The Mediterranean diet is considered as the foremost dietary regimen and its adoption is associated with the prevention of degenerative diseases and an extended longevity. The preeminent features of the Mediterranean diet have been agreed upon and the consumption of olive oil stands out as the most peculiar one. Indeed, the use of olive oil as the nearly exclusive dietary fat is what mostly characterizes the Mediterranean area. Plenty of epidemiological studies have correlated that the consumption of olive oil was associated with better overall health. Indeed, extra virgin olive oil contains (poly)phenolic compounds that are being actively investigated for their purported biological and pharma-nutritional properties. On 18 and 19 May 2018, several experts convened in Jaen (Spain) to discuss the most recent research on the benefits of olive oil and its components. We reported a summary of that meeting (reviewing several topics related to olive oil, not limited to health) and concluded that substantial evidence is accruing to support the widespread opinion that extra virgin olive oil should, indeed, be the fat of choice when it comes to human health and sustainable agronomy.
- Introduction
The Mediterranean dietary pattern is as an excellent model of healthy eating. The term “Mediterranean diet” (MedDiet) was coined by Ancel Keys in the early 1960s and is based on the traditional culinary practices of rural areas of Southern Italy (where he lived), Crete, other parts of Greece, and other countries of the Mediterranean basin in the 1950s–1960s [1]. There are some geographical differences in the traditional Mediterranean dietary pattern, but they all share the use of olive oil as the main culinary fat (1). Other elements included in the most frequently used definition of the MedDiet include high consumption of fruits and nuts (2), vegetables (3), legumes (4), fish (5), and whole grain cereals (6). Furthermore, a salient element of this definition is low consumption of red meat and processed meats (7) and moderate or low consumption of dairy products (8). Alcohol is consumed in modest amounts, mostly as red wine with meals (9). These nine features are used to operationally define the MedDiet as proposed by Trichopoulou et al. [2], who used the sex-specific medians of the first six elements to assign 1 point for each element when it is above the sex-specific median and the other two elements (meat and dairy) are used to assign 1 point if they are below the sex-specific median; an additional point is assigned if ethanol intake is in the range of 5–25 g/d for women and 10–50 g/d for men. Other authors have used tertiles instead of dichotomizing by medians. Many meta-analyses have summarized the findings of large and well-conducted cohort studies showing the benefits of a better adherence to a MedDiet for the prevention of non-communicable disease (NCD) [3,4,5,6,7]. Among the different cohorts that have assessed associations between adherence to the MedDiet and NCD, the Seguimiento Universidad de Navarra (SUN) project is a large and still ongoing prospective cohort of relatively young adults at baseline that was designed to assess the effects of the MedDiet in a Mediterranean setting [8]. Considering both a high adherence to the MedDiet and a physically active lifestyle, the SUN is reporting a very strong reduction in all- cause mortality [9]. The benefits of adhering to a MedDiet for the prevention of NCD have also been proven by two controlled clinical trials. One, the Lyon Diet Heart trial, used a MedDiet enriched with alpha- linolenic acid in survivors of a myocardial infarction and showed a dramatic reduction in reinfarctions compared to a control group [10]. The PREDIMED trial, an intervention with a MedDiet in primary cardiovascular prevention with either extra-virgin olive oil or mixed nuts, showed a 30% relative risk reduction in the occurrence of a first clinical cardiovascular event compared to a control group, where participants were recommended to follow a low-fat diet [11,12]. As mentioned above, olive oil, particularly extra-virgin olive oil, is the most characteristic component of the MedDiet. Several experts convened in Jaen (Spain) on 18 and 19 May 2018 to discuss the most recent research on olive oil and its components. What follows is a summary of that meeting.
