Is Olive Oil Brain Healthy? Hope Springs Eternal
/A new study links olive oil consumption with less risk of dementia-related death, but there are reasons to be skeptical.
As you all know by now, I’m always looking out for lifestyle changes that are both pleasurable and healthy. They are hard to find – especially when it comes to diet. My kids complain about this all the time – “when you say healthy food, you just mean yucky food.” And yes, French fries are amazing, and no we can’t have them three times a day.
So when I saw an article claiming that olive oil reduces the risk of dementia, I was interested. I love olive oil – I cook with it all the time. But 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… and one to believe it’s true.
The study I’m talking about this week is this one, appearing in JAMA Network Open and it follows a well-trod formula in the nutritional epi space.
Nearly 100,000 participants – all healthcare workers – filled out a food frequency questionnaire every four years. Food frequency questionnaires look something like this.
The one in this study had 130 questions touching on all aspects of diet… how often do you eat bananas, how often do you eat bacon, how often do you eat 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 7 grams of olive oil a day to those who reported eating none.
And we could stop there if we want – I’m sure big olive oil would be happy with that. Is there such a thing as big olive oil? But no, we need to dig deeper here.
Because this study has the same problems all nutritional epidemiology studies have. Number one – no one is sitting around drinking small cups of olive oil. You eat it with other foods. And it was clear from the food frequency questionnaire that people who ate more olive oil also ate less red meat, more fruits and vegetables, more whole grains, more butter and less margarine. And that’s 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 isn’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 compared to 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 – it’s 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 lots of olive oil is simply different than the type of person who doesn’t. For one thing, olive oil is expensive. 25 ounces of olive oil is on sale at my local supermarket right now for $11.00.
A similar amount of vegetable oil goes for $4.
Isn’t it interesting that food that costs more money tends to be associated with better health outcomes (I’m looking at you, red wine)? Perhaps it’s not the food – perhaps it’s the money. 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 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.
Now – I did tell you there is one reason to believe this study is true.
But it’s not really from this study.
It’s from this one – the PREDIMED randomized trial.
This is nutritional epidemiology I can get behind. Published in 2018, investigators in Spain randomized around 7500 participants to receive a liter of olive oil once a week versus mixed nuts, versus small non-food gifts. The idea here being hey – 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 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. Though it does make me wonder – can we make French fries in the stuff?
A version of this commentary first appeared on Medscape.com.