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.     

Occurrence of trans fatty acids in rats fed a trans-free diet: A free radical-mediated formation?

Trans isomers of unsaturated fatty acids are absorbed from the diet, due to their presence in diary fat and hydrogenated vegetable oils, and health concern has risen due to their effects on lipid risk factors in cardiovascular diseases. On the basis of the efficiency of the thiyl-radical-catalyzed cis/trans isomerization in vitro and the presence of many sulfur-containing compounds in the cell, the aim of this study was to demonstrate that trans geometry of lipid double bonds can be endogenously generated within membrane phospholipids. The study reports trans fatty acids occurrence in tissue and erythrocyte phospholipids of young adult rats fed a diet completely free of trans isomers. Results show that tissues are differently prone to the endogenous isomerization and that, following a free radical attack, trans fatty acids can reach very high amounts. The effectiveness of this process is considerably inhibited in the presence of all-trans retinol, confirming previous data in model membranes. Our results suggest that geometrical isomerization of unsaturated fatty acids, which causes a structural modification of membrane lipids and may influence basic membrane properties and vital biochemical functions, can occur under radical stress conditions and could be efficiently prevented by vitamin A.     

The 1990 Borden Award Lecture. Dietary regulation of fatty acid and triglyceride metabolism.

The level of circulating triacylglycerols is determined by the balance between their delivery into the plasma and their removal from it. Plasma triacylglycerols are derived either from dietary fat as chylomicrons or from endogenous hepatic synthesis as very low density lipoproteins. Their removal occurs through the action of lipoprotein lipase after which the fatty acids are either stored in adipose tissue or oxidized, primarily in skeletal muscle and heart. The composition of the diet has been shown to influence many of these processes. Hepatic fatty acid synthesis and triacylglycerol secretion are affected by the quantity and composition of dietary fat, carbohydrate, and protein. Polyunsaturated but not saturated fats reduce hepatic fatty acid synthesis by decreasing the amount of the lipogenic enzymes needed for de novo fatty acid synthesis. Dietary fish oils are particularly effective at reducing both fatty acid synthesis and triacylglycerol secretion and as a result are hypotriacylglycerolemic, particularly in hypertriacylglycerolemic individuals. In addition, dietary fish oils can increase the oxidation of fatty acids and lead to increased activity of lipoprotein lipase in skeletal muscle and heart. It appears that the hypotriacylglycerolemic effect of dietary fish oils is mediated by effects on both synthesis and removal of circulating triacylglycerols.     

Reduction in plasma cholesterol and increase in biliary cholesterol by a diet rich in n-3 fatty acids in the rat

Cholesterol and lipoprotein metabolism were investigated in a group of rats fed a fish oil-supplemented diet, a rich source of n-3 fatty acids. For comparison purposes, other groups of rats were fed either safflower oil (n-6 fatty acids) or coconut oil (saturated fatty acids). Diets were isocaloric and contained identical amounts of cholesterol. Rats fed fish oils for 2 weeks showed a 35% lower plasma cholesterol level than rats fed safflower oil, who in turn showed a 14% lower plasma cholesterol level than those fed coconut oil. The fall in plasma cholesterol level with fish oils was associated with significant falls in low density and high density lipoprotein cholesterol levels, but with no significant change in the ratio of low density to high density lipoprotein cholesterol. The fatty acid compositions of plasma, hepatic, and biliary lipids showed relative enrichment with n-3 fatty acids, reflecting the composition of the diet. The fish oil diet increased the basal secretion rate of cholesterol into bile, but the bile acid secretion rate remained unchanged. It is suggested that n-3 fatty acids reduce the plasma cholesterol level in rats by increasing the transfer of cholesterol into bile.     

Nutritional properties of trans fatty acids

The role of trans fatty acids (TFA) present in partially hydrogenated fats widely consumed in food and their link with coronary heart disease has been examined in this review. Most of the studies carried out have been on the effects of TFA on blood-lipid profile. The perceived effects of TFA intake depend on the fat or oil with which they are compared and appears to be in between that of dietary saturated fats and monounsaturated fatty acids. When compared to saturated fat, TFA intake shows lower levels of total and LDL-cholesterol in blood. But when both TFA and saturated fatty acids are compared with cis fatty acids or native unhydrogenated oil, increase in total and LDL-cholesterol are noted. The effects of TFA on HDL-cholesterol and Lp(a) are not clearly established. The undesirable effects of TFA can be overcome by inclusion of essential fatty acids at a minimum of 2 energy per cent level in the diet. The link between trans fatty acid intake and coronary heart disease (CHD) are not unequivocally established.     

