Bioavailability and antioxidant effects of olive oil phenols in humans: a review

OBJECTIVE: We reviewed the bioavailability and antioxidant effects of phenols from extra virgin olive oil. SEARCH STRATEGY: We searched the MEDLINE database for the years 1966-2002. To review the bioavailability of olive oil phenols, we selected animal and human studies that studied the absorption, metabolism, and urinary excretion of olive oil phenols. We also estimated the intake of the various phenols in the Mediterranean area. To review the antioxidant effects of olive oil phenols, we included human and animal studies on the effect of olive oil phenols on markers of oxidative processes in the body. We excluded studies without a proper control treatment and studies in which the antioxidant effects of phenols could not be disentangled from those of the fatty acid composition of olive oil. RESULTS: Bioavailability studies in humans show that the absorption of olive oil phenols is probably larger than 55-66 mol%, and that at least 5% is excreted in urine as tyrosol and hydroxytyrosol. Animal studies suggest that phenol-rich olive oil lowers oxidisability of ex vivo low-density lipoprotein (LDL) particles or lowers markers in urine of oxidative processes in the body. In five out of seven human studies, however, these effects of phenols were not found. There are no data on the phenol concentrations in plasma that are attainable by intake of olive oil. We estimated that 50 g of olive oil per day provides about 2 mg or approximately 13 micromol of hydroxytyrosol-equivalents per day, and that the plasma concentration of olive oil phenols with antioxidant potential resulting from such an intake can be at most 0.06 micromol/l. This is much lower than the minimum concentrations of these phenols (50-100 micromol) required to show antioxidant activity in vitro. CONCLUSION: Although phenols from olive oil seem to be well absorbed, the content of olive oil phenols with antioxidant potential in the Mediterranean diet is probably too low to produce a measurable effect on LDL oxidisability or other oxidation markers in humans. The available evidence does not suggest that consumption of phenols in the amounts provided by dietary olive oil will protect LDL against oxidative modification to any important extent.     

Inhibition of platelet aggregation by olive oil phenols via cAMP-phosphodiesterase

The aim of the present study was to confirm that olive oil phenols reduce human platelet aggregability and to verify the hypothesis that cAMP- and cGMP- phosphodiesterases (PDE) could be one of the targets of the biological effect. Four extracts from oils characterized by a high phenol content (HPE), and low phenol levels (LPE) were prepared and analyzed qualitatively and quantitatively by HPLC-UV and electrospray ionization-MS/MS. Human washed platelets stimulated with thrombin were used for the aggregation assay. Human platelet cAMP-PDE and recombinant PDE5A1 were used as enzyme source. Platelet aggregation and enzyme activity were assayed in the presence of HPE, LPE and individual phenols. The phenol content of HPE ranged between 250 and 500 mg/kg, whereas the LPE content was 46 mg/kg. The compounds identified were hydroxytyrosol (HT), tyrosol (TY), oleuropein aglycone (OleA) and the flavonoids quercetin (QU), luteolin (LU) and apigenin (AP). OleA was the most abundant phenol (range 23.3 to 37.7 %) and LU was the most abundant flavonoid in the extracts. Oil extracts inhibited platelet aggregation with an 50% inhibitory concentration interval of 1.23-11.2 microg/ml. The inhibitory effect of individual compounds (10 microm) including homovanillyl alcohol (HVA) followed this order: OleA>LU>HT = TY = QU = HVA, while AP was inactive. All the extracts inhibited cAMP-PDE, while no significant inhibition of PDE5A1 (50 microg/ml) was observed. All the flavonoids and OleA inhibited cAMP-PDE, whereas HT, TY, HVA (100 microm) were inactive. Olive oil extracts and part of its phenolic constituents inhibit platelet aggregation; cAMP-PDE inhibition is one mechanism through which olive oil phenols inhibit platelet aggregation.     

Bioavailability and antioxidant effects of olive oil phenolic compounds in humans: a review

Olive oil, the main source of fat in the Mediterranean diet, is a functional food which besides having a high level of monounsaturated fatty acid contains several minor components with biological properties. For some olive oil minor components, such as the antioxidant phenolic compounds, a large body of studies, mainly experimental or in animal models, have been performed. Randomized, controlled, clinical trials in humans are required to provide evidence that olive phenolic compounds contribute significantly to health benefits in order to give recommendations at population level. Here, we summarize the state of the art of the body of knowledge, and to which extent we have evidence, of the bioavailability and of the antioxidant benefits of olive oil phenolic compounds in humans.     

