Harvard University researchers recently suggested that adding just a spoonful of olive oil to your diet every day not only provides powerful health benefits but also reduces the risk of dementia by up to 28%.
The findings, however, have been dissected by another expert who believes there are more limitations to the study than conceded by the authors, and that the type of person who eats a lot of olive oil is simply different from the type of person who doesn’t. For one thing, olive oil is expensive, he observed.
But the researchers argue that while healthy “Mediterranean” diets in general tend to include a dash of olive oil, the ingredient itself stands out for its beneficial qualities.
“Typically, people who use olive oil for cooking or as a dressing have a better overall diet quality, but interestingly, we found the association to be regardless of this factor,” said Harvard University nutritionist Anne-Julie Tessier.
Science Alert reports that Tessier and colleagues combined the results of surveys on nurses and health professionals conducted from the 1970s and 1980s. All were free of heart disease and cancer when the surveys first introduced questions on olive oil consumption in 1990. In the years that followed, 4 751 of the 92 383 selected participants died from dementia-related causes.
The researchers found adults who regularly consumed more than 7g of olive oil a day (a little more than a teaspoon, or about half a tablespoon) were 28% less likely to die of dementia-related diseases than those who never or rarely consumed olive oil.
“Olive oil may exert anti-inflammatory and neuroprotective effects because of its high content of monounsaturated fatty acids and other compounds with antioxidant properties, like vitamin E and polyphenols,” Tessier and team said in their paper, which was published in JAMA Network.
Animal studies suggest specific types of fats, likes monounsaturated fatty acids found in olive oil, can have protective benefits on health, and polyphenols can help prevent the amyloid-plaques associated with Alzheimer’s disease.
Limitations include that study participants were primarily white and educated, meaning the results can’t yet be generalised across diverse populations.
Additionally, as it was an observational study, the researchers can’t directly link the outcomes to olive oil just yet.
Previous research has also suggested people who regularly consume olive oil have about a 30% lower risk of dying from a neurodegenerative disease.
Casting doubt
In a Medscape commentary titled “It would be nice if olive oil really did prevent dementia”, F Perry Wilson, an associate professor of medicine and public health and director of Yale’s Clinical and Translational Research Accelerator, said: “As is always the case in the world of nutritional epidemiology, we need to be careful… There are a lot of reasons to doubt the results of this study.”
He wrote:
Nearly 100 000 participants, all healthcare workers, filled out a food frequency questionnaire every four years with 130 questions touching on all aspects of diet: how often do you eat bananas, bacon, olive oil?
Participants were followed for more than 20 years, and if they died, the cause of death was flagged as being dementia-related or not.
Over that time frame there were around 38 000 deaths, of which 4 751 were due to dementia.
The rest is just statistics. The authors show that those who reported consuming more olive oil were less likely to die from dementia — about 50% less likely, if you compare those who reported eating more than 7g of olive oil a day, with those who reported eating none.
And we could stop there if we wanted to. But we need to dig deeper because this study has the same problems as all nutritional epidemiology studies.
Number one, no one is sitting around drinking small cups of olive oil. They consume it with other foods.
It was clear from the food frequency questionnaire that people who consumed more olive oil also consumed less red meat, more fruits and vegetables, more whole grains, more butter, and less margarine. And those are just the findings reported in the paper.
I suspect that people who eat more olive oil also eat more tomatoes, for example, though data this granular aren’t shown. So, it can be really hard, in studies like this, to know for sure that it's actually the olive oil that is helpful rather than some other constituent in the diet.
The flip side of that coin presents another issue. The food you eat is also a marker of the food you don’t eat. People who ate olive oil consumed less margarine, for example.
At the time of this study, margarine was still adulterated with trans-fats, which a pretty solid evidence base suggests are really bad for your vascular system.
So perhaps it’s not that olive oil is particularly good for you but that something else is bad for you.
In other words, simply adding olive oil to your diet without changing anything else may not do anything.
The other major problem with studies of this sort is that people don’t consume food at random.
The type of person who eats a lot of olive oil is simply different from the type of person who doesn’t. For one thing, olive oil is expensive.
