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Posted on Mon, 3 Sep 18

Do probiotics cause brain fog?

A widely reported study has been said to prove that probiotics are linked to adverse effects such as digestive problems and brain fog, but the research had important flaws. 

The research paper entitled “Brain fogginess, gas and bloating: a link  between SIBO, probiotics and metabolic acidosis” has been reported by media as evidence that probiotics could have negative health effects, but the study has limitations that prevent this conclusion. 

In the study, patients with self-reported gas, bloating were separated into two groups, those with “brain fog” and those without. Comparing the two groups, the study authors found that small intestinal bacterial overgrowth (SIBO), probiotic consumption, moderately increased urinary d-lactic acid, and delayed intestinal transit were more common in those with brain fog. After stopping probiotics and a course of antibiotics, it was reported that brain fog and gastrointestinal symptoms improved in 70% and 85% of people respectively [1]. Probiotics could cause SIBO and increase d-lactic acid production which might cause brain fog, suggested the study authors. 

News headlines read “Could probiotics cause brain fog and bloating?” but the study did not answer this question. Here are a few reasons why: 

  • The study did not provide direct evidence that probiotics cause SIBO, it is unknown if the people in the study had SIBO before starting probiotics
  • In several previous clinical trials probiotics have been consistently shown to reduce SIBO, not cause it.
  • There is no medical definition or criteria for ‘brain fog’ and there can be many causes of this non-specific symptom, so the assessment and correlation is dubious.
  • The method they used to define D-lactic acidosis was unusual and not well validated, so it is unclear if they did indeed have significantly elevated levels of D-lactic acid.
  • It may have been the antibiotic treatment that improved symptoms, not stopping probiotics.
  • The researchers did not retest to see if SIBO had resolved, so it’s also unclear if SIBO was to blame. 

Some caveats are that D-lactic acidosis due to probiotic use can occur in people with short bowel syndrome, and thus D-lactic acid producing probiotics should be avoided by these people [2]. However, not all probiotics produce D-lactic acid, in fact some probiotics can supress D-lactate-producing bacteria and have been used as a successful treatment for D-lactic acidosis [3].   

And it is possible that sub-clinical D-lactic acidosis (due to alterations in gut bacteria and impaired absorption of carbohydrates, not probiotics) could contribute to neurological disturbances in people without short bowel syndrome, but more research is needed to understand if this is the case [4]. 

Probiotics have a great safety record, but comprehensive safety evaluations have not been made for many probiotics strains and/ or doses [5]. More good quality research is needed to document probiotic safety, but this study does not provide it or prove that probiotics cause SIBO, gas, bloating and brain fog. 


1. Rao SSC, Rehman A, Yu S, Andino NM. Brain fogginess, gas and bloating: a link  between SIBO, probiotics and metabolic acidosis. Clin Transl Gastroenterol. 2018  Jun 19;9(6):162. 

2. Munakata S, Arakawa C, Kohira R, Fujita Y, Fuchigami T, Mugishima H. A case of D-lactic acid encephalopathy associated with use of probiotics. Brain Dev. 2010 Sep;32(8):691-4. 

3 .Yilmaz B, Schibli S, Macpherson AJ, Sokollik C. D-lactic Acidosis: Successful  Suppression of D-lactate-Producing Lactobacillus by Probiotics. Pediatrics. 2018  Aug 8. pii: e20180337. doi: 10.1542/peds.2018-0337. [Epub ahead of print]

4. Wallis A, Ball M, McKechnie S, Butt H, Lewis DP, Bruck D. Examining clinical similarities between myalgic encephalomyelitis/chronic fatigue syndrome and D-lactic acidosis: a systematic review. J Transl Med. 2017 Jun 7;15(1):129.

5. Sanders ME, Akkermans LM, Haller D, Hammerman C, Heimbach J, Hörmannsperger G, Huys G, Levy DD, Lutgendorff F, Mack D, Phothirath P, Solano-Aguilar G, Vaughan E. Safety assessment of probiotics for human use. Gut Microbes. 2010 May-Jun;1(3):164-85

Tags: Probiotics, SIBO

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