Extra virgin olive oil phenols and markers of oxidation in Greek smokers: a randomized cross-over study

OBJECTIVE: To examine the effect of a low phenol olive oil and high phenol olive oil on markers of oxidation and plasma susceptibility to oxidation in normolipaemic smokers. DESIGN: Randomized single-blind cross-over trial with two intervention periods. SETTING: The Medical School and University Hospital of the University of Crete, Heraklion, Crete, Greece. SUBJECTS: Twenty-five healthy males and females completed the study. INTERVENTIONS: Each intervention was of three weeks duration and intervention periods were separated by a two week washout. Seventy grams of extra virgin olive oil was supplied to each subject per day in the intervention periods. The olive oils supplied differed in their phenol content by 18.6 mg/day. Two fasting venous blood samples were taken at the end of each intervention period. RESULTS: The markers of antioxidant capacity measured in fasting plasma samples (total plasma resistance to oxidation, concentrations of protein carbonyl as a marker of protein oxidation, malondialdehyde and lipid hydroperoxides as markers of lipid oxidation and the ferric reducing ability of plasma) did not differ significantly between the low and high phenol olive oil diets. CONCLUSIONS: No effect of olive oil phenols on markers of oxidation in smokers was detected. It may be that the natural concentrations of phenols in olive oil are too low to produce an effect in the post-absorptive phase. Possible reasons for period effects and interactions between diet and administration period need attention to aid further cross-over trials of this kind. SPONSORSHIP: Unilever Research Vlaardingen, The Netherlands.     

Levels of lipochromes and other bioactives in virgin olive oil from new olive germplasm

The composition of virgin olive oils from three new olive cultivars, such as Arno, Tevere and Basento, has been investigated in this work. Our attention was focused on chlorophylls, carotenes, xanthophylls and other major bioactives, as well as on some analytical indices related to functionality, quality and typicality of the new products. Our analytical study showed how the new oil kinds, especially Arno oil, had an interesting qualitative-quantitative composition of the pigment fraction. Pheophytin a (within the chlorophyll subfraction) and lutein (within the carotenoid subfraction) were found to be the major natural pigments occurring in the new products. Interesting were also the concentrations of biophenols, tocopherols, and tocotrienols detected in the three new oil varieties, as well as the values of the remarkable ratio fatty acids ω₆/fatty acids ω₃ and biophenols/polyunsaturated fatty acids (PUFA) (and those of the analytical variables endurance and susceptibility to autoxidation). The chemometric analyses showed how different multivariate methods were effective in discriminating between oil varieties. Thus, these chemometrics can effectively be used for traceability or authentication of the new products. We concluded how the three new olive cultivars can actually lead to new advances in the olive growing sector.     

Nabali olive oil ripening and oil properties

The effect of ripening of "Nabali" olives on the yield and some properties of the extracted oil was examined. Seven samples were harvested every 2 weeks and analyzed for moisture and oil content. The extracted oil was subjected to chemical analysis. Oil content increased and there were some significant changes in the proportions of fatty acids. Nabali olive oil is relatively high in arachidic, stearic and linoleic acids.     

Virgin olive oil phenols inhibit proliferation of human promyelocytic leukemia cells (HL60) by inducing apoptosis and differentiation

Although epidemiologic evidence and animal studies suggest that olive oil may prevent the onset of cancer, the components responsible for such an effect and their mechanisms of action remain largely unknown. In the present study, we investigated the effect of a virgin olive oil phenol extract (PE) on proliferation, the cell cycle distribution profile, apoptosis, and differentiation of the human promyelocytic cell line HL60. PE inhibited HL60 cell proliferation in a time- and concentration-dependent manner, as demonstrated by the viable cell count and 3-[4,5-dimethyl(thiazol-2-yl)]-3,5-diphenyltetrazolium bromide (MTT) metabolism. Cell growth was completely blocked at a PE concentration of 13.5 mg/L; apoptosis was also induced as detected by fluorescence microscopy and flow cytometry. Determination of the cell cycle distribution by flow cytometry revealed an accumulation of cells in the G(0)/G(1) phase. Two compounds isolated from PE, the dialdehydic forms of elenoic acid linked to hydroxytyrosol (3,4-DHPEA-EDA) and to tyrosol (pHPEA-EDA), were shown to possess properties similar to those of PE; they account for a part of the powerful effects exerted by the complex mixture of compounds present in PE. The concentrations of the different compounds in PE were determined by HPLC, and the purity of 3,4-DHPEA-EDA and pHPEA-EDA was ascertained by NMR. Treatment with PE induced a differentiation in HL60 cells, which subsequently acquired the ability to produce superoxide ions and reduce nitroblue tetrazolium to formazan. These results support the hypothesis that polyphenols play a critical role in the anticancer activity of olive oil.     

