The link below isn’t an actual study (I couldn’t get my hands on it), but it’s a very interesting article that discusses and extrapolates on the study:
In the study that the paper discusses, patients who were depressed were tested for leaky gut (a condition whereby the intestines let too many things through into your bloodstream).
Leaky Gut Leads to Depression
Researchers found that 35% of patients tested positive for leaky gut. And it’s likely that more than that may have had minor cases of leaky gut that would not have shown up on the tests.