2. Future Challenges to Public Health and the Role of Virgin Olive Oil
The Global Burden of Disease analyzes the relative contribution of different causes of mortality [13]. In the Western world, NCDs account for the majority of deaths and cardiovascular diseases (CVD) are still the number one contributor. Diet and lifestyle play a major preventive role. In terms of risk factors, the three world leading factors for CVD are (a) high systolic blood pressure (SBP), (b) smoking, and (c) high body mass index (BMI). For men, high SBP is the leading risk factor in nearly all high-income countries [13,14]. On average, global population SBP decreased slightly since 1980, but trends varied significantly across regions and countries, with significant decreases in North America, Western Europe and Australasia. SBP is currently highest in low-income and middle-income countries [15]. Hypertension independently increases the risk of coronary heart disease, sudden death, stroke, heart failure, and peripheral arterial disease. Several epidemiological studies have analyzed the relationship between monounsaturated fatty acids (MUFA) consumption and hypertension [16]. The OmniHeart study—published in 2005—compared three diets: one rich in carbohydrates, another one rich in vegetable proteins, and a third diet rich in MUFA. The diets rich in vegetable proteins and MUFA-compared with the carbohydrate one-reduced blood pressure and improved lipid profiles [17]. The International Study of Macro/Micronutrients and Blood Pressure (INTERMAP) is a multi-centre cross-sectional study of 4680 men and women that reported a significant inverse relationship between total MUFA intake and diastolic blood pressure (DBP) [18]. A Spanish study showed that intakes of 13 g/d of oleic acid from vegetables was associated with reductions in both SBP and DBP [18]. In the PREDIMED, when assessing 24-h ambulatory blood pressure, SBP decreased −2.3 mm Hg (95% confidence interval (CI), −4.0–−0.5) and DBP decreased −1.2 mm Hg (95% CI, −2.2–−0.2) with extra-virgin olive oil after one year of follow-up [19]. A systematic review was recently published that evaluated clinical trials of olive oil and its effects on blood pressure in individuals that were free of cardiovascular events. The paper reported that olive oil diets led to a decrease in blood pressure, while capsules providing different amounts of EVOO did not produce any effect. In particular, DBP decreased, on average, by −0.73 mm Hg (95% CI, −1.07–−0.40)) after the use of olive oil. This reduction was mainly due to extra virgin olive oil (EVOO) consumption from 10 to 50 mL per day: −1.44 mm Hg (95% CI, −1.89–−1.00); p < 0.001. However, the decrease was not statistically significant for SBP [20]. Obesity is the current most important pandemic. High BMI is the leading risk factor in women in nearly all countries in the Americas, north Africa, and the Middle East, and in many other high- income countries [21]. In Western Europe, BMI recently increased by 20% in males and 18% in females [22]. High BMI is an important risk factor for mortality and morbidity from CVD, diabetes, cancers, and musculoskeletal disorders [23,24]. Several prospective studies e.g., the EPIC [25,26] and the SUN analyzed whether the MedDiet was related to weight and BMI [27]. They were pooled in several meta-analyses [28,29] that associated adherence to MedDiet with a long-term reduction in BMI. This association was stronger when energy restriction was also applied. In addition, the PREDIMED trial, which allowed ad libitum total energy intake, showed a slight reduction in body weight gain after a median 5-year follow-up [30]. Several observational studies have also associated olive oil use with lower BMI, independently of the diet [31,32]. As per a systematic review, a diet enriched with olive oil reduced weight more than control diets: −0.92 kg (95% CI, −1.16–−0.67) and diminished BMI by −0.90, (95% CI, −0.91– −0.88), p heterogeneity < 0.001. The benefits were seen when olive oil was supplemented as part of the overall dietary pattern and not when given via capsules [33].