Olive oil phenols are absorbed in humans

Animal and in vitro studies suggest that olive oil phenols are effective antioxidants. The most abundant phenols in olive oil are the nonpolar oleuropein- and ligstroside-aglycones and the polar hydroxytyrosol and tyrosol. The aim of this study was to gain more insight into the metabolism of those phenols in humans. We measured their absorption in eight healthy ileostomy subjects. We also measured urinary excretion in the ileostomy subjects and in 12 volunteers with a colon. Subjects consumed three different supplements containing 100 mg of olive oil phenols on separate days in random order. Ileostomy subjects consumed a supplement with mainly nonpolar phenols, one with mainly polar phenols and one with the parent compound oleuropein-glycoside. Subjects with a colon consumed a supplement without phenols (placebo) instead of the supplement with oleuropein-glycoside. Ileostomy effluent and urine were collected for 24 h after supplement intake. Tyrosol and hydroxytyrosol concentrations were low (< 4 mol/100 mol of intake) in the ileostomy effluent, and no aglycones were detected. We estimated that the apparent absorption of phenols was at least 55-66% of the ingested dose. Absorption was confirmed by the excretion of tyrosol and hydroxytyrosol in urine. In ileostomy subjects, 12 mol/100 mol and in subjects with a colon, 6 mol/100 mol of the phenols from the nonpolar supplement were recovered in urine as tyrosol or hydroxytyrosol. In both subject groups, 5--6 mol/100 mol of the phenols was recovered from the polar supplement. When ileostomy subjects were given oleuropein-glycoside, 16 mol/100 mol was recovered in 24-h urine, mainly in the form of hydroxytyrosol. Thus, humans absorb a large part of ingested olive oil phenols and absorbed olive oil phenols are extensively modified in the body.     

Bioavailability of polycyclic aromatic hydrocarbons from buried shoreline oil residues thirteen years after the Exxon Valdez oil spill: a multispecies assessment

Seven taxa of intertidal plants and animals were sampled at 17 shoreline sites in Prince William Sound ([PWS]; AK, USA), that were heavily oiled in 1989 by the Exxon Valdez oil spill (EVOS) to determine if polycyclic aromatic hydrocarbons (PAH) from buried oil in intertidal sediments are sufficiently bioavailable to intertidal prey organisms that they might pose a health risk to populations of birds and wildlife that forage on the shore. Buried residues of EVOS oil are present in upper and middle intertidal sediments at 16 sites. Lower intertidal (0 m) sediments contain little oil. Much of the PAH in lower intertidal sediments are from combustion sources. Mean tissue total PAH (TPAH) concentrations in intertidal clams, mussels, and worms from oiled sites range from 24 to 36 ng/g (parts per billion) dry weight; sea lettuce, whelks, hermit crabs, and intertidal fish contain lower concentrations. Concentrations of TPAH are similar or slightly lower in biota from unoiled reference sites. The low EVOS PAH concentrations detected in intertidal biota at oiled shoreline sites indicate that the PAH from EVOS oil buried in intertidal sediments at these sites have a low bioavailability to intertidal plants and animals. Individual sea otters or shorebirds that consumed a diet of intertidal clams and mussels exclusively from the 17 oiled shores in 2002 were at low risk of significant health problems. The low concentrations of EVOS PAH found in some intertidal organisms at some oiled shoreline sites in PWS do not represent a health risk to populations of marine birds and mammals that forage in the intertidal zone.     

