Olive oil and postprandial hyperlipidemia: Implications for atherosclerosis and metabolic syndrome

1 Olive oil and postprandial hyperlipidemia: Implications for atherosclerosis and metabolic syndrome Sergio Montserrat-de la Paz* ,a , Beatriz Bermudez b , Magdalena P Cardelo a , Sergio Lopez a , Rocio Abia a , Francisco JG Muriana a a Laboratory of Cellular and Molecular Nutrition, Instituto de la Grasa, CSIC. Ctra. de Utrera Km. 1, 41013 Seville, Spain b Department of Cell Biology, Faculty of Biology, University of Seville. C/ Profesor Garcia Gonzalez s/n, 41012 Seville, Spain *Corresponding Author: Sergio Montserrat-de la Paz Laboratory of Cellular and Molecular Nutrition, Instituto de la Grasa, CSIC. Ctra. de Utrera Km. 1, Campus Universitario Pablo de Olavide, Edificio 46, 41013 Seville (Spain) Tel: +34 954 611 550 (Ext. 360) E-mail: dela[email protected] Short title: Olive oil and postprandial hyperlipidemia Page 2 of 43Food & Function Postprint of Food and Function 2016, 7, 4734-4744, doi: 10.1039/C6FO01422D 2 ABSTRACT Olive oil is the primary source of fat in the Mediterranean diet, which is associated with a significant improvement in health status, as measured by reduced mortality from several chronic diseases. The current pandemic of obesity, metabolic syndrome, and type 2 diabetes is intimately associated with an atherogenic dyslipidemic phenotype. The core components of the dyslipidemia of the metabolic syndrome, which most likely initiate atherosclerosis, are the “lipid triad” of high plasma triglycerides, low levels of high-density lipoproteins, and a preponderance of small, dense low-density lipoproteins at fasting. However, postprandial (non-fasting) TG (postprandial hyperlipidemia) are also recognized as an important component for atherosclerosis. Herein, the purpose of this review was to provide an update on effects and mechanisms related to the olive oil on postprandial hyperlipidemia and its implications for the onset and progression of atherosclerosis and metabolic syndrome. Keywords: Olive oil, postprandial hyperlipidemia, atherosclerosis, metabolic syndrome, lipoproteins, cardiovascular diseases Page 3 of 43 Food & Function 3 OLIVE OIL Generalities Virgin olive oil plays a pivotal role as the main source of fat in the Mediterranean diet, declared by the UNESCO as an Intangible Cultural Heritage since 2013. This diet that has traditionally been linked to longevity in Mediterranean populations and is associated with a significant improvement in health status, as measured by reduced mortality from several chronic diseases. 1 According to EC Regulations (EU Regulation 1348/2013; EU Regulation 29/2012; EEC Regulation 1513/2001), virgin olive oils are those obtained from the mesocarp of the drupe from the fruit of the olive tree (Olea europaea L.), obtained exclusively from fresh and healthy olives by physical procedures under low thermal conditions (<27°C). Spain is by far the largest producer of virgin olive oil in the world, accounting for approximately 50% (≈1,600,000 tons) of total global production in 2015. 2 Extra virgin olive oil is a virgin olive oil whose free acidity, expressed as oleic acid, is not more than 0.8 gram per 100 grams and organoleptic characteristics (flavour and colour) are excellent (for olive oil classification and definitions see Ref. 3 ). The levels of fatty acids (FAs) in virgin olive oils vary during the different maturation stage of the olives, the variety, and the growing conditions. It is generally accepted that cooler areas will give oil with higher monounsaturated FAs (MUFAs) content than warmer climates. 