In a new study published this month by Augusta University in Clinical and Transitional Gastroenterology Journal, researchers found the first known link between probiotic use, bacterial overgrowth in the small intestine and symptoms of brain fogginess.

This study is noteworthy as it challenges the perception probiotics are safe, a perception generated from a 2015 meta-analysis of 57 studies that indicated probiotics cause no harm and reinforced by physicians who recommend probiotics to patients while doubting their efficacy but believing they 'do no harm'. 

In the study, patients reported confusion and difficulty concentrating while also experiencing symptoms like bloating, pain and gas. All consumed probiotics. 

These patients were found to have large colonies of bacteria in their small intestine, which produced high levels of D-lactic acid. D-lactic acid is temporarily toxic to brain cells and interferes with cognition. The issue here is that the bacteria did not colonize the large intestine as expected. 

Some patients' in the study had such high levels of D-lactic acid, that brain fogginess could last hours after eating. Some had to stop working because of their symptoms.

Patients were treated with antibiotics, their probiotics were stopped and dietary advice provided. 85% of patients saw a complete resolution of their brain fogginess after treatment reaffirming that symptoms were related to D-lactic acidosis and small intestinal bacterial overgrowth.