Lipid Technology May 2010, Vol. 22, No. 5 103

Feature

Beneficial effects of vaccenic acid onpostprandial lipid metabolism and dyslipidemia:Impact of natural trans-fats to improve CVD risk

Ye Wang, M. Miriam Jacome-Sosa, Donna F. Vine and Spencer D. Proctor

Ye Wang and M. Miriam Jacome-Sosa are PhD Candidates, Dr. Donna F. Vine is an Assistant Professor, and Dr. Spencer D. Proctor isan Associate Professor from the Metabolic and Cardiovascular Disease Laboratory, Alberta Institute for Human Nutrition, AlbertaDiabetes Institute, 4-10 Agriculture/Forestry Centre, University of Alberta, Edmonton, Alberta, T6G 2P5, Canada. Tel: 1-780-4924672.Fax: 1-780-4929270; E-mail: flora.wang@ualberta.ca; jacome@ualberta.ca; donna.vine@ualberta.ca; spencer.proctor@ualberta.ca

Summary

Abnormal non-fasting (postprandial) lipid metabolism has been recognized as a significant contributor to dyslipidemia and cardiovasculardisease (CVD) risk. Clinically, impaired metabolism of lipoproteins following a meal (e.g. chylomicrons) has been demonstrated in anumber of chronic diseases, including obesity, insulin resistance, as well as type 1 and 2 diabetes. Given the proposed effects of dietarytrans fat to contribute to a lipid profile that increases CVD risk, there has been a public health campaign in many countries to eliminatethese fatty acids from the food supply. In contrast, our group has recently reported novel lipid-lowering benefits of a major naturally-occurring trans fatty acid vaccenic acid (VA, shorthand lipid name 18:1 trans-11), in an animal model of dyslipidemia and the metabolicsyndrome. Studies to date have shown that dietary supplementation of VA effectively reduces not only fasting lipids, but also postprandialtriacylglycerol and chylomicron concentrations in obese JCR:LA-cp rats. Evidence from animal studies to date suggest that VA may down-regulate hepatic fatty acid synthesis and directly influence lipogenesis in the intestine. The discovery of new bioactive properties of VA issupported by clinical studies which have provided increased momentum for industry applications. In this review we summarize the emergingbeneficial view of natural trans fats that have distinct and differential properties compared to those synthetically produced in partiallyhydrogenated vegetable oils (PHVO), with a particular focus on fasting and postprandial lipid metabolism in CVD risk.

Introduction

Cardiovascular disease (CVD) remains the leading cause of deathand is responsible for an estimated 28% of total mortalities glob-ally each year. The non-pharmaceutical management of CVD [asrecommended by the World Health Organization (WHO)] con-tinues to focus on lifestyle changes including: smoking cessa-tion, weight control, regular physical activity, moderate alcoholintake and special attention to dietary habits and macronutri-ent intake. Over the past few decades, health researchers haveargued that a diet rich in saturated and industrial trans fats con-tributes significantly to the prevalence of obesity, type-2 dia-betes and CVD. As a result, the American Heart Association(AHA) currently recommends choices of foods richer in poly-unsaturated fatty acids (PUFA), such as fatty fish, whole grainsand nuts, and mono-unsaturated fatty acids (MUFA) such as oliveoil, whilst restricting the intake of whole fat dairy and red meatproducts [1]. In addition, foods prepared with partially hydroge-nated vegetable oils (PHVO) such as shortenings are now alsobeing targeted in order to eliminate dietary trans fat intake.However, in many countries, dietary guidelines for trans fathave overlooked the types of trans fat and their complexity dueto the nature of their origin. Indeed, trans fats produced as aresult of industrial modification have a different fatty acid com-position [mainly trans-9 18:1 (elaidic acid) and trans-10 18:1] com-pared to those naturally present in dairy products [trans-11 18:1(vaccenic acid or VA) and cis-9, trans-11 18:2 (rumenic acid, a con-jugated linoleic acid or CLA)], and these different isomers appearto mediate different physiological properties in human metabo-lism. Consistent with this, our own group as well as other

researchers, have provided data suggesting that select naturaltrans fats (such as VA) may even reduce CVD risk, particularlyduring conditions of dyslipidemia. We would advocate that a“broad-brush” approach of public health policy that does not dif-ferentiate between natural trans-fats and PHVO may be mislead-ing for industry and consumers [2].