3. Extra-Virgin Olive Oils and Prevention of NCDs
3.1. Extra-Virgin Olive Oil and Cancer
3.1.1. Breast Cancer
Breast cancer is the second most frequently diagnosed malignant tumor and it is the most common malignancy among women [34]. Worldwide, more than 2 million new breast cancer diagnoses were made in 2018. In terms of mortality, breast cancer is the fifth cause of death among different tumor types (627,000 deaths estimated in the world in 2018) and the third cause of death among women in developing countries, and the fourth one in the most developed areas [35]. Historically, a lower incidence of breast cancer has been observed in Mediterranean countries than in other European countries or the USA [36]. In this sense, diet might play a role in breast cancer incidence because the diets of these countries are quite different. In a meta-analysis of the association between a higher adherence to the traditional MedDiet and the incidence of postmenopausal breast cancer, including data from cohort studies, the combined estimate for overall postmenopausal breast cancer incidence was 0.94 (95% CI 0.88–1.01) [37]. A significant inverse association was observed when a higher vs. a lower adherence to the traditional MedDiet were compared for estrogen receptor negative (ER-) breast cancer (Hazard Ratio (HR) 0.60 (95% CI, 0.39–0.93)). Even though moderate alcohol use (preferably in the form of red wine and consumed with meals) is a characteristic trait of the MedDiet, there is strong evidence suggesting that higher alcohol consumption is associated with a higher risk of breast cancer [38]. In the aforementioned meta-analysis, two out of the five studies incorporated alcohol consumption in the assessment of the adherence to the traditional MedDiet. Interestingly, when the studies in which the alcohol intake component for calculating the adherence to the MedDiet had not been
considered, the HR for postmenopausal breast cancer incidence among women with a higher adherence to the traditional MedDiet compared to women with a lower adherence became 0.92 (95% CI 0.87–0.98). It is also noteworthy that only one of the studies included in the meta-analysis included participants from Mediterranean countries. This latter study was done in the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort [39] by Buckland and colleagues and included participants from Italy, Greece and Spain. The authors found that better adherence to the traditional MedDiet was associated with a significant 7% reduction in the risk of postmenopausal breast cancer, compared to women in the lowest adherence group (HR = 0.93, 95% CI 0.87–0.99). With regards olive oil, to our knowledge, there is only one prospective cohort study that assessed the association between olive oil consumption and the risk of breast cancer [40]. In that study, data from Mediterranean countries from the EPIC cohort were included and no significant association was observed. Nevertheless, it has to be underscored that no distinction between varieties of olive oil was done, i.e., no specific association between extra-virgin olive oil consumption or refined olive oils consumption and the risk of postmenopausal breast cancer was reported. In addition, evidence from case-control studies suggests that olive oil intake could be inversely associated with the risk of breast cancer [41]. In the PREDIMED trial, participants at high CVD were allocated in a 1:1:1 ratio to a nutritional intervention fostering the adherence to a traditional MedDiet supplemented with either extra-virgin olive oil or mixed nuts or to a control group. Participants in the control group were advised to follow a low-fat diet [42]. The main outcome in the trial was the incidence of CVD, but incidence of breast cancer was included as a secondary outcome [43]. Among 4152 women included in the analysis, women allocated to the MedDiet supplemented with extra-virgin olive oil group exhibited a 68% relatively lower risk of incident breast cancer (HR = 0.32, 95% CI 0.13–0.79) compared to the control group. Women allocated to the MedDiet group supplemented with nuts showed a HR = 0.59 (95% 0.26–1.35), compared to the control group. In particular, dose-response observational analyses found a HR of 0.72 (95% CI 0.57–0.90) for each additional 5% of calories from extra- virgin olive oil. In terms of breast cancer, the main limitation of the PREDIMED trial was the small number of observed incident cases (only 35). However, the observed risk reduction with extra-virgin olive oil was so dramatic that even with such small number of cases, differences were highly significant.
3.1.2. Colorectal Cancer
It has been estimated that 1.4 million new cases of colorectal cancer are being annually diagnosed among men and 733,000 among women [35]. For women, colorectal cancer is the third leading cause of cancer death. As for breast cancer, historically, a lower incidence of colorectal cancer has been observed in Mediterranean countries than other European countries or the USA [36]. Moreover, similarly to breast cancer, a higher adherence to the traditional MedDiet has been proposed as a potential protective factor against colorectal cancer. In a meta-analysis of prospective cohort studies [44], a higher adherence to the traditional MedDiet was associated with a 14% lower risk of colorectal cancer (HR = 0.86, 95% CI 0.80–0.92). Unfortunately, to our knowledge, no large prospective cohort studies or large case-control studies have been recently conducted aiming to assess the association between extra-virgin olive oil consumption and the risk of colorectal cancer. It must be underlined that studying cancer development as related to diet is very difficult to carry out in humans due to the scantiness (or absence) of surrogate markers that dietary interventions could modulate. Therefore, even if we have robust epidemiological studies (with consistent results, good control for confounding and corroborated by a plethora of in vivo investigations) the true nature and extent of olive oil’s contribution to chemoprevention might never be confirmed in humans. Nevertheless, older studies by Machowetz et al. [45] and Salvini et al. [46] performed in healthy males and postmenopausal women, respectively, have reported reduced oxidative DNA damage after short-term use of phenol-rich olive oil, which might translate into a lower cancer risk.
