Last year, a study came out showing that beef jerky and other cured meats, could trigger mania in bipolar disorder (paper, popular article). It was a pretty big deal, getting coverage in the national press and affecting the advice psychiatrists (including me) gave their patients.

The study was pretty simple: psychiatrists at a mental hospital in Baltimore asked new patients if they had ever eaten any of a variety of foods. After getting a few hundred responses, they compared answers to controls and across diagnostic categories. The only hit that came up was that people in the hospital for bipolar mania were more likely to have said they ate dry cured meat like beef jerky (odds ratio 3.49). This survived various statistical comparisons and made some biological sense.

The methodology was a little bit weird, because they only asked if they'd ever had the food, not if they'd eaten a lot of it just before becoming sick. If you had beef jerky once when you were fourteen, and ended up in the psych hospital when you were fifty-five, that counted. Either they were hoping that "ever had beef jerky at all" was a good proxy for "eats a lot of beef jerky right now", or that past consumption produced lasting changes in gut bacteria. In any case, they found a strong effect even after adjusting for confounders and doing the necessary Bonferroni corrections, so it's hard to argue with success.