In humans and rats, eating cured meats might induce manic episodes.
For people with bipolar disorder, manic episodes can be euphoric, but they can also be terrifying. In the throes of mania, some people feel like they are superhuman. They start new projects and stay up all night to work on them. In the worst cases, they cease thinking coherently: They might attempt to walk into the sea or fly off the roof.
Though medications can help manage the symptoms, no pill is perfect, and all of them have side effects. Bipolar disorder appears to be at least partly genetic, but environmental factors also play a role, perhaps by switching different genes on and off, which might spark manic episodes. And the thing that might be switching on some of these genes, according to a new study, is rather surprising: a category of preservatives in beef jerky called nitrates.
For the study, recently published in the journal Molecular Psychiatry, researchers asked people being treated for psychiatric disorders at the Sheppard Pratt Health System in Baltimore whether they had ever eaten dry cured meat, undercooked meat, or undercooked fish. Those who had eaten cured meats—which include jerky and meat sticks—were three and a half times more likely to be in the group that was hospitalized for mania compared with the control group.
Meanwhile, cured meats were not significantly associated with other types of psychiatric disorders, such as major depression, and none of the other foods participants were asked about was significantly correlated with mania.
Here is where I pause and let you compose your tweet: “Correlation does not imply causation!”
This is true, and even though the authors controlled for a lot of different factors, it doesn’t prove that beef jerky caused mania. After all, it could just be that people who were already about to go manic were staying up late, ambling down to 7-Eleven, and stocking up on Slim Jims. (The authors controlled for the subjects’ mothers’ education, which is a proxy for socioeconomic status, but not a perfect one.)
So they decided to test this hypothesis further. They turned to rats.
First, they fed one group of rats normal food, and fed another group a piece of beef jerky every other day. They tried to make the rat-adjusted version roughly proportionate to the amount that a human would eat for a snack. Within two weeks, the jerky-eating rats began sleeping irregularly and behaving more excitedly. In other words, they seemed manic.
“We were able to get an effect in rats that was pretty consistent with what we were seeing in people,” said Robert Yolken, a professor of neurovirology at the Johns Hopkins University School of Medicine and one of the authors of the study.
They found the same difference when they compared rats that were given jerky with rats that were given a special, nitrate-free meat. The nitrate-free-meat rats? Normal. The beef-jerky rats? Extremely hyper.
Yolken and his team then looked inside the brains and guts of the rats. Their brains exhibited gene and molecular-pathway changes that are similar to the kind seen in people with bipolar disorder. What’s more, the manic rats, the ones with nitrate in their diet, had different kinds of bacteria living in their guts than the control rats did.
The researchers don’t know exactly why the nitrates had this effect. Nitrates have antibacterial properties, and Yolken thinks the preservative might have been altering the microbiomes of the rats and the humans. In past research, he and his colleagues found that when people who were hospitalized for a manic episode were given probiotics, they were less likely to be rehospitalized in the next six months.
It’s not totally clear how these microbiome changes affect the brain. According to the researchers, the bacteria might be sending signals through the vagus nerve, which connects the gut and brain. Or they could be releasing chemicals called butyrates that travel through the circulatory system to the brain, where they influence the production of mood-setting hormones called neurotransmitters.
Yolken and his colleagues “took the innovative approach of finding something in humans and testing it in the animal model,” said Melvin McInnis, an expert in bipolar disorder at the University of Michigan who was not involved in the study. “That brings credibility to the study in a way that would lead us to take these results as valid.”
Based on this study alone, Yolken said, it’s too early to say that people with bipolar disorder should avoid nitrates.
But there are already other health risks associated with eating too much processed meat. As McInnis put it, “Eating a lot of foods with nitrates is not a good idea anyway, and this adds another piece of evidence to that.”
Either way, we need many more studies before we can conclusively prove that anything in beef jerky is sparking manic episodes. In the meantime, this research adds to the pile of evidence that there are strong connections between what we eat and how we feel.
Olga Khazan is a staff writer at The Atlantic.