3.2. Extra-Virgin Olive Oil and Cardiovascular Disease
Southern European countries have the lowest incidence rates of coronary heart disease [47,48] in spite of a high prevalence of classical cardiovascular risk factors [49,50]. As mentioned above, the near totality of evidence comes from epidemiological studies, with the notable exceptions of the Lyon Heart Study and the PREDIMED randomized trial. An inverse relationship between olive oil intake and coronary heart disease mortality and incidence has been reported within the frame of EPIC cohorts [39,40,41,51]. The Three-City study also found an inverse relationship between olive oil and stroke in women [52]. In this regard, Guasch-Ferré et al. reported that olive oil—specifically extra-virgin—intake in the context of a MedDiet was associated with a reduction in the risk of CVD and mortality in older high cardiovascular-risk individuals [53]. A recent meta-analysis of 32 cohort studies showed that olive oil, but not MUFAs, consumption was associated with a reduced risk of all-cause mortality, CVD events, and stroke when comparing the upper to the lower tertile of consumption [54]. This finding indicates that the minor constituents of olive oil could be responsible for its health benefits [55,56].
Modulation of Classic Cardiovascular Risk Factors
As mentioned, a traditional MedDiet, supplemented with virgin olive oil or nuts, reduced blood pressure and the risk of hypertension in high cardiovascular risk subjects [19,57]. In addition, in the context of the PREDIMED trial, DBP decreased in hypertensive women after both MedDiet interventions [58]. Lower values of DBP in the two groups promoting the MedDiet with extra virgin olive oil or nuts versus the control group were also observed in PREDIMED’s 4 year-follow up [59]. Moreover, the 5-year PREDIMED intervention with unrestricted-calorie MedDiet was associated with a decrease in weight and less gain in central adiposity compared with the control low-fat diet [30]. In this regard, the effect of the MedDiet on weight loss was previously reported in the DIRECT trial, where participants following a calorie-restricted MedDiet achieved a higher weight loss compared to low-fat and low-carbohydrate diets [60,61]. With regards (poly)phenol-rich extra-virgin olive oil, a meta-analysis of 69 experimental studies showed moderate effects for lowering SBP, with no effects on DBP [62]. In agreement, a decrease in SBP after virgin olive oil intake was described in hypertensive and coronary heart disease patients [63].
The benefits of olive oil (poly)phenols on circulating HDL cholesterol were evaluated by the European Food Safety Authority (EFSA) and the conclusion was that evidence was insufficient to establish a cause-effect relationship [64]. In a recent meta-analysis concerning the effects of phenolic compound-rich olive oil on cardiovascular risk factors, no effects were observed with regard to lipid profile. In this regard, the small number (n = 8) of included studies should be underscored [62]. It is noteworthy that a number of high quality randomized controlled trials have reported a dose-dependent increment in HDL-cholesterol after virgin olive oil intake [55,56], even though the mechanisms behind these purported effects are still unclear. The EFSA allowed a health claim with regard to the protection conferred by olive oil phenolic compounds (5 mg/day) against LDL oxidation [65]. In this regard, it has been highlighted that oxidation of the LDL particle might play a role in atherosclerosis onset and progression [66,67], but clear-cut human evidence is lacking, trials of antioxidants have failed to demonstrate benefits [68], and the contribution of oxidized LDL to atherosclerosis is still elusive. Yet, a meta-analysis of 300 experimental studies reported that oxidized LDL concentration decreased after high-phenolic olive oil intake [62]. In the EUROLIVE Study, a randomized cross-over controlled trial in healthy subjects, a linear decrease of in vivo lipid oxidative damage was observed along with the phenolic content of olive oil [56]. In addition, other studies have shown a decrease in the ability of LDL to be oxidized after the consumption of phenolic-rich olive oils [56,63,69,70]. Regarding HDL, recent studies have demonstrated that their function can be more relevant than their concentrations [71]. Hernáez et al. reported a rise of cholesterol efflux after the regular intake of virgin olive oil within the framework of the EUROLIVE Study [70]. In agreement, a traditional MedDiet, especially when supplemented with virgin olive oil, improved HDL function [72]. Interestingly, a recent article
