Trans-9-octadecenoic acid is biologically neutral and does not regulate the low density lipoprotein receptor as the cis isomer does in the hamster

Lamb Meat Quality and Carcass Evaluation of Five Autochthonous Sheep Breeds: Towards Biodiversity Protection

Simple Summary

The biodiversity protection represents a challenge of the agenda ONU 2030 for Sustainable Development Goals (SDG). Autochthonous sheep breeds, including Altamurana, Bagnolese, Gentile di Puglia, Laticauda and Leccese, reared in continental Southern Italy, are strongly affected by extinction risk; thus, it is urgent to find new solutions to valorise their products and obtain sustainable and smart food from local farms. The objective of the present study was to assess the lamb carcass commercial quality, chemical and fatty acid composition of Altamurana, Bagnolese, Gentile di Puglia, Laticauda and Leccese autochtonous sheep breeds. Data on EU Mediterranean classification showed that the carcasses from both Bagnolese and Laticauda breeds were classified in the heaviest category C. Moreover, breed can influence the meat fatty acids profile, which is resulted rich in conjugated linoleic acid with good nutritional properties; thus, lambs’ meat from autochthonous sheep breeds can be considered interesting for human consumption. The present study can help to protect the biodiversity of livestock heritage of Southern Italy and open a new field for the valorisation and promotion of their derived products.

Abstract

In the present study, the evaluation of the carcasses and meat quality, in terms of chemical composition and fatty acid profile, of lambs from five autochthonous sheep breeds (Altamurana, Bagnolese, Gentile di Puglia, Laticauda, and Leccese) reared in continental Southern Italy, were studied. All the carcasses were evaluated according to the EU Mediterranean classification system for carcasses weighing less than 13 kg. Meat chemical composition and fatty acids profile were assessed on both loin and leg commercial cuts. Fatty acid composition of loin resulted in differences among breeds, displaying lower values of saturated fatty acid in Altamurana, Bagnolese, and Leccese breeds and the highest content of polyunsaturated fatty acid in the Altamurana breed. Principal component analysis grouped lamb according to fatty acid content and to conjugated linoleic acid (CLA), omega n-3 and n-6 fatty acids; thus, Altamurana, Bagnolese, and Leccese breeds are characterized by the highest values of CLA content. Our data demonstrated that lamb meat from autochthonous breed has good carcass quality and the content of CLA, n-3, and n-6 was valuable for human consumption; therefore, the valorisation of local meat quality can help to avoid the extinction of the autochthonous breed offering to the market and consumer’s high nutritive products.

Differential effects of the trans-18:1 isomer profile of partially hydrogenated vegetable oils on cholesterol and lipoprotein metabolism in male F1B hamsters

Trans-fatty acid consumption from partially hydrogenated vegetable oil (PHVO) has been positively associated with multiple cardiovascular disease risk factors and events. This study was designed to examine the effects of trans-fatty acid isomer profile of PHVO on plasma lipids and lipoproteins and hepatic expression of key genes involved in cholesterol and fatty acid metabolism. Thirty-three male F(1)B strain Syrian Golden Hamsters were allocated to 1 of 3 hypercholesterolemic diets containing (5% by weight): 1) tristearin [control fat (CON)]; 2) partially hydrogenated high-oleic acid sunflower oil (PH-SUN); or 3) partially hydrogenated high-linoleic acid safflower oil (PH-SAF). PH-SUN contained more trans-4 to trans-10 18:1 compared with PH-SAF, which contained more trans-11 to trans-16 18:1. The addition of both PHVO to the diet increased plasma total cholesterol concentrations relative to CON, but only PH-SUN increased the plasma ratio of non-HDL:HDL cholesterol compared with CON. PH-SUN increased VLDL (total, large, and medium) and IDL particle concentrations while decreasing total, medium, and small HDL particle concentrations relative to CON. Both PHVO diets increased the hepatic cholesterol ester concentration, whereas the hepatic TG concentration was lower in PH-SUN compared with PH-SAF and CON. Levels of hepatic LDL receptor, HMG-CoA reductase, and sterol response element binding protein 1 mRNA were specifically reduced in the PH-SUN group compared to the CON group. Expression of SREBP1c was upregulated in both PHVO groups compared to CON, whereas only the PH-SAF group had higher levels of the lipogenic enzymes acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase-1 compared to CON. These results indicate that differences in the trans-fatty acid profile of PHVO can differentially affect lipid and lipoprotein metabolism.