Dietary lipids and blood cholesterol: quantitative meta-analysis of metabolic ward studies.

OBJECTIVE: To determine the quantitative importance of dietary fatty acids and dietary cholesterol to blood concentrations of total, low density lipoprotein, and high density lipoprotein cholesterol. DESIGN: Meta-analysis of metabolic ward studies of solid food diets in healthy volunteers. SUBJECTS: 395 dietary experiments (median duration 1 month) among 129 groups of individuals. RESULTS: Isocaloric replacement of saturated fats by complex carbohydrates for 10% of dietary calories resulted in blood total cholesterol falling by 0.52 (SE 0.03) mmol/l and low density lipoprotein cholesterol falling by 0.36 (0.05) mmol/l. Isocaloric replacement of complex carbohydrates by polyunsaturated fats for 5% of dietary calories resulted in total cholesterol falling by a further 0.13 (0.02) mmol/l and low density lipoprotein cholesterol falling by 0.11 (0.02) mmol/l. Similar replacement of carbohydrates by monounsaturated fats produced no significant effect on total or low density lipoprotein cholesterol. Avoiding 200 mg/day dietary cholesterol further decreased blood total cholesterol by 0.13 (0.02) mmol/l and low density lipoprotein cholesterol by 0.10 (0.02) mmol/l. CONCLUSIONS: In typical British diets replacing 60% of saturated fats by other fats and avoiding 60% of dietary cholesterol would reduce blood total cholesterol by about 0.8 mmol/l (that is, by 10-15%), with four fifths of this reduction being in low density lipoprotein cholesterol.     

The retina is more susceptible than the brain and the liver to the incorporation of trans isomers of DHA in rats consuming trans isomers of alpha-linolenic acid

Trans polyunsaturated fatty acids are formed during heat treatments of vegetable oils from polyunsaturated fatty acids containing cis double bonds. After dietary intake, they are distributed in the body and are incorporated into nervous tissues including the retina. Since nervous tissues are known to be rich in n-3 fatty acids such as docosahexaenoic acid (DHA), we studied the ability of the retina and the brain to incorporate trans isomers of DHA formed in vivo from the dietary precursor trans alpha-linolenic acid. Wistar rats were fed with trans isomers of alpha-linolenic acid for 21 months. A linear incorporation of trans DHA and a decrease in cis DHA was observed in the retina, whereas no major changes were observed in the brain. In parallel to the modifications in retinal cis and trans DHA levels, the retinal functionality evaluated by the electroretinogram showed defects in animals that consumed trans alpha-linolenic acid. These results suggest that the mechanisms leading to the incorporation of cis and trans fatty acids are quite different in the retina when compared to the brain and the liver, the retina being more susceptible to changes in the dietary lipid contribution.     

Microbial biohydrogenation of oleic acid to trans isomers in vitro

Ruminant products are significant sources of dietary trans fatty acids. Trans fatty acids, including various conjugated linoleic acid isomers, have been shown to act as metabolic modifiers of lipid metabolism. Trans fatty acids originate from biohydrogenation of dietary unsaturated fatty acids by gut microbes; however, the exact synthetic pathways are unclear. It was our goal to examine the biohydrogenation pathway for oleic acid, where oleic acid is hydrogenated directly to stearic acid. Our objective in this study was to trace the time course of appearance of 13C in labeled oleic acid to determine if trans monoenes are formed from the 13C-labeled oleic acid or if the 13C appears only in stearic acid as described in reviews of earlier work. Enrichments were calculated from the mass abundance of 13C in major fatty acid fragments and expressed as a percentage of total carbon isotopomers. Significant 13C enrichment was found in stearic acid, oleic acid, trans-6, trans-7, and in all trans C18:1 in positions 9-16. We concluded that the biohydrogenation of oleic acid by mixed ruminal microbes involves the formation of several positional isomers of trans monoenes rather than only direct biohydrogenation to form stearic acid as previously described.     