concluded that high phenolic extra virgin Olive Oil may improve some cardiovascular risk factors [73]. Olive oil intake contributes to the homeostasis of the thrombogenic profile by improving the production of coagulation factors and biomarkers related to platelet aggregation [74]. Olive oil intake attenuates the pro-thrombotic state in the postprandial phase [75,76,77,78] and this is very likely due to the effect of hydroxytyrosol, as ad-hoc human experiments demonstrated [79]. Moreover, phenolic-rich olive oil intake also improved the postprandial pro-thrombotic state (activated coagulation factor VII, tissue plasminogen activator, tissue factor, plasminogen activator inhibitor type-1, and fibrinogen) in several randomized controlled trials in healthy subjects [80] and in hypercholesterolemic patients [74]. In a long-term randomized crossover trial with olive oil consumption, a decrease in plasma fibrinogen in women with initially high fibrinogen concentrations was also reported [81].
Type 2 diabetes includes chronic and uncontrolled hyperglycemia, which leads to serious injury of nerves and blood vessels [82]. In the ATTICA study, adherence to a MedDiet was related to a proper homeostasis of hydrocarbon metabolism in normoglycemic subjects [83]. In addition, a reduction in type 2 diabetes incidence after an intervention based on a traditional MedDiet rich in extra virgin olive oil in elderly high cardiovascular risk volunteers was observed in the initially non-diabetic type 2 subset of participants in the PREDIMED trial [84].
A significant association between certain dietary patterns and lower risk of type 2 diabetes was reported in several studies included in recent meta-analyses [12,85]. In one of these meta-analyses, Schwingshackl et al. concluded that adherence to the MedDiet reduced the risk of developing type 2 diabetes by 19% [12]. Another systematic review of eight meta-analyses of randomized controlled trials and five randomized controlled trials suggested that a MedDiet pattern can be useful for the prevention and management of type 2 diabetes mellitus and pre-diabetic states [86]. The authors found that adherence to the MedDiet was associated with lower glycated hemoglobin (HbA1c) levels and a better profile of cardiovascular risk factors, as compared with a control diet, such as a low-fat one. Two meta-analyses showed that a higher adherence to MedDiet was linked to a reduction in the risk of future diabetes by 19–23% [86]. Another meta-analysis of cohort studies reported that although diets associated with the prevention of type 2 diabetes may vary in their composition, they share some mutual components, including whole grains, fruit, vegetables, nuts, legumes, protein sources such as white meat and seafood, little or moderate alcohol consumption, and reduced intake of red and processed meats and of sugar-sweetened beverages, and abundant use of healthy oils, notably olive oil [87]. Clinical trials of the effects of olive oil on carbohydrates metabolism are scant. Carnevale et al. recently reported, in impaired fasting glucose patients, that virgin olive oil improved post-prandial glucose probably mediated by incretin up-regulation. In this regard, the highest quintile of olive oil intake in comparison with the lowest one has also been associated with a lower risk of type 2 diabetes mellitus; substituting other types of fats and salad dressings with olive oil has been inversely associated with type 2 diabetes mellitus onset [88]. In a cross-sectional study, insulin resistance was found to be lower in individuals who consumed olive oil versus sunflower oil or a mixture of vegetable oils [89]. In particular, a crossover randomized trial reported that a daily intake of extra virgin olive oil (25 mL/day) for eight weeks was inversely associated with fasting plasma glucose and HbA1c, and a number of circulating inflammatory adipokines (such as visfatin), in overweight patients with type 2 diabetes [90].
