There is a strong positive association between smoking and the occurrence of small intestinal bacterial overgrowth (SIBO), according to the results of a study published in the Journal of Clinical Medical Research.1 In addition to smoking, abdominal pain, bloating, and anemia also were strongly associated with SIBO.

Small intestinal bacterial overgrowth occurs when there is an overabundance of colonic bacteria (>105 colony-forming units/mL) in the small intestine causing inflammation and malabsorption.2 Breath tests, either the lactulose breath test or glucose breath test, have replaced jejunal aspirate cultures as the primary mode to diagnose SIBO.3

To examine the characteristics of patients undergoing breath tests for SIBO, researchers from Montreal conducted a retrospective study that included all patients tested with a clinical suspicion of SIBO and who presented with symptoms between 2012 and 2016. Common symptoms of SIBO include bloating, flatulence, abdominal pain, diarrhea, fatigue, and vitamin deficiencies.

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A total of 136 patients were included in the study (63% women; mean age, 51.2 years). Patients were administered either 10 g of lactulose or 75 g of glucose; exhaled hydrogen and methane levels were measured every 15 minutes for 3 hours. A breath test was considered positive for SIBO if a peak in exhaled hydrogen or methane production occurred in less than 90 minutes. A peak was defined “as an increase in the level of hydrogen of more than 20 ppm above the basal level and 10 ppm for methane.” SIBO was diagnosed in 33.8% of patients.

Patients with SIBO were more likely to be active smokers (odds ratio [OR]=6.66) and report bloating (OR=5.39), abdominal pain (OR=4.78), and anemia (OR=4.08). No association was found between SIBO and gender, age, or weight. Interestingly, none of the patients with SIBO had conditions that are typically associated with the disorder, such as cirrhosis, scleroderma, celiac disease, and chronic pancreatitis. Despite the lack of association, the researchers recommended ruling out these conditions in patients with positive SIBO tests.

SIBO is traditionally managed with antibiotic therapy. Antibiotics were prescribed in 43% of patients with a positive breath test and 2.2% of patients with a negative breath test. The most commonly prescribed antibiotic was metronidazole. However, no difference in symptom improvement was found in patients with or without antibiotic therapy.

One explanation for the lack of response to therapy is that all the patients were referred to their physician, who then decided whether or not treatment was clinically indicated. “Since the choice of antibiotic regimen was left to the discretion of the treating physician, there was no control over the treatment or not of our patients. Indeed, this could also explain our negative results concerning antibiotic treatment on SIBO symptoms,” wrote the researchers.

Bloating and abdominal pain were good predictors of a positive breath test, concluded the authors. Anemia, they noted, is recognized as a consequence of SIBO and “is explained mainly by vitamin B12 deficiency. In addition to having objectified a positive association of anemia with SIBO, 3 out of 4 patients with vitamin B12 deficiency had a positive breath test for SIBO.”

This is the first study to confirm that active smoking is a major risk factor for SIBO. Tobacco has been shown to alter the intestinal microbiome, and smokers have comorbidities that could increase the likelihood of having a positive breath test. “However, it is possible that the positivity in smokers is due to noncompliance with the instructions to refrain from smoking before the test,” the authors concluded.


  1. Kiow JLC, Bellila R, Therrien A, Sidani S, Bouin M. Predictors of small intestinal bacterial overgrowth in symptomatic patients referred for breath testing. J Clin Med Res. 2020;12(10):655-661. doi:10.14740/jocmr4320
  2. Khoshini R, Dai SC, Lezcano S, Pimentel M. A systematic review of diagnostic tests for small intestinal bacterial overgrowth. Dig Dis Sci. 2008;53(6):1443-1454. doi: 10.1007/s10620-007-0065-1
  3. Rezaie A, Buresi M, Lembo A, Lin H, McCallum R,Rao S, Schmulson M, et al. Hydrogen and methane based breath testing in gastrointestinal disorders: the North American consensus. Am J Gastroenterol. 2017;112(5):775-784. doi: 10.1038/ajg.2017.46