4 The composition of virgin olive oil includes minor compounds (unsaponifiable fraction) that could range from one to 3% of the oil. 5 The constituents of minor compounds are present in low concentrations but they are responsible for the unique and delicate flavour of virgin olive oil (aldehydes, alcohols, esters, Page 4 of 43Food & Function 4 hydrocarbons, ketones, furans, and others). This fraction contains important bioactive compounds. Brief description of olive oil composition FAs are carboxylic acids and often contain a long, unbranched aliphatic chain. FAs are categorized as saturated (SFAs), MUFAs, and polyunsaturated (PUFAs) based on their structural and chemical properties. SFAs do not contain any double bonds or other functional groups along the chain, which is fully saturated with hydrogen atoms. The principal dietary SFAs are palmitic (16:0) and stearic (18:0) acids, which are composed of 16 and 18 carbon atoms, respectively. MUFAs contain one pair of carbon atoms linked by a cis double bond. Oleic acid (18:1n−9), which contains 18 carbon atoms with a double bond at the 9 th carbon from the methyl end of the FA molecule, is the major dietary MUFA and represents 55 to 83% of the total FAs in virgin olive oil (Table 1). Carbon chains containing 2 or more cis double bonds, with the first double bond located between either the 3 rd and 4 th or the 6 th and 7 th carbon atoms from the methyl end of the FA molecule, belong to the n−3 or n−6, respectively, PUFA families. These families cannot be synthesized by the human body (double bonds can be introduced into all positions of the FA chain with the exception of the n−3 and n−6 positions) and therefore must be obtained from the diet as -linolenic acid (18:3n−3) and linoleic acid (18:2n−6) or their long-chain PUFA derivatives. Of these FAs, eicosapentaenoic acid (EPA, 20:5n−3), docosahexaenoic acid (DHA, 22:6n−3), dihomo-γ-linolenic acid (20:3n−6), and arachidonic acid (AA, 20:4n−6) are the most metabolically significant. 6 The concentrations of SFAs (palmitic + stearic acids) and PUFAs (α-linolenic + linoleic acids) in virgin olive oil range from 8 to 26% and from 3 to 22% of the total FAs, respectively. Page 5 of 43 Food & Function 5 Oleic acid is the primary component of virgin olive oil (≈83% oleic acid in position sn−2 of the triglycerides, TGs) and is also found in peanut oil (≈59% oleic acid) and canola oil (≈37% oleic acid). Oleic acid is a key component of TGs and membrane lipids. 7 Importantly, oleic acid is the most common FA in nature, as well as in our diet (generally, oleic acid supplies an amount of calories equivalent or greater than the amount provided by SFAs and PUFAs combined). Tight restrictions on SFA consumption (<10% of total daily calories; less than 7% for high-risk individuals) and PUFA consumption (<5%) have been recommended. By contrast, oleic acid may provide up to 20–25% of total daily calories. The unsaponifiable fraction of virgin olive oil contains highly bioactive compounds (>200 constituents) (Table 2). Despite their wide variety and nutritional significance, they commonly account up to 3% of the total oil composition (reaching individual concentrations as smaller as ppm). 8 This fraction is fundamental in contributing to specific characteristics of virgin olive oil, such as its oxidative stability and its special flavour (aroma and taste) as well as its colour. 9 Among the several minor compounds of virgin olive oil (Figures 1–3), the most abundant fraction is hydrocarbons (squalene and, in smaller amounts, the carotenoids β-carotene and lutein). Other minor compounds of virgin olive oil include phytosterols, such as β-sitosterol, ∆5-avenasterol, and campesterol; triterpenic compounds in the form of dialcohols (erythrodiol and uvaol) or acids (oleanolic and maslinic acids); and phenolic compounds, representing the polar fraction. The main classes of phenolic compounds in virgin olive oil are secoiridoids, as ester derivatives of elenolic acid in either its aglyconic form or glycosylated with hydroxytyrosol (oleuropein) or tyrosol (ligustroside); simple phenols, in the form of alcohols (tyrosol and hydroxytyrosol) or acids (p-coumaric, o-coumaric, caffeic, ferulic, sinapic, gallic, Page 6 of 43Food & Function 6 gentisic, syringic, vanillic, protocathechuic, and p-hydroxybenzoic acids); hydroxyisochromans formed by the reaction between hydroxytyrosol and benzaldehyde or vanillin; flavonoids (luteolin, apigenin, and quercetin); and the lignans (+)-pinoresinol, 1-(+)-acetoxypinoresinol, and 1-(+)-hydroxypinoresinol. The lipophilic phenols include tocopherols α- β-, γ-, and δ-tocopherols) and tocotrienols α- β-, γ-, and δ-tocotrienols), with α-tocopherol as the predominant constituent in virgin olive oil. It is important to note that only virgin olive oil contains minor compounds, since mostly of them disappear during refining processes. 6,9 Digestion of triglycerides of olive oil and absorption of oleic acid and other fatty acids In general, the first event in the transformation of insoluble oil into soluble and absorbable lipids is the formation of an initial emulsion (chyme) by mastication in the mouth where the dispersion of TGs happens. The surface area of TGs is then increased, which benefits their emulsion (formation of lipid droplets) in the stomach. During the initial gastric process, partially emulsified TGs are attached by lingual and gastric lipases. 10 Gastric lipase activity does not contribute to the hydrolysis of phospholipids (PLs) and cholesteryl esters (CEs), and is functional in the pH range of 3 to 6. In the stomach, this enzyme hydrolyses only 10 to 30% of ingested TGs because of an inhibition process induced by the long-chain free FAs (FFAs) generated, which are mostly protonated at gastric pH. It explains the limited lipolysis of TGs under gastric conditions regarding the complete TGs hydrolysis by pancreatic lipase in the duodenum. 11,12 Page 7 of 43 Food & Function 7 During gastric lipolysis, FFAs have higher affinity for the surface than the core of the lipid droplets. 13 There is a considerable fusion between lipid droplets, probably due to the presence of FFAs, monoglycerides (MGs), and diglycerides (DGs) that are known to be fusogenic. This change in the lipid composition of the droplet surface during lipolysis could modify the interfacial tension or the surface pressure and could then interfere with gastric lipase binding and activity. 14 The accumulation of FFAs at the droplet surface leads to an inhibition of lipolysis by gastric lipase. The mechanism by which this happens is attributed to the formation of clusters at the surface of the lipid droplets. In the intestine, the smaller size of lipid droplets increases proportionally the lipid surface exposed to the pancreatic lipase. 15 The FAs released from the initial gastric lipolysis and the amino acids and peptides formed by gastric proteolytic activity stimulate specific receptors in intestinal epithelial cells to secrete cholecystokinin. It stimulates gallbladder contraction delivering bile salts to the duodenum. The fat droplets covered with bile salts are not accessible to pancreatic lipase, but the co-lipase enzyme allows the pancreatic lipase molecule to bind to the lipid aqueous interface and facilitates the stabilization of emulsified TGs. Pancreatic lipase cleaves the sn-1 and sn-3 positions of TGs obtaining sn-2 MGs and FFAs. 16 In olive oil, up to 83% of total sn-2 positions in TGs are occupied by oleic acid, which means that olive oil acts as a supplier of oleic acidrich hydrocarbon skeletons and free oleic acid for energy or for cellular synthesis of TGs and PLs. 17 Absorption of lipid molecules, such as sn-2 MGs and oleic acid, takes place along the epithelial cells of the small intestine, mainly in the proximal jejunum but also in parts that are more distal. The epithelial cells of