The PREDIMED trial recently reported that the random allocation of diabetics to a MedDiet with extra virgin olive oil was associated with a significant reduction in the need of starting glucose-lowering medication (Relative risk: 0.78, 95% CI, 0.62 to 0.98) [91].
4. Extra-Virgin Olive Oil and Health: Molecular Mechanisms
4.1. Cancer
Following the numerous epidemiological reports that show reverse associations between olive oil use and cancer risk, several investigators undertook mechanistic studies to substantiate this interesting hypothesis. Although the provision of antioxidants actually increases cancer risk, most research is being focusing on oxidative stress (now called redox code [92]), whose exact role in cancer etiology is, however, still elusive. Yet, modulation of the redox code by plants’ secondary metabolites might play some roles in cancer and cancer stem cells progression, as well as therapy [93]. Over the last few years, many in vitro and animal studies have demonstrated that olive oil phenolics (alcohols and secoiridoids) possess anticarcinogenic properties, which are likely not mediated by direct antioxidant actions. Several mechanisms have been investigated, including inhibition of angiogenesis [94], of proliferation and invasion [95,96,97,98], induction of apoptosis [99,100], and anti-inflammatory actions [101]. It is likely that if olive (poly)phenols have chemopreventive properties, these are due to a variety of molecular actions rather than to a single one. One example is that of oleocanthal (OC), which was shown as cytotoxic to human melanoma cells, but not to normal dermal fibroblasts; its molecular actions include the inhibition of ERK1/2 and AKT phosphorylation and downregulation of Bcl-2 expression [102]. Of note, OC inhibits cell growth more effectively than classic pharmaceutical COX inhibitors; in addition, OC inhibits colony formation and induces apoptosis-by PARP cleavage, activation of caspases 3/7, and chromatin condensation—in hepatocellular carcinoma (HCC) and colorectal cancer (CRC) cells. Yet, OC is not toxic to primary normal human hepatocytes. In addition, despite being known as an antioxidant molecule, OC induces DNA damage, dose-dependently increases intracellular ROS production, and causes mitochondrial depolarization [103]. Finally, OC has the potential to inhibit the growth of hormone-dependent breast cancer and improve sensitivity to tamoxifen therapy [104]. Oleuropein (OL) also inhibits HepG2 (human hepatoma) viability via the induction of apoptosis (i.e., upregulation of BAX and downregulation of Bcl-2), activation of the caspase pathway, and modulation of the phosphatidylinositol 3 kinase/protein kinase B (PI3K/AKT) signaling pathway, in turn suppressing the expression of activated AKT [105]. Of note, a combination of OLE and cisplatin shows synergistic effects, i.e., as compared with the addition of individual molecules effects in HepG2, leading to an increase of NO and of the pro-nerve growth factor (NGF)/NGF ratio. This is accompanied by a dose-dependent upregulation of caspase-3 and a concomitant downregulation of MMP-7 gene expression [105]. With regards hydroxytyrosol (HT), some papers reported chemoprevention via prevention of DNA damage in PBMC and inhibition of breast (MDA and MCF-7), prostate (LNCap and PC3) and colon (SW480 and HCT116) cancer cell proliferation [106]. HT also reduces papillary (TPC-1, FB-2) and follicular (WRO) thyroid cancer cell proliferation and viability by promoting apoptotic cell death via intrinsic pathways. However, these experiments were performed with high doses of HT, which diminishes their physiological relevance [107]. HT and two of its colonic metabolites (phenylacetic and hydroxyphenylpropionic acids) are able to arrest the cell cycle and promote apoptosis in HT-29 and Caco-2 cells [108]. Oleacin, another phenolic compound found in EVOO, has been described to inhibit tumor-initiating cells by metabolic and epigenetic mechanisms using concentrations close to physiologic metabolism [109,110,111]. Cellular senescence, which impairs the proliferation of damaged or premalignant cells, also plays a role in aging and age-related diseases (including cancer) and is an interesting therapeutic target [112]. Indeed, modulation of the senescence-associated inflammatory phenotype has been recently suggested as an important mechanism of action of olive oil phenols. In a recent study performed in pre-senescent human lung (MRC5) and neonatal human dermal (NHDF) fibroblasts, a four-to-six weeks supplementation with 1 μM HT or 10 μM OLE