Impact of dietary fat type within the context of altered cholesterol homeostasis on cholesterol and lipoprotein metabolism in the F1B hamster

Cholesterol status and dietary fat alter several metabolic pathways reflected in lipoprotein profiles. To assess plasma lipoprotein response and mechanisms by which cholesterol and dietary fat type regulate expression of genes involved in lipoprotein metabolism, we developed an experimental model system using F1B hamsters fed diets (12 weeks) enriched in 10% (wt/wt) coconut, olive, or safflower oil with either high cholesterol (0.1%; cholesterol supplemented) or low cholesterol coupled with cholesterol-lowering drugs 10 days before killing (0.01% cholesterol, 0.15% lovastatin, 2% cholestyramine; cholesterol depleted). Irrespective of dietary fat, cholesterol depletion, relative to supplementation, resulted in lower plasma non-high-density lipoprotein (non-HDL) and HDL cholesterol, and triglyceride concentrations (all Ps < .05). In the liver, these differences were associated with higher sterol regulatory element binding protein-2, low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and 7α-hydroxylase messenger RNA (mRNA) levels; higher scavenger receptor B1 and apolipoprotein A-I mRNA and protein levels; lower apolipoprotein E protein levels; and in intestine, modestly lower sterol transporters adenosine triphosphate-binding cassette (ABC) A1, ABCG5, and ABCG8 mRNA levels. Irrespective of cholesterol status, coconut oil, relative to olive and safflower oils, resulted in higher non-HDL cholesterol and triglyceride concentrations (both Ps < .05) and modestly higher sterol regulatory element binding protein-2 mRNA levels. These data suggest that, in F1B hamsters, differences in plasma lipoprotein profiles in response to cholesterol depletion are associated with changes in the expression of genes involved in cholesterol metabolism, whereas the effect of dietary fat type on gene expression was modest, which limits the usefulness of the experimental animal model.

Individual trans octadecenoic acids and partially hydrogenated vegetable oil differentially affect hepatic lipid and lipoprotein metabolism in golden Syrian hamsters

Trans fatty acids (TFA) from industrial sources [i.e. partially hydrogenated vegetable oil (PHVO)] have been associated with several chronic human diseases, especially coronary heart disease (CHD). The possible contribution of individual TFA to overall CHD risk remains largely unknown. The objective of the present study was to investigate the effects of 2 major trans 18:1 isomers, trans-9 18:1 [elaidic acid (EA)] and trans-11 18:1 [vaccenic acid (VA)] on plasma lipid biomarkers of CHD risk. Thirty-two male Golden Syrian hamsters were randomly assigned to 1 of 4 dietary treatments: 1) control "Western" diet; 2) PHVO supplement; 3) EA supplement; and 4) VA supplement. Fat supplements were incorporated into the respective treatment diets at 2.5 g/100 g of diet. Compared with the control diet, the PHVO diet increased the plasma ratios of total:HDL-cholesterol and nonHDL:HDL-cholesterol by 17 and 23%, respectively. In contrast, these values decreased by 27 and 46% after the EA treatment and 8 and 14% after the VA treatment, respectively, indicating an improvement (reduction) in CHD risk. With regard to liver lipids, the EA diet reduced the content of (n-3) and (n-6) PUFA relative to the other treatments, suggesting an inhibition of enzymes common to the 2 biosynthesis pathways. Overall, results demonstrate that the hypercholesterolemic effects of PHVO are not dependent on the presence of EA or VA and that other bioactive components in PHVO must be responsible for its associated adverse health effects.

