Probiotic use causes “brain fog,” gas, bloating, researchers say, but NPA refutes findings


The Natural Products Association (NPA; Washington, DC) did not mince words in its critique of the study.

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Probiotic use was a consistent factor among 30 subjects who reported experiencing brain fogginess in addition to unexplained abdominal bloating, pain, and gas, according to a new study published in Clinical and Translational Gastroenterology1. The purpose of the study was to determine whether “brain fogginess,” defined as feelings of confusion or trouble concentrating, was associated with D-lactic acidosis and small intestinal bacterial overgrowth (SIBO).

D-lactic acidosis is a form of metabolic acidosis that typically occurs when someone has short-bowel syndrome, a complication from surgical resection of the small intestine to treat conditions like Crohn’s disease and intestinal cancer. D-lactic acidosis is a result of carbohydrates malabsorption and an increase in the load of carbs delivered to the colon where carbohydrate-fermented bacteria reside. In the colon, these bacteria then convert the excess carbohydrates into lactate, which is then absorbed where D-lactate accumulates in the blood. When acidosis develops, symptoms include a reduced mental state. SIBO, a condition involving excess bacteria in the small intestine, often results in chronic diarrhea and malabsorption of nutrients.

The subjects in this study did not have short-bowel syndrome or any other history of small bowel or colonic surgery, known intestinal dysmotility, recent antibiotic use, known neurological or neuropsychiatric problems, celiac disease, or renal failure. Researchers defined brain fogginess as two or more of the following symptoms for more than three months: mental confusion, cloudiness, impaired judgment, poor short-term memory, and difficulty with concentration. Subjects reported that these symptoms could last between 30 minutes to several hours, typically following a meal. A majority of the subjects reporting brain fogginess (94%) reported postprandial fatigue and weakness, while 13.3% reported having symptoms so severe they had to quit their jobs.

Of the 38 subjects, only eight did not report symptoms of brain fogginess. All subjects who reported brain fogginess also reported taking probiotics, ranging in time between three months and three years and involving multiple bacterial varieties, including Lactobacillus and/or Bifidobacterium species-both of which produce D-lactic acid-or Streptococcus thermophilus. Eleven subjects also reported eating cultured yogurt daily, and two reported eating upwards of 20 oz of homemade cultured yogurt daily. Only one person in the non-brain fogginess group reported taking probiotics.

D-lactic acidosis was found in 23 subjects in the brain fogginess (BF) group (76.7%) and in 2 in the non-BF group (28%). Nineteen subjects in the BF group (63.3%) also had SIBO, a percentage significantly higher than the non-BF group of which 25% had SIBO. Overall, the prevalence of these conditions was significantly higher in the BF group than the non-BF group. Subjects were told to stop taking probiotics, and after a course of antibiotics, 70% of subjects reported improvements in their symptoms and 85% said their BF was gone.

The researchers said one reason that subjects were experiencing BF resulting from D-lactic acidosis and SIBO was intestinal dysmotility, which means the gut cannot properly move its contents, slowing the passage of food and bacteria in the gastrointestinal tract, leading the bacteria to colonize in the stomach or small intestine rather than the colon where it belongs. For this reason, the researchers said they believe consumers should be more discriminate in the way they take probiotics because, while probiotics can be beneficial to health in a variety of situations, issues like constipation may not be solved by probiotics and instead made worse.


Study Limitations

Researchers acknowledge that the study has some severe limitations-most significantly, that it was observation based and lacked a control group. They also failed to quantify or characterize brain fog more specifically, something they claim they aim to do in future research. These limitations cast valid skepticism on the results, which industry trade groups have been quick to point out.

The Natural Products Association (NPA; Washington, DC) did not mince words in its critique of the study. “We’ve seen better science represented at grade school fairs,” said Daniel Fabricant, PhD, president and CEO of NPA, in a press release. “To call this science is insulting to the thousands of scientific studies out there confirming the safe and effective use of probiotics. Consumers should always consult with their doctors or medical professionals before using probiotics, but to suggest they are making people confused or disoriented based on this laughable study is absurd.”


  1. S.S.C. Rao et al. “Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis.” Clinical and Translational Gastroenterology, vol. 9 (2018): 162
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