aglycone (OLE) prevented TNFα-induced inflammation, decreased the number of β-galactosidase-positive cells and p16 protein expression, IL-6, metalloprotease secretion, COX-2, and α-smooth-actin levels. In NHDF, OLE and HT treatment counteracted the senescence-associated increases of cyclooxygenase 2 (COX2) expression, nuclear transcription factor-kappa B (NFκB) protein level, and nuclear localization [113]. In short, an imbalance of the redox code increased inflammation, and deregulation of cell cycle concur to alter cellular replication [114]. Major players include the activation of some transcription factors, such as NF-κB, signal transducer and activator of transcription 3 (STAT3), MAPK, and the hypoxia-inducible factor 1α (HIF1α). These transcription factors dictate the production of inflammatory mediators, e.g., cytokines and chemokines and the activation of COX2, in turnrecruiting and activating leukocytes and triggering the inflammasome of tumor cells, resulting in more inflammatory mediators being produced and a cancer-related inflammatory microenvironment being generated and propagated in a vicious cycle [79]. Therefore, we can hypothesize that olive (poly)phenols would primarily act as anti-inflammatory molecules, but their manifold actions on cell signaling could also contribute to chemoprevention. One important cautionary note concerns the exceedingly high concentrations of (poly)phenols that are frequently employed in in vitro studies. Due to the low bioavailability of such compounds, extrapolation of in vitro data to human physiopathology is often questionable. In this respect, the most appropriate “high-concentration” studies are those carried out on GI tract cells. Other data should be interpreted in light of this caveat. In summary, olive oil and its (poly)phenols might play important roles in lessening cancer risk, as observed in the Mediterranean area [115]. Whether the association between olive oil consumption and chemoprevention is causal or casual is worth further investigation, but—even though human trials are impractical—the advice to consume olive oil as the predominant source of fat to lower cancer risk rests on solid bases.
4.2. Cardiovascular Disease
The molecular and cellular mechanisms underlying the cardioprotective effects of olive oil and its components are manifold [116,117]. Some of them are discussed below.
4.2.1. Extra Virgin Olive Oil (EVOO) and Atherosclerosis
Atherosclerosis is a common finding in cardiovascular diseases. In a subcohort of the PREDIMED that evaluated intima-media thickness (IMT), an inverse association between extra virgin olive oil (EVOO) consumption and IMT was described [118], suggesting a protective role of olive oil against the development of carotid atherosclerosis in persons at high cardiovascular risk [119]. In animal models, the administration of EVOO and some of its components such as hydroxytyrosol and squalene [120] reduced atherosclerotic lesions [121,122]. Recently, the EVOO secoiridoid oleacein was found to reduce secretion of metalloproteinases from carotid plaques [123]. Undoubtedly, this area needs to be expanded with new image technology to solve some current controversies.
4.2.2. EVOO and Endothelial Dysfunction
Endothelial dysfunction plays a key role in the development of atherosclerotic cardiovascular disease. The consumption of a MedDiet rich in extra-virgin olive oil improved endothelial function compared to the same yet low-fat diet in patients with prediabetes and diabetes participating in the CORDIOPREV clinical trial [124]. Australian men and women consuming a MedDiet rich in extra- virgin olive oil for 6 months showed significantly lower SBP and improved endothelial function [125]. Lower values of DBP were found in the PREDIMED group receiving EVOO [59]. In hypertensive rats, administration of EVOO slowed down the age-dependent increase in systolic blood pressure with concomitant decreases in nitric oxide (NO) and 8-isoprostane [126]. This beneficial effect has been attributed to the phenolic compounds present in virgin olive oil, since these compounds reverted the decreased endothelial NO synthase phosphorylation, and consequently intracellular NO levels, and increased endothelin-1 synthesis in ECV304 incubated with high glucose and fatty acids [127]. However, in Wistar rats fed extra-virgin olive oil with or without phenolic compounds, aorta vascular endothelial adhesion molecule-1 and E-selectin were lower compared to control animals. These results suggest that these changes are independent of phenolic compounds [128].
