Two of the biggest health concerns of the modern age – ultra-processed foods (UPFs) and microplastics – have one thing in common: we don’t actually know if or how they harm our health. But researchers are proposing a bold new idea: that these problems are actually linked, and UPFs contain especially high levels of microplastics that worsen mental health.
“We know ultra-processed foods have microplastics. We know that ultra-processed foods are also associated with depression and anxiety. We thought that it would be helpful to put out the hypothesis that potentially microplastics could be mediating that association, with the caveat being, it’s a very hard topic to study,” says  at the University of Ottawa in Canada.
How plausible is this idea, and can it be tested? Fabiano and his colleagues say their hypothesis hinges on a few key strands of emerging evidence. The first is that UPFs contain high levels of microplastics. While defining a UPF is a murky business – with some people arguing it includes anything that can’t be made in a home kitchen – it is generally taken to mean foods made mostly from ingredients extracted from other foods or synthesised in laboratories, which can include cookies, soft drinks and breakfast cereals.
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Such industrial processing is likely to expose food to plastic machinery, conveyer belts and packaging that can shed microplastic fragments – those less than 5 millimetres long – argue the researchers. Supporting this, the team points to a recent study that found highly processed protein products contain more microplastics than less processed ones – for instance, chicken nuggets contain than chicken breasts.
“It is very, very plausible that the more processed the food is, the more chemical contaminants and microplastics it’s bound to have,” says at the Food Packaging Forum, a non-profit organisation based in Switzerland.
But she says we don’t yet have enough evidence to say this definitively – the study highlighted by Fabiano and his colleagues only looked at 16 foods and only a handful of similar studies have been conducted, so larger studies are needed, she says. What’s more, even if UPFs are generally rich in microplastics, that doesn’t mean all of them are, or that all unprocessed foods are low in microplastics, she says.
The next piece of the puzzle is whether UPFs harm mental health. Fabiano points to a that looked at UPF intake and mental health symptoms across 10 million people. It found that those who consumed high levels of UPFs had about a 20 per cent higher risk of depressive symptoms and were around 50 per cent more likely to experience anxiety compared with those whose diet contained no or low levels of UPFs.
While this suggests a link between UPFs and poor mental health, untangling causation is tricky. For instance, people with poorer mental health may be more likely to eat UPFs, rather than the other way around, says Fabiano. What’s more, other factors could account for the link. “You can’t control for: Do they or don’t they go cycling every morning? Do they take the dog for a walk? Do they go to the gym? These can all boost mental well-being,” says  at Deakin University in Australia.
Stronger evidence comes from about . In these studies, researchers randomly assigned people with depressive symptoms to either consume their usual diet or switch to a Mediterranean one low in UPFs. Supporting the observational data, they found that those who cut down on UPFs saw greater improvements in depressive symptoms. “These trials are the gold-standard for getting towards causality,” says Berk. But these studies mostly focus on depression and are relatively small, so larger trials that assess a wider range of mental disorders are needed, says Fabiano.
Finally, if it turns out that UPFs really harm mental health, how likely is it that microplastics within them are to blame, rather than their nutritional content? Here is where making the link gets most difficult. There’s no evidence yet connecting microplastics to mental health outcomes, says Berk. That’s partly because it’s so difficult to accurately measure microplastic levels. “You’ve got to make sure you’re not contaminating samples with the lab equipment, and it requires highly specialised techniques,” says at the London School of Hygiene and Tropical Medicine.
Meanwhile, a leading explanation for why UPFs are harmful is that they’re high in fat, salt and sugar and low in fibre, all of which increase inflammation that can impair brain function and make people feel worse, says Fabiano. This probably plays a larger role than microplastics, he says. Nevertheless, figuring out whether microplastics play a role, and to what degree, is crucial, says Yates.
To do so, researchers could develop a system that scores the microplastic levels across various food types. “You could have a scale from say, 1 to 100, and assign different foods different scores,” says Fabiano. Then, they could estimate people’s microplastic intake from food diaries and relate that to the processing level of their food, along with their mental health symptoms.
Muncke and Yates say that such studies will be important, along with animal studies that show how microplastics affect physiology and brain function. “Eventually, with enough data, I do think we can get an answer,” says Muncke.
Brain Medicine
