Researchers looking for a connection between foodborne viruses and psychiatric disorders instead found a surprising link between mania and cured meats such as salami, hot dogs, and beef jerky.
The discovery is reported in a paper published in the journal Molecular Psychiatry and adds to evidence of potential links between diet and mental health. Although the study is observational and thus cannot identify causation, the researchers suggest that a chemical used to preserve meats, called nitrates, may be implicated.
The research was led by neurovirologist Robert Yolken from Johns Hopkins University in Baltimore, US. Over a 10-year period he and colleagues collected data from 1101 people aged between 18 and 65, with and without psychiatric disorders. Those with mental illness were recruited from the Baltimore health system, with the remainder signed up after responding to posters in health centres and universities.
After adjusting for other demographic factors, Yolken and colleagues found that participants hospitalised for mania were 3.5 times more likely to have a dietary history that included nitrate-cured meat than the control group.
The pattern was not repeated among people hospitalised with other psychiatric disorders, such as schizophrenia, major depression, or bipolar episodes without mania.
The researchers investigated further in a series of animal experiments, feeding rats normal chow or beef, with and without nitrates. The rats consuming nitrate-fortified diets consistently displayed disturbed sleep and hyperactive behaviour within two weeks on the diet.
When scaled up, the total nitrates consumed per day was equivalent to what a human might eat in a snack, such as a hot dog or salami.
Further analysis of gut microbes showed altered patterns of bacteria in the rats on the nitrate diets. The researchers also report several differences in brain patterns that have previously been identified in bipolar disorder.
“There’s growing evidence that germs in the intestines can influence the brain,” Yolken says. “And this work on nitrates opens the door for future studies on how that may be happening.”
He adds that their findings could add new insights to the multiple factors that contribute to the complex condition.
People with bipolar disorder experience periods of depression alternated with mania. Manic episodes can last for weeks to months, with heightened mood and energy levels. It can lead to risky behaviour and delusional thinking and result in frequent hospitalisations.
Commonly diagnosed in the early to mid-20s, bipolar disorder affects over three percent of young Australians and is associated with a genetic vulnerability and stressful life events.
A growing body of population studies has found associations between dietary patterns and psychiatric disorders – including a 2011 Australian study in women with bipolar disorder.
The diet associated with greater odds of bipolar disorder included processed meats, which is consistently identified as part of an unhealthy diet that confers risk of mental illness.
Converging research has identified several biological pathways by which diet might influence the brain, including its impact on inflammation, oxidative stress and gut bacteria.