We aren’t provided data on household income in this study, but we can see that the heavy olive oil users were less likely to be current smokers and they got more physical activity.
Now, the authors are aware of these limitations and do their best to account for them. In multivariable models, they adjust for other stuff in the diet, and even for income (sort of; they use census tract as a proxy for income, which is really a broad brush), and still find a significant though weakened association showing a protective effect of olive oil on dementia-related death.
But still – adjustment is never perfect, and the small effect size here could definitely be due to residual confounding.
There is one reason to believe that this study is true, but it's not really from this research. It’s from the PREDIMED randomised trial.
This is nutritional epidemiology published in 2018, where investigators in Spain randomised around 7 500 participants to receive a litre of olive oil once a week vs mixed nuts, vs small non-food gifts, the idea being that if you have olive oil around, you’ll use it more.
And people who were randomly assigned to get the olive oil had a 30% lower rate of cardiovascular events. A secondary analysis of that study found that the rate of development of mild cognitive impairment was 65% lower in those who were randomly assigned to olive oil.
That’s an impressive result.
So, there might be something to this olive oil thing, but I’m not quite ready to add it to my “pleasurable things that are still good for you” list just yet.
Study details
Consumption of Olive Oil and Diet Quality and Risk of Dementia-Related Death
Anne-Julie Tessier, Marianna Cortese, Changzheng Yuan, et al
Published in JAMA Network Open on 6 May 2024
Abstract
Importance
Age-standardised dementia mortality rates are on the rise. Whether long-term consumption of olive oil and diet quality are associated with dementia-related death is unknown.
Objective
To examine the association of olive oil intake with the subsequent risk of dementia-related death and assess the joint association with diet quality and substitution for other fats.
Design, Setting, and Participants
This prospective cohort study examined data from the Nurses’ Health Study (NHS; 1990-2018) and Health Professionals Follow-Up Study (HPFS; 1990-2018). The population included women from the NHS and men from the HPFS who were free of cardiovascular disease and cancer at baseline. Data were analysed from May 2022 to July 2023.
Exposures
Olive oil intake was assessed every four years using a food frequency questionnaire and categorised as (1) never or less than once per month, (2) greater than 0 to less than or equal to 4.5 g/d, (3) greater than 4.5 g/d to less than or equal to 7 g/d, and (4) greater than 7 g/d. Diet quality was based on the Alternative Healthy Eating Index and Mediterranean Diet score.
Main Outcome and Measure
Dementia death was ascertained from death records. Multivariable Cox proportional hazards regressions were used to estimate hazard ratios (HRs) and 95% CIs adjusted for confounders including genetic, sociodemographic, and lifestyle factors.
Results
Of 92 383 participants, 60 582 (65.6%) were women and the mean (SD) age was 56.4 (8.0) years. During 28 years of follow-up (2 183 095 person-years), 4751 dementia-related deaths occurred. Individuals who were homozygous for the apolipoprotein ε4 (APOE ε4) allele were 5 to 9 times more likely to die with dementia. Consuming at least 7 g/d of olive oil was associated with a 28% lower risk of dementia-related death (adjusted pooled HR, 0.72 [95% CI, 0.64-0.81]) compared with never or rarely consuming olive oil (P for trend < .001); results were consistent after further adjustment for APOE ε4. No interaction by diet quality scores was found. In modelled substitution analyses, replacing 5 g/d of margarine and mayonnaise with the equivalent amount of olive oil was associated with an 8% (95% CI, 4%-12%) to 14% (95% CI, 7%-20%) lower risk of dementia mortality. Substitutions for other vegetable oils or butter were not significant.
Conclusions and Relevance
In US adults, higher olive oil intake was associated with a lower risk of dementia-related mortality, irrespective of diet quality. Beyond heart health, the findings extend the current dietary recommendations of choosing olive oil and other vegetable oils for cognitive-related health.
Medscape article – It Would Be Nice if Olive Oil Really Did Prevent Dementia
See more from MedicalBrief archives:
More olive oil, less butter, reduces risk of CVD mortality – large Harvard analysis
Dietary fat intake and risk of stroke — Two large 27-year studies
Mediterranean diet linked to 41% reduced risk of late-stage AMD