the small intestine show an apical membrane with a brush border made up of many microvilli, which have a Page 8 of 43Food & Function 8 width of about 100 nm and being much smaller (5-20 nm) the spaces between them. Lipid metabolites generated throughout the digestion of olive oil are more polar than the parent TGs, but they still have a limited solubility in the aqueous environment of the intestinal lumen. Micelles and sub-micelles are the main vehicles for approximating sn-2 MGs and FFAs successfully to the membrane of the microvilli. There, the acidic microclimate (pH 5.3-6) promotes both micellar dissociation and FA protonation, facilitating the passive diffusion of FFAs across the cellular membrane. 18 Enterocytes may also take up FFAs via energy dependent and carrier-mediated processes. It is known the existence of two isotypes (I and L) of FA binding proteins (FABPs), which differ in their binding specificity. I-FABP binds strongly to FFAs, whereas L-FABP preferentially binds to sn-2 MGs. 19 These carriers play an important role at low FFA concentrations (probably to ensure sufficient uptake of lipid nutrients), whereas passive diffusion predominates at high FFA concentrations. Other proteins, including GP330 (megalin), CD36, SR-BI, and caveolin can bind lipids and related metabolites. 20 Differences in the rate of absorption of FFAs have been described as a function of chain length and of number and place of double bonds. 21 Short-chain (2-4 carbon atoms) and medium-chain (6-12 carbon atoms) FAs are more rapidly absorbed than FAs of more than 14 carbon atoms, because they do not need micellar solubilisation, just bound to albumin and are transported directly to the liver by the portal vein. 22 Assembly of intestinal lipoproteins containing triglycerides from olive oil ingestion In the enterocyte, FFAs (mainly oleic acid) from absorption and the pool of endogenous metabolism, together with sn-2 MGs, are used for re-synthesis of TGs. This process is initiated with the activation of FFAs to the corresponding acyl-CoA. In this form, FFAs are sequentially transferred to sn-2 MGs by MG and DG Page 9 of 43 Food & Function 9 acyltransferases. These enzymes form a complex called ‘‘triglyceride synthetase’’, 23 and the pathway favours the stereospecific reacylation at the sn-1 position. It contributes to 80% of the intestinal TG re-synthesis in the fed state. Acyl-CoA can also be transferred to α-glycerophosphate (derived from glucose metabolism) by the phosphatidic acid pathway, which accounts for the remaining 20%. 21 The composition of these novel TGs closely resembles the composition of TGs from olive oil. 24 These TGs are coated with cholesterol, PLs, and one molecule of apolipoprotein (apo) B48 at the rough and smooth endoplasmic reticulum in a microsomal TG transfer protein (MTP)-dependent step, 25 and further processed in the Golgi apparatus before being released as chylomicrons (CMs) by the enterocyte through exocytosis. It occurs through the basolateral membrane of enterocytes and CMs enter the lymphatic capillaries of intestinal microvilli that drain into lymphatic channels, reaching the systemic circulation through the thoracic duct. 26 The body can also secrete very lowdensity lipoproteins (VLDLs). While CMs are of intestinal origin and formed after the ingestion of fatty meals, VLDLs are the major lipoproteins secreted by the liver during fasting. 21 Both CMs and VLDLs are considered TG-rich lipoproteins (TRLs). POSTPRANDIAL HYPERLIPIDEMIA Generalities Postprandial hyperlipemia is a normal and transient physiological phenomenon that occurs in response to the ingestion of a fatty meal. Dietary lipids are absorbed as described above and intestinally secreted TRLs have the function to stabilize the absorbed dietary lipids for transport in the aqueous plasma environment and to provide cells with exogenous FAs by receptor (e.g., apoB48 receptor, LDL receptor, and LDL receptor-related protein) or non-receptor-dependent mechanisms for energy and numerous metabolic pathways. 