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

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

Butters Varying in trans 18:1 and cis-9,trans-11 Conjugated Linoleic Acid Modify Plasma Lipoproteins in the Hypercholesterolemic Rabbit

The experiment was designed to study the effects of butters differing in conjugated linoleic acid (CLA) and trans 18:1 contents on lipoproteins associated with the risk of atherogenesis. New Zealand White male rabbits (9.6 weeks; 2.1 kg) were assigned for 6 or 12 weeks to three diets (n = 6 per diet) made of conventional pellets with 0.2% cholesterol and with 12% fat provided from a butter poor in trans-10 and trans-11 18:1 and in CLA (standard group), or rich in trans-10 18:1 (trans-10 18:1 group) or rich in trans-11 18:1 and in cis-9,trans-11 CLA (trans-11 18:1/CLA group). Blood samples were collected at the end of dietary treatments. Lipoproteins were separated by gradient-density ultracentrifugation. Lipid classes were determined enzymatically and apolipoproteins A-I and B by radial immunodiffusion. Mainly in the 12-week rabbits, higher plasma triglycerides and apolipoprotein B levels shown in the standard and trans-10 18:1 groups compared with those in the trans-11 18:1/CLA group are associated with higher plasma levels of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) also shown in these two groups. In the 12-week rabbits, a shift towards denser LDL, considered as more atherogenic, was shown only in the trans-10 18:1 group. In these animals, the VLDL + LDL to HDL ratio was 1.7-2.3 times higher in the trans-10 18:1 group than in the other groups (P = 0.076). These results suggest a rather neutral effect of trans-11 18:1/CLA butter towards the risk of atherogenesis, whereas trans-10 18:1 butter would tend to be detrimental.

Dietary fatty acids differentially modulate messenger RNA abundance of low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and microsomal triglyceride transfer protein in Golden-Syrian hamsters

Dietary fatty acids modulate plasma and intracellular cholesterol concentrations. Circulating non-high-density lipoprotein cholesterol (nHDL-C) concentration is determined by rates of hepatic very low-density lipoprotein assembly and secretion, and clearance of subsequent metabolic products. The effect of dietary fat (butter, traditional margarine, soybean oil, and canola oil) was assessed with respect to plasma lipids, hepatic lipid composition, and messenger RNA (mRNA) abundance of low-density lipoprotein (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, sterol regulatory element-binding protein (SREBP) 2, and microsomal triglyceride transfer protein (MTP) in the Golden-Syrian hamster (Charles River Laboratories, Wilmington, MA). Hamsters were fed with a nonpurified diet (6.25 fat g/100 g) with 0.1 g cholesterol/100 g (control diet) or control diet with an additional 10 g experimental fat/100 g for 12 weeks. Hamsters fed with the control diet, unsaturated fats (canola and soybean oils), and margarine, relative to butter, had significantly lower total cholesterol and nHDL-C and triglyceride concentrations. Additional dietary fat, regardless of fatty acid profile, resulted in higher hepatic cholesterol concentrations. In contrast, relative to the control diet-, butter-, or margarine-fed hamsters, these changes were associated with a 4- and 8-fold higher LDL receptor and 5- and 9-fold higher SREBP mRNA abundance, in hamsters fed with canola and soybean oils, respectively. MTP mRNA, a marker of very low-density lipoprotein particle formation, was higher in canola- and soybean oil-fed hamsters relative to the control diet-fed hamsters, although differences were modest. These results suggest that the substitution of canola and soybean oils for butter results in lower nHDL-C concentrations that may be related to increased mRNA abundance of the LDL receptor, SREBP-2, and MTP genes.

Butter naturally enriched in conjugated linoleic acid and vaccenic acid alters tissue fatty acids and improves the plasma lipoprotein profile in cholesterol-fed hamsters