Stabilization of olive oil by microwave heating

Olive fruits of Picual and Shemlalli cultivars were exposed to microwaves generated from an oven at low and moderate power settings for 9 and 12 min. The gross chemical compositions of the non-microwaved and microwaved fruits of both cultivars were determined. In general, the data show that microwave treatment did not cause any change in the fruit components of Picual and Shemlalli cultivars. The stability of extracted olive oil samples from the fruits of both cultivars was followed up by determining the lipase and lipoxygenase activities during storage. The effect of microwave heating on the lipase and lipoxygenase activities of olive oil samples was dependent upon the oven power level, the moderate power setting of the microwave oven produced the least free fatty acid and hydroperoxide contents respectively. The results demonstrated that microwave treatment obviously decreased the enzyme activities during storage and consequently increased the stability of olive oil.     

Postprandial triacylglycerols from dietary virgin olive oil are selectively cleared in humans

The aims of the present study were to evaluate the effect of a meal rich in virgin olive oil on triacylglycerol composition of human postprandial triacylglycerol-rich lipoproteins (fraction Sf > 400), and to assess the role of the triacylglycerol molecular species concentration and polarity on lipoprotein clearance. Fasting (0 h) and postprandial blood samples were collected hourly for 7 h from eight healthy normolipidemic subjects after the ingestion of the meal. Plasma and lipoprotein triacylglycerol concentrations increased quickly over fasting values and peaked twice at 2 and 6 h during the 7-h postprandial period. The triacylglycerols in the lipoprotein fraction at 2 h generally reflected the composition of the olive oil, however, the proportions of the individualmolecular species were altered by the processes leading to their formation. Among the major triacylglycerols, the proportion of triolein (OOO; 43.6%) decreased (P < 0.05), palmitoyl-dioleoyl-glycerol (POO; 31. 1%) and stearoyl-dioleoyl-glycerol (SOO; 2.1%) were maintained and linoleoyl-dioleoyl-glycerol (LOO; 11.4%) and palmitoyl-oleoyl-linoleoyl-glycerol (POL; 4.6%) significantly increased (P < 0.05) compared with the composition of the triacylglycerols in the olive oil. Smaller amounts of endogenous triacylglycerol (0.8%), mainly constituted of the saturated myristic (14:0)and palmitic (16:0) fatty acids, were also identified. Analysis of total fatty acids suggested the presence of molecular species composed of long-chain polyunsaturated fatty acids of the (n-3) family, docosapentaenoic acid, [22:5(n-3)] and docosahexaenoic acid (DHA), [22:6(n-3)] and of the (n-6) family [arachidonic acid, [20:4(n-6)]. The fastest conversion of lipoproteins to remnants occurred from 2 to 4 h and was directly related to the concentration of the triacylglycerols in the lipoprotein particle (r = 0.9969, P < 0.05) and not with its polarity (r = 0.1769, P > 0.05). The rates of clearance were significantly different among the major triacylglycerols (OOO, POO, OOL and POL) (P < 0.05) and among the latter ones and PLL (palmitoyl-dilinoleoyl-glycerol, POS (palmitoyl-oleoyl-stearoyl-glycerol) and OLL (oleoyl-dilinoleoyl-glycerol) (P < 0.01). OOO was removed faster and was followed by POO, OOL, POL, PPO (dipalmitoyl-oleoyl-glycerol), SOO, PLL, POS and OLL.     

Olive oil phenols inhibit human hepatic microsomal activity

We have examined the inhibition of human hepatic microsomal androstenedione 6beta-hydroxylation and both reductive and oxidative 17beta-hydroxysteroid dehydrogenase (17beta-HSD) activity by complex phenols found in olive oil. Structurally similar compounds were also examined for comparison. Androstenedione 6beta-hydroxylase activity was inhibited by oleuropein glycoside, hydroxytyrosol and gallic acid. Oleuropein glycoside, hydroxytyrosol, gallic acid and dihydroxybenzoic acid also inhibited reductive 17beta-HSD activity. Oxidative 17beta-HSD activity was not inhibited by any of the compounds tested; however gallic acid stimulated activity by approximately 30%. Androstenedione 6beta-hydroxylase activity showed atypical kinetics. For oleuropein glycoside, hydroxytyrosol and gallic acid the apparent K(i) values were determined to be 80, 77 and 70 micromol/L, respectively. Analysis of structural features of inhibitory compounds established that a 3,4-dihydroxyphenyl ethanol structure was required for inhibition of androstenedione 6beta-hydroxylase for this group of compounds.     