Effects on plasma lipoproteins of monounsaturated, saturated, and polyunsaturated fatty acids in the diet of African green monkeys

Work by other investigators has shown that an increase in dietary content of monounsaturated fatty acids can result in a decreased plasma low density lipoprotein (LDL) cholesterol concentration. This observation, combined with the epidemiologic evidence that monounsaturated fat-rich diets are associated with decreased rates of death from coronary heart disease, suggests that inclusion of increased amounts of mono-unsaturated fat in the diet may be beneficial. The present study was carried out in a primate model, the African green monkey, to evaluate the effects of dietary monounsaturated fat on plasma lipoprotein cholesterol endpoints. Two study periods were carried out in which the fatty acid compositions of the experimental diets were varied. All diets contained 35% of calories as fat. In the first experimental period, a mixture of fats was used to set the dietary fatty acid composition to be approximately 50-60% of the desired fatty acid, either saturated, monounsaturated, or polyunsaturated (n-6). In the second experimental period, pure fats were used (palm oil, oleic acid-rich safflower oil, and linoleic acid-rich safflower oil) to maximize the difference in fatty acid composition. The effects of the more exaggerated dietary fatty acid differences of period 2 were similar to those that have been reported in humans. For the group fed the diet enriched in monounsaturated fat compared to saturated fat, whole plasma and LDL cholesterol concentrations were significantly lower while high density lipoprotein (HDL) cholesterol concentrations were not affected. For the group fed the diet enriched in polyunsaturated fat compared to saturated fat, both LDL and HDL cholesterol concentrations were significantly lower than in the group fed saturated fat. LDL cholesterol concentrations were comparable in the monounsaturated and polyunsaturated fat groups and the percentage of cholesterol in LDL was lowest in the monounsaturated fat fed group. Trends were similar for the mixed fat diets, although no statistically significant differences in plasma lipoprotein endpoints could be attributed to monounsaturated fatty acids in this dietary comparison. Since effects on plasma lipoproteins similar to those seen in humans were identified in this primate model, relevant mechanisms for the effects of dietary fatty acids on lipoprotein endpoints related to coronary artery atherosclerosis, per se, can subsequently be examined.     

Trans fatty acids enhance amyloidogenic processing of the Alzheimer amyloid precursor protein (APP)

Hydrogenation of oils and diary products of ruminant animals leads to an increasing amount of trans fatty acids in the human diet. Trans fatty acids are incorporated in several lipids and accumulate in the membrane of cells. Here we systematically investigate whether the regulated intramembrane proteolysis of the amyloid precursor protein (APP) is affected by trans fatty acids compared to the cis conformation. Our experiments clearly show that trans fatty acids compared to cis fatty acids increase amyloidogenic and decrease nonamyloidogenic processing of APP, resulting in an increased production of amyloid beta (Aβ) peptides, main components of senile plaques, which are a characteristic neuropathological hallmark for Alzheimer's disease (AD). Moreover, our results show that oligomerization and aggregation of Aβ are increased by trans fatty acids. The mechanisms identified by this in vitro study suggest that the intake of trans fatty acids potentially increases the AD risk or causes an earlier onset of the disease.     

Metabolic and health effects of isomeric fatty acids

PURPOSE OF REVIEW: Isomeric fatty acids have the same number of carbon and hydrogen atoms, but may have distinct metabolic and health effects. Two well-known examples of isomeric fatty acids are cis and trans monounsaturated fatty acids, and conjugated isomers of linoleic acid (CLA). The purpose of this review is to summarize recent findings from human studies on the metabolic and health effects of these two classes of isomeric fatty acids. RECENT FINDINGS: Apart from an unfavorable effect on serum lipoproteins, trans monounsaturated fatty acids from hydrogenated oils may increase plasma markers for a low-grade inflammatory state. From epidemiological studies, however, it is not possible to conclude if effects of ruminant and industrial trans fatty acids on cardiovascular risk are different. In contrast to in-vitro and animal studies, there are no indications that in humans the two most common CLA isomers (cis9,trans11-CLA and trans10,cis12-CLA) affect body composition differently. Longer-term supplementation, however, may slightly decrease body fat mass without apparent effects on plasma markers for glucose and lipid metabolism. Other studies have even reported adverse effects of CLA supplementation on insulin resistance and lipid peroxidation. SUMMARY: Evidence is increasing that trans monounsaturated fatty acids from hydrogenated oils increase plasma markers of low-grade chronic inflammation. From epidemiological studies, however, it is not clear if effects of ruminant and industrial trans fatty acids on cardiovascular risk are different. Effects of CLA on body composition remain controversial and more research is needed before the widely available CLA supplements should be advocated as an adjunct to control body weight.     