Butter, which is naturally enriched in cis-9, trans-11 conjugated linoleic acid (rumenic acid; RA) and vaccenic acid (VA), has been shown to be an effective anticarcinogen in studies with animal models; however, there has been no examination of the effects of a naturally derived source of VA and RA on atherosclerosis-related biomarkers. The current study was designed to determine the effect of a diet containing VA/RA-enriched butter on plasma lipoproteins and tissue fatty acid profiles in cholesterol-fed hamsters. Male Golden Syrian hamsters were fed diets containing 0.2% cholesterol and 20% added fat as: 1) Control, 20% standard butter (CT); 2) 5% standard butter + 15% VA/RA-enriched butter (EB); 3) 15% standard butter + 5% partially-hydrogenated vegetable oil (VO). After 4 wk, plasma lipoproteins were isolated, cholesterol quantified, and tissue fatty acid profiles determined. Tissue concentrations of VA and RA were increased by consumption of the EB diet compared with both the CT and VO diets, whereas the VO diet increased their concentration compared with the CT diet only. Total and LDL cholesterol concentrations were significantly reduced in hamsters fed EB and VO compared with CT, whereas VLDL cholesterol concentrations were reduced in hamsters fed EB compared with those fed CT and VO. HDL cholesterol concentrations did not differ among treatments. The ratio of potentially atherogenic lipoproteins [VLDL + intermediate density lipoproteins (IDL) + LDL] to antiatherogenic HDL was significantly lower in hamsters fed VA/RA-enriched butter (0.60) than in those fed either control diet (1.70) or the diet containing partially hydrogenated vegetable oil (1.04). Thus, increasing the VA/RA concentration of butter results in a plasma lipoprotein cholesterol profile that is associated with a reduced risk of atherosclerosis.

Dietary trans-18:1 raises plasma triglycerides and VLDL cholesterol when replacing either 16:0 or 18:0 in gerbils

To compare the relative impact of trans-18:1 with the two main dietary saturated fatty acids it replaces, plasma lipid response was assessed in Mongolian gerbils fed diets rich in 16:0 (24%en),18:0 (10%en), or trans-18:1 (4 or 6%en). The diets were designed such that the 18:0-rich diet substituted 7%en as 18:0 for 16:0, whereas 4%en and 6%en from trans-18:1 was substituted for 16:0 in the two trans diets. The control group was fed a diet formulated according to the fatty acid balance of American Heart Association (AHA), but provided 40%en as fat. Gerbils (n = 10 per dietary group) were fed one of the five diets for 8 weeks. The control diet, with 4 times the polyunsaturated fatty acids (PUFA) content and a P:S ratio about 10 times greater than the test diets, resulted in the lowest plasma TC, LDL cholesterol (LDL-C) and VLDL cholesterol (VLDL-C). Among the test diets, plasma TC and TG were lowest with the 18:0-rich diet. TC in gerbils fed the 16:0-rich diet and 4%en-trans were 20% higher than the 18:0-rich diet, while the 6%en-trans diet was 35% higher. VLDL-C was significantly higher in the 6%en-trans diet compared to all other groups at 8 weeks. Both trans fatty acid diets elevated plasma TG approximately 2- and 3-fold, respectively, compared to the 16:0-rich and 18:0-rich diets at 8 weeks. Further, plasma TG continued to rise over time with trans fatty acids compared to 16:0 or 18:0. Thus, in the fatty acid-sensitive gerbil, impaired TG metabolism represents a major aspect of the hyperlipemia caused by trans fatty acid substitution for major saturated fatty acids.

Trans fatty acids affect lipoprotein metabolism in rats

This study was designed to investigate the effects of oleic (CIS), palmitic (SAT) and trans fatty acids (TRANS) on cholesterol metabolism. Rats fed the TRANS diet had lower plasma total cholesterol (P < 0.005) and non-HDL-cholesterol (non HDL-C) concentrations (P < 0.005) compared with their CIS-fed counterparts. Plasma HDL-C was highest in rats fed the SAT diet (P = 0.01). An in vivo assay of reverse cholesterol transport (RCT) was performed whereby radiolabeled cholesterol was delivered to the liver as acetylated LDL and the reappearance of label into plasma and HDL was determined. Plasma radioactivity in TRANS-fed rats was lower than in their SAT-fed counterparts (P = 0.01), and consistent with the cholesterol distribution in plasma, the difference was due to lower [(3)H]-cholesterol in lower density lipoproteins. Despite diet-induced differences in the cholesterol and phospholipid concentrations and fatty acid composition of HDL, the amount of label in HDL did not differ among groups, suggesting that consumption of these diets resulted in HDL populations with similar capacity to participate in RCT. The present findings suggest that dietary trans fatty acids regulate the metabolism of apolipoprotein B-containing lipoproteins in rats and that the effect may be masked in species possessing high plasma cholesteryl ester transfer protein (CETP) activity. These results reinforce the important role of CETP activity in determining the distribution of plasma cholesterol in response to dietary trans fatty acids.

