A large-scale analysis of people who eat mushrooms suggests they have a lower risk of developing depression.

The association is still a mystery, and for now, the authors say the data should be interpreted with caution. There's always a chance the results are a mere correlation, especially since eating more mushrooms didn't seem to lower the odds of depression any further.

That said, this is one of the first large observational studies on general mushroom consumption and depression. It includes the diet and mental health data of more than 24,000 adults across the United States from 2005 to 2016.

The findings don't differentiate between various types of mushrooms, but they are consistent with several small clinical trials on lion's mane mushrooms (Hericium erinaceus), which found eating certain types of fungi can help reduce depression and anxiety.

"The study adds to the growing list of possible health benefits of eating mushrooms," says public health scientist Joshua Muscat from the Pennsylvania State University.

What it is specifically about some mushrooms that makes them good for our health is still a puzzle. 

White button mushrooms (Agaricus bisporus) are the most commonly eaten fungi in the US and are full of potassium, which is thought to help lower anxiety. Other edible mushrooms like lion's mane are known to contain neurotrophic factors linked to brain health, as well as anti-inflammatory agents, which are thought to help alleviate symptoms of depression. 

But nutrition science is tricky business. Mushrooms host a variety of vitamins, minerals and antioxidants that could be contributing to their apparent antidepressant effects. Teasing out which factors are at play will require many more molecular, clinical, and epidemiological studies. 

Nevertheless, there's a powerful antioxidant known as ergothioneine contained in mushrooms that scientists have their eye on. Humans can only get it through diet, and mushrooms have it in the highest concentrations of any fresh foods we consume. 

In recent animal models, this antioxidant has been found to cross the bloodstream barrier that separates the brain from the rest of the body, which suggests ergothioneine could have some effect on neurological health.

Other animal models suggest this antioxidant plays a role in gut health, too, where there are also neurons that can also impact a person's mood. Whether the same can be said of humans remains to be investigated.

"Mushrooms are the highest dietary source of the amino acid ergothioneine – an anti-inflammatory which cannot be synthesized by humans," says epidemiologist Djibril Ba from Penn State.

"Having high levels of this may lower the risk of oxidative stress, which could also reduce the symptoms of depression."

Still, that's just a potential explanation. More research among larger cohorts will need to study what is different about specific mushrooms and how those differences ultimately impact human health.

The data in this case came from the US National Health and Nutrition Examination Survey, in which participants nationwide were asked to recall how many mushrooms they'd eaten in the two days prior. Their depression was then measured using a standardized patient health questionnaire. 

The observed association between mushroom consumption and lower odds of depression was independent of other confounding factors, like social status, economic status, lifestyle risk factors, self-reported disease, and medication use.

The people most likely to eat mushrooms were college-educated, non-Hispanic white women, according to the authors. But the link to depression was only clear when they compared mushroom eaters to non-eaters.

Within the cohort of mushroom eaters, those who ate them relatively a lot, didn't seem to show any additional benefits.

In a further analysis of the data, the authors compared those who ate one serving of mushrooms per day with those who ate one serving of red or processed meat. Interestingly, the substitution was not associated with lower odds of depression.

Clearly, there's still a lot we don't know about the relationship between mushrooms and mental health. But given how often the relationship keeps popping up in studies, it's worth exploring more.

"These findings highlight the potential clinical and public health importance of mushroom consumption as a means of reducing depression and preventing diseases," the authors conclude.

The study was published in the Journal of Affective Disorders.