27 In healthy people, the levels of Page 10 of 43Food & Function 16 and non-fasting levels of plasma TGs. 28 However, the clinical efficacy, mechanisms of action, safety, and tolerability of these newer agents requires further testing in clinical trials. Conclusions MetS is a major and escalating public health and clinical challenge worldwide in the wake of urbanization. The complexity of the molecular pathophysiology of MetS requires rational therapeutic and dietary strategies. Olive oil is a natural fruit product that contains a unique composition of oleic acid and minor constituents. Within this context, the consumption of olive oil has shown a broad range of promising activities in the postprandial disturbances. Nonetheless, further efforts are needed to mechanistically define the biochemical and biological postprandial activities of olive oil on atherosclerosis and MetS. Conflicts of interest The authors state no conflict of interest. Acknowledgements This study was supported by research Grant AGL2011-29008 (Spanish Ministry of Science and Innovation, MICINN). S.M. has the benefit of a FPI fellowship (BES- 2012-056104) of MICINN. B.B. and S.L. acknowledges financial support from “V Own Research Plan” (University of Seville) and the Spanish Research Council (CSIC)/Juan de la Cierva, respectively. Page 17 of 43 Food & Function 17 ABBREVIATIONS AA: arachidonic acid Apo: apolipoprotein CE: cholesteryl ester CHD: coronary heart disease CM: chylomicron CVD: cardiovascular disease DG: diacylglycerol DHA, docosahexaenoic acid EPA: eicosapentaenoic acid FA: fatty acid FABP: fatty acid binding protein FFA: free fatty acid HDL: high-density lipoprotein IL: interleukin LDL: low-density lipoprotein MCP: monocyte chemotactic protein MetS: metabolic syndrome MG: monoacylglycerol MI: myocardial infarction MTP: microsomal triglyceride transfer protein MUFA: monounsaturated fatty acid NAD: nicotinamide adenine dinucleotide NAM: nicotinamide NAMPT: nicotinamide phosphoribosyltransferase PL: phospholipid Page 18 of 43Food & Function 18 PPAR: peroxisome proliferator-activated receptor PUFA: polyunsaturated fatty acid SFA: saturated fatty acid T2D: type-2 diabetes TG: triglyceride TNF: tumour necrosis factor TRL: triglyceride-rich lipoprotein VLDL: very low-density lipoprotein Page 19 of 43 Food & Function 19 References 1. 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Alberti and P.Z. Zimmet. Definition, diagnosis and classification of diabetes mellitus and its complications. Part 1: diagnosis and classification of diabetes mellitus provisional report of a WHO consultation, Diabetic Med., 1998, 15, 539– 553. 52. B. Balkau and M.A. Charles. Comment on the provisional report from the WHO consultation: European Group for the Study of Insulin Resistance (EGIR), Diabetic Med., 1999, 16, 442–443. 53. J.I. Cleeman. Executive summary of the third report of the National Cholesterol Education Program (NCEP) expert panel on detection, evaluation, and treatment of high blood cholesterol in adults (adult treatment panel III), J. Am. Med. Assoc., 2001, 285, 2486–2497. Page 26 of 43Food & Function 32 Figure Legends Figure 1. Chemical structures of squalene, carotenoids, sterols, triterpenic dialcohols, and triterpenic acids in virgin olive oil. 6 Figure 2. Chemical structures of secoiridoids, phenyl-alcohols, and phenyl-acids in virgin olive oil. 6 Figure 3. Chemical structures of hydroxyisochromans, flavonoids, lignans, tocopherols, and tocotrienols in virgin olive oil. 6 Figure 4. Serum TG levels after the ingestion of fatty meals. (A) Serum TG levels reach a peak at 3-4 h and slowly return to initial