Effect of dietary elaidic versus vaccenic acid on blood and liver lipids in the hamster

Male hamsters (30 per group) were fed five different semi-purified diets ad libitum. The diets, containing 30% of energy (en%) as fat, differed in their dietary fat composition (specified fatty acids exchanged at 10 en%) and were fed for 4 weeks. The five fatty acids compared in mixed triglycerides were elaidic acid (C18:1 9t), vaccenic acid (C18:1 11t), their cis-counterpart oleic acid (C18:1 9c), medium-chain fatty acids (MCFA; C8:0 and C10:0), and palmitic acid (C16:0). Compared with oleic acid, dietary MCFA and palmitic acid tended to increase blood cholesterol levels in the hamsters. The effect of elaidic and vaccenic acid on blood cholesterol did not differ from that of oleic acid. When elaidic acid and vaccenic acids were compared directly, the ratio of LDL/HDL-cholesterol in plasma was significantly higher in hamsters fed vaccenic acid than in those fed elaidic acid, and elaidic acid was incorporated at low levels, but more efficiently than vaccenic acid at the sn-2 position of platelet phospholipids. Biological consequences of this low incorporation are considered unlikely as levels of arachidonic acid (C20:4 n-6) and docosohexaenoic acid (C22:6 n-3) in the platelet phospholipids of all dietary groups did not differ. With respect to the effect on the LDL/HDL-cholesterol ratio, elaidic acid may be preferable to vaccenic acid. We conclude that this animal study does not provide evidence for the suggestion, based on epidemiological observations, that elaidic acid would be more detrimental to cardiovascular risk than vaccenic acid.

Cholesterol balance and metabolism in mice with loss of function of Niemann-Pick C protein

Type C Niemann-Pick disease is due to a mutation in Niemann-Pick C (NPC) protein, a putative determinant of intracellular cholesterol transport. This study quantifies cholesterol balance in vivo across all tissues in mice with this defect. Cholesterol balance in the heterozygous animal is normal, but in the homozygous mouse the whole animal cholesterol pool expands continuously from birth, reaching 5, 442 mg/kg at 7 wk. The size of this pool in each organ is proportional to the rate at which each tissue clears low-density lipoprotein-cholesterol. Despite this expansion, however, cholesterol synthesis is increased so that whole animal synthesis equals 180 mg. day-1. kg-1. Forcing additional cholesterol into the liver through the clathrin-coated pit pathway increases the hepatic cholesterol pool in control mice, all of which is esterified, while there is a much greater increase in this pool in mutant mice, all of which is unesterified. These findings are consistent with the view that there is a block in sterol movement from the lysosome to the sites of regulation in NPC disease and have important implications for understanding the function of the NPC protein in intracellular cholesterol metabolism, in general, and in the brain, in particular.

Dietary monounsaturated fatty acids promote aortic atherosclerosis in LDL receptor-null, human ApoB100-overexpressing transgenic mice