4.2.3. EVOO and Transcriptomics
Acute intake of high phenolic EVOO was able to modify the transcriptome of peripheral blood mononuclear cells through the modulation of different pathways associated with the pathophysiology of cardio-metabolic disease. These beneficial effects were more pronounced in healthy subjects who used high phenolic EVOO [129], thus providing further support to notion that olive phenolic compounds could be employed to treat chronic inflammatory states [130].
4.2.4. EVOO and Plasma Lipids
When 91 healthy men and women were randomized to consume 50 g daily of extra virgin coconut oil, EVOO, or unsalted butter for four weeks, the latter significantly increased total cholesterol/HDL-C ratio and non-HDL-C compared with coconut oil. Coconut oil and EVOO use lead to similar results for both parameters. These results evidence the complex mixture that virgin oils obtained from fruits represent [131]. In a prospective longitudinal and comparative study where 18 postmenopausal women participated in two periods of dietary intervention with either butter or extra virgin olive oil (28 days each), the latter decreased total/HDL-cholesterol and triglycerides/ HDL-cholesterol [132]. The effects of corn oil (CO) versus extra-virgin olive oil intake was tested in 54 men and women consuming 54 g of either oil for 21 days in a randomized double-blind controlled-feeding crossover trial. CO reduced non-high-density lipoprotein cholesterol compared to EVOO. There were no differences on HDL-cholesterol but APOA1 increased more with EVOO compared with CO intake [133]. The post-prandial 2-h lipid profile of 25 healthy subjects randomly allocated (in a cross-over design) to Mediterranean-type meal with 10 g of either EVOO or CO showed a significantly lower increase of LDL-C and ox-LDL in the EVOO group compared with the CO one [134]. In a random sub-sample of individuals from the PREDIMED study, after 1-year of intervention, the group consuming the MedDiet enriched in EVOO showed increased LDL resistance against oxidation and decreased the degree of LDL oxidative modifications compared to the low-fat group [135]. In the same study, the branch receiving EVOO showed increased cholesterol efflux capacity, decreased cholesteryl ester transfer protein activity and increased HDL ability to esterify cholesterol, paraoxonase-1 arylesterase activity, and HDL vasodilatory capacity, resulting in a more functional HDL [72]. HDL cholesterol efflux capacity was higher in 47 healthy European male volunteers consuming 25 mL/d EVOO of high phenolic content compared to when the same subjects received a phenolic—poor EVOO for 3 weeks [70]. Likewise, in apolipoprotein-E-deficient mice, 7 µL/mouse/day of EVOO for 2 months stimulated cholesterol efflux rate from mouse peritoneal macrophages [121]. Overall, these findings propose a better status of plasma lipoproteins by the administration of EVOO. According to lipidomics analyses, plasma ceramide levels decreased in participants receiving the MedDiet, including EVOO in the PREDIMED trial [136]. The intervention of EVOO significantly changed 20 lipid species compared with the control group. However, the latter changes were not associated with subsequent CVD risk [137] and the true contribution of such modifications to the observed risk reduction requires further investigations. In summary, the effects of EVOO on plasma lipids are yet to be fully elucidated and can only be evaluated in substitution/replacements scenarios. This is true for all kinds of lipid trials and data should be interpreted in light of these limitations.
4.2.5. EVOO and Adipose Tissue Metabolism
The adipose tissue is one of the targets of cardiometabolic prevention because inflammation, maybe the elusive oxidative stress, and secretion of adipocytokines contribute to cardiovascular risk. Much research has devoted to the study of phenolic compounds in these cells. In this regard, in 3T3-L1 adipocytes, HT was able to promote mitochondrial function by stimulating mitochondrial biogenesis through increasing the promoter transcriptional activation and protein expression of peroxisome proliferator-activated receptor coactivator 1 alpha [138]. HT, in nutritionally relevant amounts, was also able to augment the glutathione-driven antioxidant enzymatic pathways in the adipose tissue.