Incorporation of dietary triacylglycerols from olive oil and high-oleic sunflower oil into VLDL triacylglycerols of hypertensive patients.

OBJECTIVES: To establish whether the ingestion of diets enriched with olive oil or high-oleic sunflower oil may produce changes in the composition of VLDL triacylglycerols from hypertensive patients. It could be relevant for the uptake and metabolism of triacylglycerol-derived metabolites by extrahepatic tissues. DESIGN: Patients were assigned to the diets in a random-order sequence. SUBJECTS: The participants were 24 hypertensive patients recruited from a religious community. INTERVENTIONS: The study was conducted over two four week periods with a four week washout period between both MUFA diets. RESULTS: Dietary olive oil kept in balance the content of saturated fatty acids and decreased the content of arachidonic acid in VLDL triacylglycerols. HOSO diet reduced the content of palmitic acid and increased the content of linoleic acid. There was also a decrease in trioleate-glycerol and an increase in tripalmitate-glycerol of VLDL after the MUFA diets, but these effects were more pronounced in the HOSO group. Intake of olive oil decreased the content of disaturated triacylglycerols and increased the content of dioleate-containing triacylglycerols. A decrease in palmitate-dioleate-glycerol after dietary HOSO was observed. Olive oil (but not HOSO) promoted the presence of long-chain PUFA of n-3 family at the sn-2 position of VLDL triacylglycerols. CONCLUSIONS: Our data indicate that olive oil and HOSO, providing a similar concentration of MUFA (oleic acid), differ in the formation of VLDL triacylglycerols in hypertensive patients.     

Antioxidant and anti-atherogenic activities of olive oil phenolics

The aim of the current study was to investigate the antioxidant and cellular activity of the olive oil phenolics oleuropein, tyrosol, hydroxytyrosol, and homovanillic alcohol (which is also a major metabolite of hydroxytyrosol). Well-characterized chemical and biochemical assays were used to assess the antioxidant potential of the compounds. Further experiments investigated their influence in cell culture on cytotoxic effects of hydrogen peroxide and oxidized low-density lipoprotein (LDL), nitric oxide production by activated macrophages, and secretion of chemoattractant and cell adhesion molecules by the endothelium. Inhibitory influences on in vitro platelet aggregation were also measured. The antioxidant assays indicated that homovanillic alcohol was a significantly more potent antioxidant than the other phenolics, both in chemical assays and in prolonging the lag phase of LDL oxidation. Cell culture experiments suggested that the olive oil phenolics induce a significant reduction in the secretion of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 (and a trend towards a reduced secretion of monocyte chemoattractant protein-1), and protect against cytotoxic effects of hydrogen peroxide and oxidized LDL. However, no influence on nitric oxide production or platelet aggregation was evident. The data show that olive oil phenolics have biochemical and cellular actions, which, if also apparent in vivo, could exert cardioprotective effects.     

Virgin olive oil: a key healthy component of the Mediterranean diet

Oxidative stress is in the basis of some diseases such as atherosclerosis, and is considered to be very important from the point of view of ageing. Biological membranes are very sensitive to oxidative stress because the presence of carbon-carbon double bonds in the lipid tails of their phospholipids. The type of dietary fat influences several biochemical parameters at the membrane level because membranes adapt their lipid composition to some extent in response to dietary fat. It is well know that dietary fat may modulate membrane susceptibility to oxidation, thus probably affecting in a direct or indirect way the susceptibility to oxidative stress-related phenomena. In this review we summarize more than 15 years of research on the role of dietary fat, namely virgin olive oil, from the point of view of mitochondrial oxidative stress, ageing and atherosclerosis prevention.     

Anticarcinogenic compounds of olive oil and related biomarkers

Olive oil, one of the oldest vegetable oils consumed without any refining, is associated with a reduced risk of a number of common cancers. Minor constituents of virgin olive oil have been suggested to be among the major chemopreventive components. A brief overview is presented of recent findings concerning the bioavailability of certain important olive oil minor components including efficient antioxidant polyphenols, the triterpene hydrocarbon squalene and β-sitosterol, considered as putative nutritional biomarkers, in relation to the incidence of cancer.