In mice with genetically engineered high levels of plasma low density lipoprotein (LDL), we tested the hypothesis that an increase in the dietary content of monounsaturated fatty acids but not of polyunsaturated fatty acids would promote atherosclerosis. The mouse model used was an LDL receptor-null, human apoB100-overexpressing strain. Six experimental groups of 19 to 38 mice of both sexes were established when the animals had reached 8 weeks of age. For the next 16 weeks, individual groups were fed either a commercial diet or prepared diets including fat as 10% of energy, with 5 different fatty acid enrichment patterns including the following: saturated (sat), cis and trans monounsaturated (mono), and n-3 and n-6 polyunsaturated (poly). Highly significant differences (ANOVA, P<0. 0001) in LDL cholesterol (in mg/dL) were found, with the rank order at 16 weeks being trans mono (mean, 1390)>sat (922)=cis mono (869)=n-6 poly (868)>n-3 poly (652)>commercial diet (526). Significant elevations in very low density lipoprotein cholesterol were also found in the trans and cis mono and sat groups, and triacylglycerol concentrations were also elevated in all groups. High density lipoprotein cholesterol concentrations were consistently low (20 to 50 mg/dL) in all groups. Highly significant differences (ANOVA, P<0.0001) in atherosclerosis, quantified by measurement of aortic cholesteryl ester concentration (mg/g protein) among dietary fatty acid groups were found, with the order being trans mono (mean, 50.4)>sat (35.6)=cis mono (34.6)>n-6 poly (18. 3)=n-3 poly (9.7)=commercial diet (7.8). Therefore, in this mouse model of hypercholesterolemia, dietary cis or trans monounsaturated fat did not protect against atherosclerosis development, whereas aortic atherosclerosis in either of the polyunsaturated fat groups was significantly less than in the saturated fat group.

Free fatty acids modulate LDL receptor activity in BHK-21 cells

It has been shown that dietary fatty acids affect serum low density lipoprotein (LDL) levels, but the mechanism responsible for this effect is still under debate. Here we investigate the effect of different free fatty acids on LDL receptor activity in BHK-21 cells. These cells possess a classical LDL receptor strongly regulated by substances like 25-OH-cholesterol or lovastatin. Preincubation of cells for 24 h with both oleic (cis 18:1) and its trans counterpart, elaidic acid, enhanced 125I-LDL binding, internalization and degradation, being oleic acid more effective than elaidic acid. Among polyunsaturated fatty acids (PUFA) of the n-6 series arachidonic acid (20:4) enhanced LDL receptor activity more than linoleic acid (18:2), and among PUFA of the n-3 series docosahexaenoic (22:6) and eicosapentaenoic acids (20:5) were more effective compared to alpha-linolenic acid (18:3). Conversely, preincubation of cells with saturated fatty acids, palmitic (16:0) and stearic (18:0) acids, decreased binding, internalization and degradation of 125I-LDL. Scatchard analysis of binding data obtained with palmitic and oleic acids showed that these two fatty acids affect LDL receptor number without altering receptor affinity. The regulatory effect of free fatty acids on LDL receptor activity in BHK-21 cells is consistent with the hypothesis that the ability of fatty acids to modulate LDL-cholesterol levels in vivo is mediated, at least in part, by an action on receptor-dependent uptake of LDL.

Dietary fatty acids and the regulation of plasma low density lipoprotein cholesterol concentrations

Epidemiologic studies over the past 25 years have shown that the level of dietary fat intake is positively correlated with the average serum cholesterol value and mortality from coronary heart disease (CHD). A number of different investigators demonstrated that in addition to total fat, the fatty acid composition of diets influenced serum total cholesterol (TC) in humans. In general, saturated fatty acids were found to elevate the serum cholesterol concentration, and unsaturated fatty acids were found to decrease this value. The lipoprotein fraction most affected was the level of cholesterol carried in low density lipoprotein (LDL-C). It has now been demonstrated that the steady-state level of LDL-C is predominantly dictated by metabolic events in the liver. As the amount of dietary cholesterol entering the body is increased, there is expansion of the sterol pool in the liver cell and down regulation of LDL receptors (LDLR) that are primarily responsible for clearing LDL-C from the blood stream. When dietary cholesterol intake is kept constant, however, long-chain saturated fatty acids further suppress hepatic LDLR activity, whereas several unsaturated fatty acids have the opposite effect. These regulatory events depend upon the availability of the various fatty acids to shift intracellular cholesterol between a regulatory and storage pool of cholesterol, and this effect is mediated by the enzyme acyl-CoA:cholesterol acyltransferase (ACAT).

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.

