Why We Asked the Question

Our research group has spent years trying to understand how the Mediterranean diet protects the aging brain. Extra virgin olive oil is the crown jewel of that diet, the ingredient most closely associated with its health benefits. Yet for a long time, scientists studying olive oil and cognition were doing something rather odd: treating all olive oil as the same thing.

Walk into any supermarket in Spain, where our study is based, and you will see bottles labelled "virgin olive oil", "extra virgin olive oil", and simply "olive oil", the latter being what we call common olive oil (COO), a refined product that undergoes industrial processing and loses most of its polyphenols and other bioactive compounds along the way. These are chemically and nutritionally very different products. But most studies lumped them together.

We wanted to fix that. And we wanted to go one step further: to look not just at the brain, but at what lives between the olive oil and the brain,  the gut microbiome.

The Gut as a Middleman

The idea that gut bacteria could influence brain health might sound far-fetched, but it is one of the most exciting frontiers in modern biomedical research. The so-called gut-brain axis is a two-way communication highway involving the immune system, the nervous system, and a cascade of metabolites that microbes produce when they digest what we eat.

We knew that olive oil, especially the polyphenol-rich virgin variety, had shown promising effects on gut microbiota in animal studies. But almost no human research had connected the dots: olive oil type → gut microbiota → cognitive changes. That gap is what drove our study.

What We Did

We worked within the PREDIMED-Plus cohort, a large Spanish trial involving older adults at high cardiovascular risk. From this cohort, we selected 656 participants aged 55 to 75 who had provided stool samples and completed detailed dietary questionnaires. We then followed their cognitive performance, assessed across memory, attention, executive function, and language, over two years.

Crucially, we separated virgin olive oil (VOO) consumption from common olive oil (COO) consumption, and we sequenced the gut bacteria from stool samples at baseline using 16S rRNA technology to get a detailed microbial fingerprint of each participant.

What We Found — and What Surprised Us

The results were striking, and in some ways counterintuitive.

Higher consumption of virgin olive oil was associated with better cognitive performance over the two-year follow-up  across multiple domains. It was also linked to a more diverse, richer gut microbiome. That part was broadly consistent with what we had hoped to find.

But common olive oil told the opposite story. Higher COO consumption was associated with accelerated cognitive decline and lower microbial diversity. Two oils with near-identical fat profiles, yet with starkly different effects on both the gut and the brain. The likely culprit: the polyphenols. Virgin olive oil retains compounds like hydroxytyrosol and oleuropein that are stripped away during the refining process that produces common olive oil. These molecules appear to nourish the microbiome in ways that then benefit the brain.

The most intriguing finding came from our mediation analysis. We identified a bacterial genus called Adlercreutzia as a significant mediator — meaning that part of the cognitive benefit from virgin olive oil appeared to travel through changes in this specific microbe. Adlercreutzia accounted for roughly 20% of the total effect of VOO on general cognitive function. This is, to our knowledge, the first time this taxon has been identified as a potential player in the olive oil–brain relationship in humans.

Why This Matters for World Microbiome Day — and for SDG 3

World Microbiome Day exists to remind us how profoundly the invisible world of microbes shapes our health. Our study is a vivid illustration of that message: the trillions of bacteria in your gut are not passive bystanders. What you eat changes who they are, and who they are may change how well you think.

This connects directly to the UN's Sustainable Development Goal 3 — Good Health and Well-Being — and particularly to the challenge of healthy aging. Dementia affects tens of millions of people worldwide, and that number is projected to nearly triple by 2050. Prevention strategies that work at the population level, that are affordable, culturally embedded, and enjoyable, are desperately needed. Dietary guidance that is specific and actionable "choose virgin olive oil over others" could be one such strategy, though we are careful to note that our observational study cannot prove causation.

What Comes Next

We are under no illusions about the limitations of this work. Our participants are older Spanish adults who already consume a lot of olive oil , the most common oil in their kitchens. Generalizability to other populations remains an open question. The exposure (diet) and mediator (gut microbiota) were measured at the same time point, which limits our ability to establish a clean temporal sequence. And while we adjusted for many confounders, residual confounding can never be ruled out entirely.

What we hope this study does is open a door. The 'olive oil–gut–brain' axis is real, biologically plausible, and now supported by human data. Future work should examine polyphenol content directly, compare olive oil to other commonly consumed fats globally, and  ideally,  test these associations in controlled dietary intervention trials.

A Personal Note

Our team is based at the Universitat Rovira i Virgili in Reus, Spain — deep in the heart of olive oil country. For many of us, olive oil is not just a research subject; it is the smell of our grandmothers' kitchens. There is something moving about science that starts with a drizzle of oil on a plate and ends with questions about memory, aging, and the extraordinary inner world of the gut.

On World Microbiome Day, we celebrate the fact that those trillions of microbes are not just fascinating — they may be among our most powerful allies in the quest for a healthier, longer, and cognitively richer life.

Read the full paper: Ni et al. "Total and different types of olive oil consumption, gut microbiota, and cognitive function changes in older adults." Microbiome (2026) 14:68. https://doi.org/10.1186/s40168-025-02306-4