Effect of radical stress and ageing on the occurrence of trans fatty acids in rats fed a trans-free diet

In a previous paper, we demonstrated that tissue trans fatty acids can not only derive from the diet but also be endogenously formed. The central focus of this study was to prove that the in vivo isomerization occurs via a radical process. Two different models of radical insult were used: CCl(4) and AAPH injection to rats fed a diet completely free of trans isomers. Following this acute radical stress, a significant increase in unnatural trans fatty acid content of erythrocyte, kidney, and heart, but not liver, was observed. These results can be mainly explained by the high content, particularly in the liver, of antioxidant vitamins A and E that exhibit also an "anti-isomerizing" effect. Since during ageing cellular components are exposed to increasing radical insults, the observation of a significant trans fatty acid accumulation in 30-month-old rats could confirm that the in vivo formation of unnatural isomers is due to a radical process. Trans fatty acids can influence the physical characteristics of bilayer microdomains, affecting membrane properties and functions; thus, knowledge of biological radical species responsible for cis/trans isomerization and their possible sources can provide protective systems for preserving lipid geometry.     

Dietary cyclic fatty acids derived from linolenic acid do not exhibit intrinsic toxicity in the rat during gestation

Heating oils and fats may lead to cyclization of polyunsaturated fatty acids, especially those showing multiple double bonds like linolenic acid. Cyclohexenyl and cyclopentenyl fatty acids are subsequently present in some edible oils and these were suspected to induce metabolic disorders. When fed during gestation in the rat, cyclic fatty acids were historically reported to induce high mortality of the neonates. Nevertheless, none of these studies have been performed with cyclic fatty acids fed as triacylglycerols, limiting the nutritional value of the conclusions. Therefore, we assessed the toxicity of a diet containing 0.7% of cyclic fatty acids fed as triacylglycerols during gestation and the first days of life in the rat. In this work, we report no deleterious effect of cyclic fatty acids in the mothers and neonates. However, cyclic fatty acids induced a tremendous insulinopenia in the mothers and pups that was associated with the reduction of food intake in the gestating females. Such a finding may be a plausible explanation for the adverse effects of cyclic fatty acids observed previously with higher doses of cyclic fatty acids. Based on present data, on previous ones showing elimination of cyclic fatty acids, and considering their low amounts in the diet, we suggest that cyclic fatty acids formed from cyclization of linolenic acid are not a major concern for human safety.     

Production of butter fat rich in trans10-C18:1 for use in biomedical studies in rodents

Trans fatty acids are suspected to be detrimental to health, particularly to cardiovascular function. Trans fatty acids include a wide range of fatty acids, with isomers of C18:1, conjugated and non-conjugated C18:2 as major components. A vaccenic acid (trans11-C18:1) + rumenic acid (cis9,trans11-CLA)-rich butter has been shown previously to exhibit health beneficial effects, but less is known concerning another trans-C18:1 present in hydrogenated vegetable oil-based products and sometimes in milk fat, the trans10-isomer. The present experiment was conducted to produce butters from milk of variable fatty acid composition for use in biomedical studies with rodents, with the overall aim of evaluating the specific effect of trans10-C18:1 and trans11-C18:1 + cis9,trans11-CLA on cardiovascular function. Milks from lactating dairy cows fed two types of maize-based diets supplemented (5% of dry matter)--or not--with sunflower oil were collected, and used to manufacture butters either rich in trans10-C18:1 (14% of total fatty acids, 64.5% of fat content) or rich in trans11-C18:1 + cis9,trans11-CLA (7.4 and 3.1% of total fatty acids, respectively, 68.5% of fat content), or with standard fatty acid composition (70% of fat content). Additionally, total saturated fatty acid percentage was reduced by more than one third in the enriched butters compared with the standard butter. An understanding of the role of nutrition on milk fatty acid composition in cows allows for the production of dairy products of variable lipid content and composition for use in biomedical studies in animal models and human subjects.     