Linoleic and alpha-linolenic acids differently modify the effects of elaidic acid on polyunsaturated fatty acid metabolism and some immune indices in rats

To explore whether the metabolic responses to trans, compared with cis, fatty acids depend on the source of dietary polyunsaturated fatty acids (PUFA), male Sprague-Dawley rats, 5 weeks old, were fed on diets containing 30 g oleic (cis) or elaidic (trans) acids/kg in combination with either 70 g perilla oil (alpha-linolenic acid) or safflowerseed oil (linoleic acid)/kg for 3 weeks in separate experiments. The dietary fats were adjusted to have the same level of total PUFA. The dietary manipulation did not influence the growth indices, but spleen weight was greater when the dietary PUFA source was perilla oil. The incorporation of trans fatty acid into liver phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine and adipose tissue lipids, particularly phospholipids, was significantly higher when rats were fed on safflowerseed oil compared with perilla oil. However, only limited differences were observed in the effects of cis and trans fatty acids on the proportions of PUFA in liver phospholipids. Splenic production of prostaglandin E2 was reduced by trans fatty acid when safflowerseed oil was the PUFA source, but no trans effect was observed on leukotriene C4 production. Dietary PUFA significantly influenced the concentration of plasma immunoglobulins (Ig) but the effect of geometry was only seen in IgG which was increased by trans acid. Dietary trans fatty acid increased the CD4+:CD8+ T-lymphocyte ratio in the spleen, reflecting a decreasing trend of the proportion of CD8+, when combined with perilla oil. These observations indicate that the type of PUFA simultaneously ingested specifically influences the effect that trans acid exerts on PUFA metabolism, eicosanoid production and some immune indices.

Replacing dietary palmitic acid with elaidic acid (t-C18:1 delta9) depresses HDL and increases CETP activity in cebus monkeys

The question whether dietary trans fatty acids affect lipoprotein metabolism similarly to specific saturated fatty acids was investigated in 11 normolipemic cebus monkeys by exchanging 5% dietary energy (%en) between elaidic (t-C18:1 delta9) and palmitic acid (16:0) in two test diets (30%en fat + 100 mg cholesterol/1000 kcal diet) conforming to the American Heart Association (AHA) Step 1 guidelines. These were compared with a normal control diet rich in saturated fat and cholesterol (38%en fat + 180 mg cholesterol/1000 kcal diet). The control diet was fed initially for 14 wk, followed by each of the the two test diets in a crossover design. Plasma lipid concentrations were determined four times between the 6th and 14th wk. Turnover studies (using 125I-HDL and 131I-LDL) were conducted after 9 wk in each dietary period. Relative to the control diet, both test diets significantly reduced plasma total cholesterol (TC), HDL cholesterol (HDL-C) and VLDL plus LDL cholesterol (LDL-C) concentrations; triglyceride (TG) concentrations tended to be lower. However, the trans diet resulted in a significantly greater reduction in HDL-C than the palmitate diet (124 +/- 17, 117 +/- 18 and 106 +/- 13 mg/dL for the control, palmitate and trans diets, respectively). The palmitate diet significantly decreased the TC/HDL-C ratio by 11% when compared with the control diet (1.68 +/- 0.17 vs. 1.89 +/- 0.30), whereas the trans diet had no effect (1.81 +/- 0.20 vs. 1.89 +/- 0.30). Kinetic studies revealed that, relative to the control diet, both test diets significantly lowered the LDL apolipoprotein B (apoB) pool size, principally reflecting an increase in the LDL apoB fractional catabolic rate (FCR) related to the reduced cholesterol intake. Between the two test diets, no significant differences in LDL kinetic parameters were observed. Both test diets significantly decreased HDL apoA1 concentrations in comparison with the control diet, which was partly explained by an increase in the fractional catabolic rate of HDL. Of the two test diets, the trans diet was associated with a 9.5% greater HDL FCR than the palmitate diet (P < 0.08) and a significant increase in plasma cholesteryl ester transfer protein (CETP) activity (% transfer 114 +/- 7 vs. 91 +/- 7; P < 0.03). Thus, palmitic acid- and elaidic acid-rich diets produced identical effects on LDL metabolism in normocholesterolemic cebus monkeys fed diets with low levels of cholesterol, whereas elaidic acid depressed HDL-C, attributable to both increased CETP activity and HDL clearance.

