Small intestinal fungal overgrowth (SIFO) is characterized by a high quantity of fungi (such as yeast) in the small intestine, while SIBO is caused by elevated numbers of bacteria. Despite the conditions sounding similar, their risk factors, symptoms, and treatments differ.
Prescription antacids and altered gut motility may increase the risk of developing SIBO, but they aren’t associated with SIFO.
Studies have found that the rates of unexplained GI symptoms are similar in participants regardless of fungal colonization, which contrasts with the relationship between bacterial overgrowth and GI distress.
Also, while SIBO is conventionally treated with antibiotics, SIFO requires antifungal medications. Probiotics may prevent or suppress intestinal overgrowth of Candida (a type of yeast), but they don’t prevent SIBO.
However, in both cases, emerging research suggests that the composition of the fungi or bacteria — not simply a higher number of either — has a greater impact on whether the overgrowth might lead to symptoms.
Hydrogen and methane are produced exclusively by gut microbes, so an abnormal rate or quantity of gas production can indicate the presence of SIBO. The ratio of these gasses determines whether a person is predominantly a hydrogen or methane producer. When it comes to SIBO, these gasses could also provide clues to which microbes are thriving (overgrowing) in the small intestine.
While hydrogen is produced primarily by bacteria, most methane is produced by another group of microbes called Archaea, which belong to a domain of life separate from Bacteria or Eukarya (the cells that make up complex organisms, such as humans.) Archaea in the human gut consume hydrogen and carbon dioxide for energy, generating methane as a result, which is why they’re also known as methanogens.
An estimated third of healthy adults produce more methane than hydrogen, so methane hasn’t been identified as a clear cause of GI symptoms. Recent theories suggest that methane plays a role in constipation by slowing the GI system, however, and suggest differentiating between intestinal methanogen overgrowth and SIBO.
A recent meta-analysis reported that the odds of having methane-positive SIBO (possibly indicating methanogen overgrowth) were approximately two to three times higher in constipation-predominant IBS compared with diarrhea-predominant or mixed IBS, according to data from five studies.
Other research indicates that individuals with methane-positive breath tests are five times more likely to have constipation than people who primarily produce hydrogen. However, because slowing down of the GI system may support the growth of methanogens, it’s unclear which comes first: the methanogens or the constipation.
After fasting overnight, a breath test begins with the ingestion of a carbohydrate — most often glucose or lactulose when testing for SIBO — and the hydrogen and methane content of the person’s exhaled breath is measured at specific intervals over the next few hours. However, a number of factors can reduce the accuracy of breath tests and lead to a false positive or false negative.
The person being tested could reduce the accuracy of the test in the following ways:
Using antibiotics within 4 weeks of testing
Using laxatives or gut-motility drugs within a week of testing
Not fasting long enough (8 to 12 hours) before the test
Eating fermentable foods, such as complex carbohydrates, the day before the test
Smoking before the test
The carbohydrate used during the test also affects the results. Glucose is more likely to lead to a false negative than lactulose, but glucose is regarded as a more accurate diagnostic tool. Lactulose is more likely to lead to a false positive by quickly reaching the large intestine, where hydrogen will be produced. Using too large a dose can speed up intestinal transit even more, leading to an early peak in gas production that could lead to a false positive.
Clinicians and researchers may also set their own cutoffs for the level of gas production required to diagnose SIBO, which would obviously have an impact on results.