Trans fatty acids and cardiovascular diseaserisk

Trans fatty acids (TFA) refers to a group of unsaturated fatty acidswith at least one double bond in the trans configuration. Esti-mates from a few years ago indicate that approximately 80–90%of TFA consumption in North America was industrial TFA (oriTFA) from PHVO-containing margarines and shortenings, whilethe remaining 10-20% TFA was thought to be derived from rumi-nant sources (rTFA). Over the past decade, population-based stud-ies have linked the intake of TFA with an increased incidence ofCVD, diabetes and inflammatory disorders [2]. TFA has oftenbeen implicated to cause increased low-density lipoprotein(LDL)-cholesterol (or “bad” cholesterol), decreased high-densitylipoprotein (HDL)-cholesterol (or “good” cholesterol) and low-grade inflammation, all of which are associated with acceleratedCVD development [1, 2]. Based on these research findings, theconcept that trans fats are detrimental to CVD risk has beenwidely advocated [2]. Nevertheless, given the emerging evidencesuggesting a neutral or beneficial role of rTFA-enriched dairy faton blood lipid profiles, this “broad-brush” of eliminating all diet-ary TFA may not be necessary. Clearly it is now critical for indus-

DOI 10.1002/lite.201000016

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try, research scientists and the consumers to further resolve thehealth implications of the different types of trans fat associatedwith possible exacerbation of CVD risk factors.

Postprandial lipid metabolism andcardiovascular disease risk

The development of CVD is thought to be attributed to abnormallipid metabolism and a pro-inflammatory status that affectsheart and blood vessel function. In excess, LDL-cholesterol (LDL-C)particles are thought to permeate into the blood vessel wallwhere they deliver cholesterol and accumulate. Within the bloodvessel, LDL-C can become oxidized which is thought to trigger alow-grade inflammatory response and thickening of the arterialwall (atherosclerosis). More severe complications can occur if theadvanced unstable fatty accumulations (plaque) rupture, releas-ing thrombulus particles which may block blood supply toimportant organs such as the heart and brain, causing a myocar-dial infarct or stroke respectively. This line of thinking underpinsthe concept that high circulating LDL-C is regarded as an impor-tant and reliable indicator of CVD development and risk. Sinceblood lipid profile [e.g. LDL-C and triacylglycerols (TAG)] canchange substantially after a meal, the clinical measurement ofLDL-C is typically carried out following a 12 hour fast. However, itis also important to note that not all individuals with heart dis-ease have high concentrations of LDL-C, suggesting that other fac-tors may be involved in this process. For example, postprandial(or non-fasting) lipids have been shown to have a significant cor-relation with CVD events, rather than LDL-C alone [3].

The postprandial phase refers to the time following the inges-tion of a meal containing lipids. Humans (in developed and pros-perous countries) are potentially in a postprandial state continu-ously for up to 16-20 hours per day. Dietary lipids (cholesteroland TAG) are digested and absorbed by the intestine via proteincarriers. Similar to the process that occurs in the liver of very-low density lipoprotein (VLDL) production, the intestine re-synthesizes lipids and incorporates them into lipid-rich particles(lipoproteins) called chylomicrons (CM) that are secreted intothe circulation. The rise in blood TAG and CM [which can bemeasured by its unique structural protein called apolipoprotein(apo) B48] can be observed within minutes following a meal, andunder normal conditions, will peak around 3–4 hours andreturn to baseline within 6-8 hours following ingestion of thelipid meal. The time taken for TAG and CM to be

,

metabolized’following a lipid meal is dependent on several factors including:the amount of lipid ingested, the rate of secretion and produc-tion of CM by the intestine, activity of hydrolytic enzymes suchas endothelial lipoprotein lipase, expression of hepatic receptorsresponsible for the removal of CM, and competition of thesepathways by other lipoprotein fractions such as LDL and VLDL.The aberrant metabolic conditions in chronic disease can have amajor impact on the factors that regulate CM clearance andhence can often impair the postprandial metabolism of CM. Theresult is a delay in the removal of CM particles from the circula-tion and an increase in non-fasting lipid parameters, such asTAG, throughout the day. The implication of this pathway inCVD development is that CM are rapidly hydrolyzed to smallerremnant particles (CM-r) that are comparative in size to LDL, andyet carry substantially more cholesterol.

