The Mediterranean diet is perhaps the best researched of all diets in terms of health outcomes and several studies have shown various benefits, most notably on stroke¹ and atrial fibrillation². This diet contains a lot of olive oil and trials of the Mediterranean diet sometimes include additional olive oil, as the monounsaturated fatty acids (MUFA), mostly oleic acid, are suspected to be behind at least some of the diet’s health benefits. While most of these large trials have been done in places where the Mediterranean diet is common, now, for the first time, a large study has examined the effect of olive oil consumption among the US population, which has far lower average consumption of olive oil. The study provides evidence that olive oil consumption is linked to reductions in mortality by several causes;³ however, it does not demonstrate causation and it remains possible that other factors may be responsible for the improvement in health outcomes.

Decrease in All-Cause and Cause-Specific Mortality

In the observational study, published in the Journal of the American College of Cardiology, researchers examined data from over 92,000 subjects in two prospective cohorts, 60,582 women in the Nurses’ Health Study and 31,801 men in the Health Professionals Follow-up Study, between 1990-2018. Over a period of 28 years, participants replied to questionnaires every four years that asked about use of olive oil (for salad dressing, baking, frying, sautéing, and spreading on bread), other vegetable oils (e.g., corn, safflower, soybean, canola oil), margarine, butter, and dairy fat. The researchers averaged the consumption of these fats during the follow-up years.

The researchers found that during the course of the study, the average consumption of olive oil increased from 1.6 g/day to 4 g/day. After adjusting for multiple confounders, compared with participants who rarely or never consumed olive oil, those in the highest quartile for olive oil consumption showed decreases in risk of death from the following causes:³

• All-causes (HR, 0.81; 95% CI, 0.78-0.84)
• CVD (HR, 0.81; 95% CI, 0.75-0.87)
• Cancer (HR, 0.83; 95% CI, 0.78-0.89)
• Neurodegenerative disease (HR, 0.71; 95% CI, 0.64-0.78)
• Respiratory disease (HR, 0.82; 95% CI, 0.72-0.93)

Perhaps more surprising was the comparison between the second quartile of the olive oil intake and the lowest quartile (rare/no olive oil intake), which showed a 12% reduction in mortality risk from all causes at the intake level of just 0.5 teaspoon or about 1.5 g/day of olive oil. If this finding is borne out in further studies, that’s an amazing benefit from an astonishingly low level of olive oil consumption. Another promising finding was a 27% reduction in risk of dementia-related mortality for those in the highest versus lowest category of olive oil consumption, which, considering the lack of proven preventatives for Alzheimer’s disease and dementia, could be of great importance from a public health perspective.

Is It Too Good to Be True?

Now for the bad news. While these findings seem pretty amazing, there is considerable skepticism regarding a causal relationship between olive oil intake and mortality beyond the standard complaints about bias and unreliability in self-reported data. As noted in an accompanying editorial,⁴ despite the study’s strong design, one major challenge of this type of observational study is residual confounding despite all attempts to control for them. For example, those with the highest olive oil consumption were more physically active, had a healthier diet, were more likely to have Southern European or Mediterranean ancestry, and were less likely to smoke. While attempts were made to mitigate these differences, residual effects of any one of those things could potentially explain some or all of the variance found in the study.

One of the main assertions of the study is that substitution of saturated and animal fats with olive oil could drive some of the effect on mortality: the study found that replacing 10 g/d of margarine, butter, mayonnaise, or dairy fat with the equivalent amount of olive oil was associated with 8-34% lower risk of total and cause-specific mortality. However, no significant differences were observed when olive oil was compared with other vegetable oils combined, which suggests that vegetable oils may provide the same health benefits as olive oil. Perhaps it is not the presence of olive oil in the diet that matters but the absence of less healthy saturated animal fats. This would suggest that adding olive oil to a diet that does not include oils or saturated fats, such as a whole food, plant-based diet, may not confer any additional benefit.

While the amount of olive oil consumed on average increased across the span of the study to 4 g/day (a bit less than one teaspoon), this pales in comparison to intake examined in the PREDIMED (Prevención con Dieta Mediterránea) trial, which looked at a Spanish population; average intake in that trial was 20 to 22 g/day of extra virgin olive oil and 16 to 18 g/day of refined/mixed olive oil.² Indeed, the intake in the US is so low that researchers did not attempt to include tracking of the type of olive oil consumed. It also remains unclear what constituent within olive oil might underlie its beneficial effects (MUFA like oleic acid, polyphenols, or something else).

Finally, the population under study is hardly representative of the US population as a whole. The subjects were all healthcare professionals, were almost exclusively white, had an average BMI of just over 25 kg/m,² and ate 4.8 to 7.2 fruits and vegetables/day. That makes it somewhat hard to generalize the findings, although one could argue that the potential benefits of substituting other dietary fats with olive oil might potentially have a larger impact in the general population, which tends to be less healthy overall.

Conclusions

So, what is the takeaway? Most would probably agree that substituting olive oil for saturated animal fat is likely to improve health, if only by reducing the intake of the latter. To that end, recommending that patients who consume little or no olive oil begin to do so is an important consideration. Would another vegetable oil be just as good? While this study cannot conclude as much, we have long known that not all oils are created equal and that the ratio of omega-6 fatty acids to omega-3s is important.⁵ And of course, functional medicine relies on multimodal treatments, not isolated therapies so, as always, it comes down to the patient in front of you and their specific diet and other unique circumstances.

References

1. Estruch R, Ros E, Salas-Salvadó J, et al. Primary prevention of cardiovascular disease with a Mediterranean diet supplemented with extra-virgin olive oil or nuts. N Engl J Med. 2018;378(25):e34. doi:1056/nejmoa1800389
2. Martínez-González MÁ, Toledo E, Arós F, et al. Extravirgin olive oil consumption reduces risk of atrial fibrillation: the PREDIMED (Prevención con Dieta Mediterránea) trial. Circulation. 2014;130(1):18-26. doi:1161/circulationaha.113.006921
3. Guasch-Ferré M, Li Y, Willett WC, et al. Consumption of olive oil and risk of total and cause-specific mortality among U.S. adults. J Am Coll Cardiol. 2022;79(2):101-112. doi:1016/j.jacc.2021.10.041
4. Larsson SC. Can small amounts of olive oil keep the death away? J Am Coll Cardiol. 2022;79(2):113-115. doi:1016/j.jacc.2021.11.006
5. Simopoulos AP. The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomed Pharmacother. 2002;56(8):365-379. doi:1016/s0753-3322(02)00253-6