylglycerols in bovine milk fat

Triacylglycerols from bovine milk fat were fractionated by reversed-phase liquid chromatography. The fatty acid and triacylglycerol compositions of each fraction were determined by capillary gas chromatography. These data were used to determine the accurate proportions of 223 individual molecular species of even-numbered triacylglycerols, accounting for 80% of total triacylglycerols (all percentages are expressed as moles per 100 mol). The three major triacylglycerols were butyroylpalmitoylacylglycerols, namely butyroylpalmitoyloleoylglycerol (4.2%), butyroyldipalmitoylglycerol (3.2%), and butyroylmyristoylpalmitoylglycerol (3.1%). Twenty-two triacylglycerols (> 1%) contained at least two of the four major long-chain fatty acids (C14:0, C16:0, C18:0, and C18:1). Among them were eight butyroyldiacylglycerols, the proportions of which reached 19% in total but only 12% when calculated on the basis of a random distribution of the fatty acids in the triacylglycerol molecules. More generally, most of the triacylglycerols that are composed of a short-chain fatty acid (C4:0 or C6:0) and two fatty acids in the range of C12 to C18 are preferentially synthesized by the mammary gland; their proportions (36% in total) were higher than the corresponding random values (24% in total). Conversely, the total amounts of simple (.4%) and mixed (2.9%) saturated long-chain (C14:0 to C18:0) triacylglycerols were much lower than those expected from random calculation (1.9 and 6.1%, respectively).

The interaction of dietary cholesterol and specific fatty acids in the regulation of LDL receptor activity and plasma LDL-cholesterol concentrations

From these brief considerations, it is clear that the steady-state LDL-cholesterol concentration is determined in a powerful way by the interaction of dietary cholesterol and specific fatty acids. There appear to be only a few saturated fatty acids and an even lesser number of unsaturated fatty acids that significantly interact with cholesterol in the liver cell to alter hepatic LDL receptor activity. These effects are uniformly seen in most experimental animals and in humans under circumstances where the experiments are properly designed. Future work is urgently needed to define the metabolic effects of the more unusual fatty acids (e.g., the trans fatty acid) and the more intimate details of how these substances regulate LDL receptor activity in the cell. It is also of considerable importance to extend these studies to the members of the same species that exhibit variable responses to these same dietary lipids. It is now clear that the magnitude of these specific responses to dietary cholesterol and specific fatty acids varies in different individuals with different genetic backgrounds from the same species. Elucidating the reasons for this variability is another area of research of considerable importance to human biology.