Imbalances in the small intestinal microbiome can drive clinical consequences in the form of small intestinal bacterial overgrowth (SIBO), characterized by higher numbers of bacteria and a distribution more commonly associated with the colon.
In recent times, SIBO has been recognized as a frequent cause of common gastrointestinal conditions that share risk factors such as maldigestion and malabsorption.
Methods used for SIBO diagnosis include the aspiration and culture of small intestinal contents from the jejunum or duodenum and, more commonly, hydrogen breath tests performed using substrates such as lactulose or glucose.
SIBO definition, diagnosis and management, however, remain a matter of debate.
Saffouri and colleagues recently showed that high-throughput sequencing analysis of the small intestinal microbiome is a better predictor of gastrointestinal symptoms when compared with duodenal aspiration and culture.
First, a distinct small intestinal microbiome composition assessed using 16S ribosomal RNA-based sequencing—and not the presence or absence of SIBO assessed by aspiration and culture of small intestinal contents—allowed for differentiation between symptomatic and asymptomatic individuals (not all symptomatic individuals had dysbiosis). This meant that the results of 16S rRNA sequencing and aspirate plus culture of duodenal contents did not correlate.
In the second part of the study, Saffouri and colleagues showed that switching a group of 16 healthy individuals with a high intake of fiber to a low-fiber diet containing a high concentration of simple sugars for seven days led to bloating and abdominal pain in 80% of participants.
Neither the resolution (50% of participants) nor appearance of SIBO (50% of participants) correlated with functional gastrointestinal symptoms. More intriguingly, asymptomatic individuals on a high-fiber diet commonly met criteria for SIBO.
In contrast, changes in the small intestinal microbiome induced by a diet high in simple sugars were predictive of symptoms and related to an alteration in duodenal permeability.
Quigley EMM. Symptoms and the small intestinal microbiome – the unknown explored. Nat Rev Gastroenterol Hepatol. 2019. doi: 10.1038/s41575-019-0171-5.
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