The nitrates could be altering the gut bacteria that, in turn, influence bipolar symptoms such as mood swings and mania, say researchers from the Johns Hopkins University School of Medicine.
The theory is supported by the discovery that bipolar sufferers are less likely to need hospital care if they take probiotics, which help re-establish the 'good' bacteria in the gut.
Researchers say that processed or cured meats were the one food type that kept cropping up when they analysed the diets of bipolar patients. They reckoned that the bipolar sufferer is three times more likely to regularly eat processed meats than is a non-sufferer.
Main researcher Robert Yolken says that the discovery could open up a new door of treatment that focuses on diet and the gut.
Yolken and his team have been working on the theory for several years after first discovering that the behaviour of rats became more manic within just a couple of weeks of being fed processed meat.
For their latest study, the researchers analysed the diets and behaviour of more than a thousand people, some of whom had psychiatric problems. Yolken began to suspect nitrates as they had previously been linked to neurodegenerative diseases, and no evidence had been found to support the then prevailing theory that bipolar disease was a genetic problem.
He doubts that occasionally eating processed meats will spark a manic episode on its own, but he suspects that regularly eating the meats could have a cumulative effect, and especially when it starts to alter the gut bacteria.