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.

Compared with dietary monounsaturated and saturated fat, polyunsaturated fat protects African green monkeys from coronary artery atherosclerosis

Atherogenic diets enriched in saturated, n-6 polyunsaturated, and monounsaturated fatty acids were fed to African green monkeys for 5 years to define effects on plasma lipoproteins and coronary artery atherosclerosis. The monkeys fed polyunsaturated and monounsaturated fat had similar plasma concentrations of LDL cholesterol, and these values were significantly lower than for LDL in the animals fed saturated fat. Plasma HDL cholesterol concentrations were comparable in animals fed saturated and monounsaturated fat and were significantly higher than in animals fed polyunsaturated fat. Thus, the monounsaturated fat group had the lowest LDL/HDL ratio. LDL particle size was largest in the saturated and monounsaturated fat groups, significantly larger than in the polyunsaturated fat group. LDL particle enrichment with cholesteryl oleate was the greatest in the animals fed monounsaturated fat, next greatest in the saturated fat-fed animals, and was least in the polyunsaturated fat-fed animals. Coronary artery atherosclerosis as measured by intimal area was less in the polyunsaturated fat compared with the saturated fat groups, was less in the animals fed polyunsaturated fat compared with the monounsaturated fat-fed animals, but did not differ between the monounsaturated and saturated fat groups. Cholesteryl ester, particularly cholesteryl oleate, accumulation in the coronary arteries was also similar between groups fed monounsaturated and saturated fat but was minimal in the animals fed polyunsaturated fat. In sum, the monkeys fed monounsaturated fat developed equivalent amounts of coronary artery atherosclerosis as those fed saturated fat, but monkeys fed polyunsaturated fat developed less. The beneficial effects of the lower LDL and higher HDL in the animals fed monounsaturated fat apparently were offset by the atherogenic shifts in LDL particle composition. Dietary polyunsaturated fat appears to result in the least amount of coronary artery atherosclerosis because it prevents cholesteryl oleate accumulation in LDL and the coronary arteries in these primates.

Hyperlipidemic effects of trans fatty acids are accentuated by dietary cholesterol in gerbils

Trans isomers of dietary fatty acids, generated during the commercial hydrogenation of unsaturated fats, may contribute to coronary heart disease (CHD) in humans by interfering with lipid metabolism. To examine this possibility in a fat-sensitive model, the Mongolian gerbil (Meriones unguiculatus) was used to compare the cholesterolemic and triglyceridemic potential of modest increments of trans fatty acids from partially hydrogenated soybean oil with other saturated fatty acids in the presence and absence of dietary cholesterol. Age-, dose-, and time-dependent effects were examined in weanling, 6-month-old, and 1-year-old gerbils. Although lipoprotein metabolism in weanling gerbils was initially refractory to trans fat, even as perturbations by saturated fatty acids were demonstrable, these gerbils eventually (after 16 weeks) developed a trans-induced hypercholesterolemia that was intermediate between the response to 16:0 and 12:0 + 14:0. The hepatic and plasma 18:1/18:2 cholesteryl ester (CE) ratio was depressed by trans in a manner similar to saturated fatty acids. The 6-month-old gerbils readily developed hypertriglyceridemia but not hypercholesterolemia, again revealing a decrease in the plasma 18:1/18:2 CE ratio. The 1-year-old gerbils revealed a dose-related (0, 5, 10%en as trans) elevation in total cholesterol (TC), and especially triglycerides (TG), that was accentuated by 0.04% dietary cholesterol. Increases in plasma lipids were again accompanied by a significant decrease in the mass of hepatic esterified cholesterol, particularly 18:1-cholesteryl esters. Thus, dietary trans-fatty acids induce age-, time-, and dose-dependent modulations in gerbil plasma lipids associated with decreased 18:1 cholesteryl esters. Further investigation with gerbils may reveal mechanisms by which trans fat consumption disturbs lipoprotein metabolism.