While current research is finding that modulating our gut microbiome through probiotics can have a number of positive health outcomes, a new study is suggesting for some people probiotics can have a dark side. The new research, from the Medical College of Georgia at Augusta University, reports a connection between symptoms associated with brain fogginess, including poor short-term memory and difficulty concentrating, and a particular bacterial overgrowth in the small intestine resulting from probiotic consumption.

The new investigation was inspired when the researchers were faced with a single patient that remarkably developed symptoms of both brain fogginess and bloating immediately after eating. Satish Rao, one of the authors on the new study, notes the patient's abdominal distention was stunningly rapid, "It happened right in front of our eyes."

The team closely examined the patient's metabolic profile and found high levels of D-lactic acid in a blood sample, with it later revealed the patient regularly consumed probiotics and yoghurt. Further research uncovered a number of patients known to both consume probiotics and suffer from brain fogginess.

Brain fogginess is not a clearly definable clinical condition, but instead more commonly refers to a variety of disorienting cognitive symptoms, such as confusion, difficulty concentrating, and poor short-term memory. The symptoms are often transient, with great difficulty in targeting exactly what triggers it.

The study gathered a cohort of subjects, all reporting symptoms of brain fog, and all with evidence of probiotic use. It was revealed that all these subjects had high levels of bacterial overgrowth in their small intestine. This overgrowth of the bacterium lactobacillus was found to produce excessive levels of D-lactic acid.

D-lactic acid is produced when certain bacteria break down sugars in food. The process can produce gases that result in bloating and abdominal discomfort, as well as D-lactic acid, which can be absorbed into the bloodstream and reach the brain. D-lactic acid has been found to be toxic to brain cells, resulting in temporary cognitive abnormalities.

"What we now know is that probiotic bacteria have the unique capacity to break down sugar and produce D-lactic acid," says Rao. "So if you inadvertently colonize your small bowel with probiotic bacteria, then you have set the stage for potentially developing lactic acidosis and brain fogginess."

The small sample size of subjects studied were then directed to cease consuming probiotics or foodstuffs containing probiotics, and administered antibiotics to reduce intestinal bacteria populations. A remarkable 85 percent of subjects suffering from symptoms of brain fog subsequently reported a complete resolution of those problems.

The study offers an intriguing counterpoint to the growing body of research centered on positive outcomes to probiotic consumption. A study in rats from the University of New South Wales last year found that probiotics were only helpful in animals with "grossly dysregulated gut health." Not only that, but the results showed that for rats on a healthy diet, the probiotics actually resulted in a small degree of memory impairment.

In both studies the mechanism behind probiotics and impaired cognitive function is still unclear. The researchers behind the current study do note limitations in what conclusions can be generated from their work. However, Rao does suggest that self-administration of probiotics should be done with caution as it could result in negative health outcomes.

"Probiotics should be treated as a drug, not as a food supplement," Rao concludes.

The research is published in the journal Clinical and Translational Gastroenterology.