levels at 6-8 h after a fatty meal. (B) Most of the day is a non-fasting state (in other words a postprandial state) in people who eat at least three fatty meals a day. 58 Figure 5. Concept of the initiation of atherosclerosis by TRLs: lipoproteins enter the subendothelial space via nonspecific transcytotic processes. This is often a nonpathologic process, because the lipoproteins leave (TRLs with greater difficulty than LDL-C) the subendothelial space again via the vasa vasorum. Lipoproteins can be easily taken up by macrophages. Circulating TRLs themselves also contribute to the presence of subendothelial macrophages. Monocytes can bin and take up TRLs, which stimulates monocytes to become activated. Subsequently, activated monocytes express adhesion molecules on the outer membrane and stimulate the expression of endothelial cellular adhesion molecules (CAMs), which allows monocytes to home the endothelium and migrate into the subendothelial space. Finally, the macrophages change into highly atherogenic foam cells when lipid uptake exceeds lipid efflux. Figure 6. Prevalence of MetS in the Spanish population aged 18 years or older from 2008 to 2010. 43 Page 33 of 43 Food & Function 33 Table 1. Chemical structure and range of major fatty acids in virgin olive oil. Fatty acid Regulations a (%) 16:0, palmitic acid 7.5-20.0 16:1n - 7, palmitoleic acid 0.3-3.5 18:0, stearic acid 0.5-5.0 18:1n-9, oleic acid 55.0-83.0 18:2n-6, linoleic acid 3.5-21.0 18:3n-3, α-linolenic acid ≤ 1.0 MUFA, monounsaturated fatty acids 55-87 SFA, saturated fatty acids 8-26 PUFA, polyunsaturated fatty acids 3-22 a International Olive oil council 6 Page 34 of 43Food & Function 34 Table 2. Minor compounds in virgin olive oil. 6 Minor compounds Concentration (mg/kg oil) Squalene β-carotene and lutein Sterols Triterpenic compounds Phenols Tocopherols and tocotrienols 800-8000 4-10 1000-3000 200-300 200-1500 250-350 Page 35 of 43 Food & Function 35 Table 3. Criteria proposed for the clinical diagnosis of MetS. 55 Clinical measures WHO (1998) EGIR (1999) ATP III (2001) Insulin resistance IGT, IFG, T2D, or lowered insulin sensitivity plus any 2 of below clinical measures Plasma insulin >75 th percentile None, but any 3 of below clinical measures Body weight Men: waist-to- hip ratio >0.90; women: waist- to-hip ratio >0.85 and/or BMI >30 kg/m 2 WC ≥94 cm in men or ≥80 cm in women WC ≥102 cm in men or ≥88 cm in women Lipids (at fasting) TGs ≥150 mg/dL and/or HDL-C <35 mg/dL in men or <39 mg/dL in women TGs ≥150 mg/dL and/or HDL-C <39 mg/dL in men or women TGs ≥150 mg/dL HDL-C <40 mg/dL in men or <50 mg/dL in women Blood pressure ≥140/90 mm Hg ≥140/90 mm Hg or on Rx against hypertension >130/85 mm Hg Glucose (at fasting) IGT, IFG or T2D IGT or IFG (but not diabetes) >110 mg/dL (includes diabetes) Page 36 of 43Food & Function 36 Table 4. Criteria proposed for the clinical diagnosis of MetS. 55 Clinical measures AACE (2003) IDF (2005) Insulin resistance IGT or IFG plus any of below clinical measures None Body weight BMI ≥25 kg/m 2 Increased WC (population specific) plus any 2 of below clinical measures Lipids (at fasting) TGs ≥150 mg/dL and HDL-C <40 mg/dL in men or <50 mg/dL in women TGs ≥150 mg/dL or on Rx against TGs. HDL-C <40 mg/dL in men or <50 mg/dL in women or on Rx to increase HDL-C Blood pressure >130/85 mm Hg ≥130 mm Hg systolic or ≥85 mm Hg diastolic or on RX against hypertension Glucose (at fasting) IGT or IFG (but not diabetes) ≥100 mg/dL (includes diabetes) IFG: impaired fasting glucose; IGT: impaired glucose tolerance; Rx: receiving treatment; WC: waist circumference. Page 37 of 43 Food & Function ! Page 38 of 43Food & Function ! Page 39 of 43 Food & Function ! Page 40 of 43Food & Function ! Page 41 of 43Food & Function ! TRLs Activation Upregulation of adhesion molecules Page 42 of 43Food & Function