Trans lipid formation induced by thiols in human monocytic leukemia cells

Trans lipids in humans originate exogenously from the ingestion of isomerized fats. An endogenous path comprising a thiyl radical-catalyzed cis-trans isomerization of cis-unsaturated phospholipids was proposed. However, whether an isomerization process might be feasible in eukaryotic cells remained to be established. Here we report the presence of trans lipids in human monocytic leukemia cell membranes (THP-1) before and after treatment with a 10 mM series of thiols. Oleic, linoleic, and arachidonic acid residues of membrane phospholipids were analyzed and, unexpectedly, an initial trans lipid content was found in control cells. Then, incubation for 24 h with thiols under physiological conditions slightly increased trans lipid content. Formation of trans isomers was also evaluated in the presence of thiol and under free radical stress induced by gamma-irradiation or by thermal decomposition of azo-compounds. The similarity of isomer trends formed under incubation and stress conditions, together with the reactivity order of fatty acid residues (arachidonic > linoleic approximately oleic), indicated a common radical path and some mechanistic considerations are advanced. These results offer the first evidence that trans lipids are formed in eukaryotic cells and confirm that thiyl radicals are harmful to the integrity of cis lipid geometry. This work motivates further studies into the relationship between lipid isomerization outcome and thiyl radicals in cellular systems, as well as the formation of trans lipids and the metabolic response to such a perturbation introduced into biological membranes.     

Replacement of Dietary Saturated Fat by PUFA-Rich Pumpkin Seed Oil Attenuates Non-Alcoholic Fatty Liver Disease and Atherosclerosis Development,with Additional Health Effects of Virgin over Refined Oil

Background and Aims

As dietary saturated fatty acids are associated with metabolic and cardiovascular disease, a potentially interesting strategy to reduce disease risk is modification of the quality of fat consumed. Vegetable oils represent an attractive target for intervention, as they largely determine the intake of dietary fats. Furthermore, besides potential health effects conferred by the type of fatty acids in a vegetable oil, other minor components (e.g. phytochemicals) may also have health benefits. Here, we investigated the potential long-term health effects of isocaloric substitution of dietary fat (i.e. partial replacement of saturated by unsaturated fats), as well as putative additional effects of phytochemicals present in unrefined (virgin) oil on development of non-alcoholic fatty liver disease (NAFLD) and associated atherosclerosis. For this, we used pumpkin seed oil, because it is high in unsaturated fatty acids and a rich source of phytochemicals.

Methods

ApoE*3Leiden mice were fed a Western-type diet (CON) containing cocoa butter (15% w/w) and cholesterol (1% w/w) for 20 weeks to induce risk factors and disease endpoints. In separate groups, cocoa butter was replaced by refined (REF) or virgin (VIR) pumpkin seed oil (comparable in fatty acid composition, but different in phytochemical content).

Results

Both oils improved dyslipidaemia, with decreased (V)LDL-cholesterol and triglyceride levels in comparison with CON, and additional cholesterol-lowering effects of VIR over REF. While REF did not affect plasma inflammatory markers, VIR reduced circulating serum amyloid A and soluble vascular adhesion molecule-1. NAFLD and atherosclerosis development was modestly reduced in REF, and VIR strongly decreased liver steatosis and inflammation as well as atherosclerotic lesion area and severity.

Conclusions

Overall, we show that an isocaloric switch from a diet rich in saturated fat to a diet rich in unsaturated fat can attenuate NAFLD and atherosclerosis development. Phytochemical-rich virgin pumpkin seed oil exerts additional anti-inflammatory effects resulting in more pronounced health effects.     

Effects of a low fat diet with and without intermittent saturated fat and cholesterol ingestion on plasma lipid, lipoprotein, and apolipoprotein levels in normal volunteers

Diets low in saturated fat and cholesterol are recommended to the American public for improving plasma lipoprotein patterns and reducing the risk of heart disease. However, since dietary intake cannot always be controlled, the effects of different degrees of dietary saturated fat lowering and occasional high saturated fat and cholesterol meals on the expected lipoprotein pattern improvement of these diets needs to be defined. In the current study, we compared lipid, lipoprotein, and apolipoprotein levels in 14 young normal volunteers on a metabolic ward when they were consuming a high saturated fat diet (42% fat), an AHA Phase II diet (25% fat), and a third diet which approximated the AHA Phase I diet (30% fat). The latter actually consisted of intermittent ingestion of meals high in saturated fat and cholesterol on the background of an AHA Phase II diet (Intermittent Saturated Fat diet). When compared to the high saturated fat diet, the AHA Phase II diet significantly reduced total, low density lipoprotein (LDL), and high density lipoprotein (HDL) cholesterol, apoB, and apoA-I levels, and improved the LDL/HDL cholesterol ratio, whereas the intermittent saturated fat diet lowered total and LDL cholesterol and apoB levels, and also improved the LDL/HDL cholesterol ratio. When compared to the AHA Phase II diet, the intermittent saturated fat diet raised total and HDL cholesterol levels. Thus, in these normal volunteers, intermittent saturated fat ingestion, in the context of an overall 30% fat diet and a 25% fat diet, did not differ with respect to the effect on improving the LDL/HDL cholesterol ratio.     