LDL has been shown to play an important role in initiatingatherosclerosis by promoting cholesterol accumulation in the

blood vessel wall. However, well established findings from ourgroup have shown that CM-r can permeate and accumulatewithin arterial tissue. Given CM-r “carry” potentially greateramounts of cholesterol per particle, they are considered to bemore atherogenic than other lipid fractions [4], which may havesignificant consequences in individuals at risk of delayed clear-ance of CM-r during impaired postprandial dyslipidemia.

JCR:LA-cp rats as a model of postprandiallipaemia

Several rodent models have been used in the research of obesityand metabolic syndrome (MetS) (5). Typically, rodents in thisfield have defects in the metabolism of leptin (a hormone thatregulates appetite) and the leptin receptor. Some of these mod-els include the ob/ob and db/db mice, the fa/fa Zucker and theJCR:LA-cp rat [5]. These models display similar symptoms asso-ciated with MetS, which include obesity, insulin resistance aswell as high levels of circulating TAG and leptin. However, theJCR:LA-cp rat is unique in that it has been used widely and spon-taneously develops macro- and micro-vascular complicationsthat resemble early human CVD. The corpulent phenotype hashigh blood TAG and cholesterol, overproduces VLDL and CM dur-ing the fasting and postprandial phase, and thus provides anopportunity to study the effect of trans-fats such as VA on post-prandial lipaemia and CVD risk [6].

Vaccenic acid and cardiovascular disease risk

Trans-11 vaccenic acid (VA) has a similar structure to that of oleicacid (cis-9 18:1) with the exception of a trans double bond at theC11 position. Curiously, it is the most prevalent trans fatty acidin dairy-derived foods following its synthesis in the gut of rumi-nant animals from PUFA substrates. Although VA synthesis iscarried out by bacteria in the gut lumen, VA tends to accumulatein adipose tissue stores. A small portion of VA is also convertedto rTFA, cis-9, trans-11 CLA in the intestine, liver and adipose tis-sue of ruminants, rodents and humans.

CLA has been widely studied and the recognition of CLA's ben-efits in cancer prevention and weight reduction has lead to arapid growth of commercialized supplements. In some coun-tries there has been a substantial investment by industry in thenatural enrichment of CLA in foods. Agricultural scientists haverevealed that increasing sunflower seeds in the diet of rumi-nants can raise the concentration of CLA in milk fat substan-tially. Interestingly, the dietary modifications of ruminantshave also resulted in a simultaneous increase in the VA contentof up to 12% of total fat. Unfortunately, there has been only lim-ited data on the putative health impact of VA in either humansor animals, due to the cost associated with producing purifiedforms of this fatty acid.

Using purified VA oil, we recently reported that dietary VAsupplementation can reduce CVD risk factors in a rodent modelof hyperlipidemia (high blood TAG and LDL-C). In a pilot study (3weeks) lean healthy and obese JCR:LA-cp rats were fed a Western-type diet containing 43% carbohydrate, 28% protein and 15% fatwith a PUFA/SFA ratio of 0.6, and n-6/n-3 PUFA ratio of 10. Trea-ted-rats were fed a diet containing 1.5% VA. After 3 weeks oftreatment with VA, obese rats (but not lean healthy rats) hadreduced concentrations of fasting plasma TAG and inflamma-

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Lipid Technology May 2010, Vol. 22, No. 5 105

tory markers, including interleukin-6 and haptoglobin [7]. In acomparable follow up chronic study design (4 months feeding inJCR:LA-cp rats) the lipid-lowering benefit of dietary VA wasenhanced including additional improvements in plasma totalcholesterol, LDL-C and free fatty acids, possibly as a result ofextended treatment period [8]. In addition, VA-fed rats alsoshowed signs of improving features of liver steatosis. We con-cluded that VA may have distinct bioactivity and potential lipidlowering properties that differentiate it from other iTFA. Morerecently, a number of other groups have also demonstrated theputative lipid-lowering properties of VA in other animal modelsof CVD [9, 10]. While no definitive clinical studies have yet beenreported, evidence in humans also tends to support the conceptthat rTFA may have either neutral or beneficial effects relative toiTFA.

