Miguel Ángel Martínez. Navarra University

The Mediterranean diet is characterized by its relatively high total fat intake (from olive oil) that makes it palatable, but low in saturated fat and rich in nutrients and dietary fiber content. It is a diet that is rich in antioxidant compounds and bioactive elements with anti-inflammatory effects, and it has a low glycemic index. These health properties help to meet nutritional requirements, reach and maintain a healthy body weight, and reduce the risk for chronic disease. There is no healthy Mediterranean diet without olive oil.

In an outcome-wide systematic review and meta-analysis of the association between olive oil and hard clinical events of chronic disease, sufficient scientific evidence was found for a protective association against cardiovascular disease (CVD), type 2 diabetes, and all-cause mortality (Martínez-González et al, Clin Nutr 2022). In line with these results, another meta-analysis published, almost simultaneously published, reported that olive oil intake was associated with reduced risk of all-cause mortality and CVD events (Xia et al, Front Nutr, 2022). The summary estimate per 5-g/d increase in olive oil intake was 4 percent (95% confidence interval [CI]: 1% to 7%, p = 0.005) relative reduction in CVD and 4 percent (95% CI: 4% to 5%, p < 0.001) reduction in all-cause mortality. In the meta-analysis by Martínez-González, a 16% reduced risk of CVD (6 to 24%), standardized for every additional olive oil consumption of 25 g/d was found, and a 22% lower relative risk of diabetes (13% to 31%, per 25 g/d) without evidence of heterogeneity was estimated. Similarly, olive oil was inversely associated with all-cause mortality (11% relative reduction; 95% CI: 7% to 15%, per 25 g/d).

Both systematic reviews pooled large prospective original studies, with long-term retention and sufficient control for confounding. More importantly, the largest randomized controlled trial (RCT) ever conducted in nutrition in Europe (PREDIMED) used Mediterranean diet and free provision of extra-virgin olive oil (EVOO) and found strong and statistically significant protection against the primary end-point (coronary heart disease, stroke, cardiovascular death), also against atrial fibrillation, peripheral artery disease, breast cancer and cognitive decline as compared to a low-fat control diet (Martínez-González et al, Prog Cardiovasc Dis.2015).

 For all-cause mortality, the inverse association reported by this RCT after 4.8 years median follow-up did not reach statistical significance, probably because longer follow-up may be needed to assess the impact of EVOO on total mortality. However, the reported multivariable-adjusted hazard ratio for all-cause death (HR = 0.90) in the original report of PREDIMED was close to the pooled estimate (HR = 0.89) observed in the recent Clinical Nutrition meta-analysis. The wider confidence intervals in PREDIMED were expected given the smaller number of fatalities (n=232) and the sizes (n=2543 in the EVOO group, n=2450 in control).

 A previous meta-analysis that evaluated the association of olive oil and all forms of cancer reported that a higher intake of olive oil was associated with a reduced risk (relative risk=0.69, 95% CI: 0.62-0.77). Nonetheless, these pooled results should be interpreted carefully, considering that they incorporated different cancer types with heterogenous prognosis and also included a large number of case-control studies, which are prone to recall bias or selection bias.

 A recent report from three Spanish cohorts, where customarily there is a high consumption of olive oil and, consequently, a wide variability in intake, consistently showed a protection against CVD; and, according to that report, the highest benefit could be potentially obtained with consumptions between 20 and 30 g/day, as observed in the largest of these 3 cohorts, the EPIC-Spain study.

 In the Harvard cohorts (Nurses’ Health Study and Health Professionals Follow-up Study), which included both fatal and non-fatal cases of CVD and used repeated measurements of diet during a long follow-up period (up to 24 years), a relative reduction of 14% (95% CI: 6%-21%) in CVD risk was found for a consumption of only >0.5 tablespoon/day as compared to no consumption. This reduction would be translated into a relative risk reduction of 27% for a difference in intake of 25 g/d.

 However, the assumption of a linear dose–response trend was not consistent with the results of the EPIC-Spain study. In the “Seguimiento Universidad de Navarra” (SUN) cohort study, only after including probable cases of CVD, the inverse linear trend did become apparent. Interestingly, in the studies conducted by Guasch-Ferre et al. in large American cohorts, replacement of saturated fats or margarines by olive oil was associated with significant benefits.

In summary, a body of scientific evidence of the highest quality supports the preventive health effects of olive oil. Such accrual of rigorous evidence is not currently available for any other food.