Dietary lipids and forages interactions on cow and goat milk fatty acid composition and sensory properties

This review summarises the known effects of dietary factors on bovine and caprine milk fatty acid composition, as well as the regulation of cow and goat mammary lipid secretion. Special attention is given to fatty acids that could play a role for human health, such as saturated fatty acids, oleic acid, n-6- or n-3-C18 to C22 polyunsaturated fatty acids, trans isomers of C18:1 and C18:2, and isomers of conjugated linoleic acid (CLA). The main dietary factors taken into account are the nature of forages, including pasture, the forage:concentrate ratio and diet starch content, and the supplementation of dairy rations with crude or processed vegetable oils or oilseeds, and vitamin E. A particular emphasis is given to studies on interactions between these dietary factors, which show that there is a considerable plasticity of ruminant milk fatty acid composition. Despite the existence of several studies on the effects of dietary factors on the sensorial quality of milk and dairy products, there is a need to evaluate more deeply how the different feeding strategies could change the nutritional, sensorial and technological aspects of milk fat quality.     

Fish oils and plasma lipid and lipoprotein metabolism in humans: a critical review

Epidemiological studies in Greenland Eskimos led to the hypothesis that marine oils rich in n-3 fatty acids (also referred to as omega (omega)-3 fatty acids) are hypolipidemic and ultimately antiatherogenic. Metabolically controlled trials in which large amounts of fish oil were fed to normal volunteers and hyperlipidemic patients showed that these fatty acids (FAs) are effective at lowering plasma cholesterol and triglyceride levels. Although more recent trials using smaller, more practical doses of fish oil supplements have confirmed the hypotriglyceridemic effect, they have shown little effect on total cholesterol levels; hypertriglyceridemic patients have even experienced increases in low density lipoprotein cholesterol (LDL-C) levels of 10-20% while taking n-3 FA supplements. Discrepancies among fish oil studies regarding the effects of n-3 FAs on LDL-C levels may be understood by noting that, in the majority of studies reporting reductions in LDL-C levels, saturated fat intake was lowered when switching from the control diet to the fish oil diet. When fish oil is fed and saturated fat intake is constant, LDL-C levels either do not change or may increase. Levels of high density lipoprotein cholesterol have been found to increase slightly (about 5-10%) with fish oil intake. Plasma apolipoprotein levels change in concert with their associated lipoprotein cholesterol levels. Although the decrease in triglyceride levels appears to result from an inhibition in hepatic triglyceride synthesis, the mechanisms leading to the increases in LDL and HDL have not been determined. Finally, fatty fish or linolenic acid may serve as alternative sources of long-chain n-3 FAs, but further studies will be needed to document their hypolipidemic and/or antiatherogenic effects.     

Reducing saturated fatty acids in Arabidopsis seeds by expression of a Caenorhabditis elegans 16:0–specific desaturase

Plant oilseeds are a major source of nutritional oils. Their fatty acid composition, especially the proportion of saturated and unsaturated fatty acids, has important effects on human health. Because intake of saturated fats is correlated with the incidence of cardiovascular disease and diabetes, a goal of metabolic engineering is to develop oils low in saturated fatty acids. Palmitic acid (16:0) is the most abundant saturated fatty acid in the seeds of many oilseed crops and in Arabidopsis thaliana. We expressed FAT–5, a membrane‐bound desaturase cloned from Caenorhabditis elegans, in Arabidopsis using a strong seed‐specific promoter. The FAT‐5 enzyme is highly specific to 16:0 as substrate, converting it to 16:1?9; expression of fat‐5 reduced the 16:0 content of the seed by two‐thirds. Decreased 16:0 and elevated 16:1 levels were evident both in the storage and membrane lipids of seeds. Regiochemical analysis of phosphatidylcholine showed that 16:1 was distributed at both positions on the glycerolipid backbone, unlike 16:0, which is predominately found at the sn‐1 position. Seeds from a plant line homozygous for FAT–5 expression were comparable to wild type with respect to seed set and germination, while oil content and weight were somewhat reduced. These experiments demonstrate that targeted heterologous expression of a desaturase in oilseeds can reduce the level of saturated fatty acids in the oil, significantly improving its nutritional value.