Vaccenic acid and postprandial lipaemia

The effect of 4-month VA dietary intervention on postprandiallipid metabolism in obese JCR:LA-cp rats was assessed by a stan-dardized oral fat challenge test, compared to CLA (the cis9,trans11 isomer) alone, and a combined dietary treatment of VAand CLA. Our approach has been to adopt an oral fat challengein rats that resembles tests used in the clinical setting. After anovernight fast (12 hours), rats are given a 5-gram food pellet con-taining 30% w/w dairy fat. Blood is sampled from the tail veinprior and at 2, 4, 5, 6, 8 and 10 hours following food ingestion.The concentration of TAG and CM (indicated by apoB48) over thepostprandial phase can then be calculated along with the corre-sponding area under the curve analysis (see summary Figure 1).Our results to date suggest that both VA and CLA have a benefi-cial impact on postprandial dyslipidemia in the JCR:LA-cp rat.We propose that under conditions of existing hyperlipidemia(such as MetS) natural trans fats may offer some conferred healthbenefit to this significant CVD risk factor.

The over-production of intestinal CM is causally associatedwith postprandial lipaemia [6]. In order to demonstrate theeffect of VA on intestinal CM production/secretion, we sampledlymph from the mesenteric duct of obese JCR:LA-cp rats prior toentering the blood stream in order to determine the direct pro-duction of CM from the intestine. The lymphatic lipid profilewas compared between rats given a lipid emulsion containingVA or without VA supplementation (controls). Interestingly wefound that TAG concentration as well as the number of CM parti-cles were both lower in rats given VA compared to controls, sug-gesting a down-regulation of CM by the intestine in the post-prandial state [8].

To further explore the metabolic pathways modulated by VA,we have investigated the impact of fatty acid synthesis in boththe liver and intestine by quantifying key enzymes involved inthis process. Acetyl-CoA carboxylase (ACC)-1 catalyzes an initialand rate limiting step in fatty acid synthesis, and fatty acidsynthase (FAS) is a multi-functional enzyme complex involved inthe production of the most common SFA, palmitic acid (C16:0).We have discovered that the protein abundance of both ACC-1and FAS are lower in VA-fed rats compared to controls [8]. Wespeculate that the improvement in fatty liver symptomsobserved in the JCR:LA-cp rat may also be partially attributed to adown-regulation in fatty acid synthesis in response to chronicdietary VA supplementation. Ongoing studies suggest that VAmay also be a natural agonist of peroxisome proliferator-acti-

vated receptors (PPAR), implying the possibility of VA's biologicalactivity mediated by PPAR-related pathways.

Conclusion

Dietary supplementation of VA, a natural trans fat found abun-dantly in dairy-derived foods, has been shown to effectivelyreduce fasting and postprandial lipid responses in animal mod-els associated with CVD risk. These novel findings warrant

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Figure 1. Postprandial lipid response following an oral fat chal-lenge in obese JCR:LA-cp rats fed the control, VA, cis-9, trans-11 CLA and VA + cis-9, trans-11 CLA diet for 16 wk. (A) Post-prandial TAG response and AUC. (B) Postprandial apoB48response and AUC. Values are means€SEM, n = 4. * P a 0.05,**P a 0.01, ***P a 0.001 compared to obese control.

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future research in elucidating the possible metabolic pathwaysmodulated by dietary VA. Furthermore, it is now vital to conductwell-controlled trials in both healthy and dyslipidemic humanpopulations in order to validate the clinically relevant bioactiveproperties of VA, particularly in those patients at significant riskof CVD.

References

[1] Lichtenstein AH, et al. Circulation 2006, 114, 82–96.

[2] Brouwer IA, et al. PLoS One. 2010, 5, e9434.

[3] Bansal B, et al. JAMA. 2007, 298, 39–46.

[4] Proctor SD, et al. Curr. Opin. Lipidol. 2002, 13, 461 –470.

[5] Russell JC, et al. Cardiovasc. Pathol. 2006, 15, 318 –330.

[6] Vine DF, et al. Atheroscler Suppl. 2008, 9, 69–76.

[7] Wang Y, et al. J. Nutr. 2008, 138, 2117–2122.

[8] Wang Y, et al. J. Nutr. 2009, 139, 2049–2054.

[9] Tyburczy C, et al. J. Nutr. 2009, 139, 257 –63.

[10] Bassett CM, et al. J. Nutr. 2010, 140, 18–24.

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