Archaea, such as Methanobrevibacter smithii are not actually bacteria at all, so perhaps methane based SIBO is not actually the right terminology for this type of overgrowth. Archaea are the predominant methane forming organisms in our digestive systems, but once thought to be bacteria, Archaea are actually prokaryotic organisms, that have features which do not resemble typical bacteria at all, so they have been reclassified from Archaebacteria to just Archaea in recent years.
Although these ancient organisms are not the only methane producers in our intestines, with rivals like certain Clostiridium and Bacteroides species, Archaea are the main focus of methane based SIBO treatment, which can be a slippery slop when considering targeted treatments for high methane SIBO.
Unlike hydrogen based SIBO which can often result in issues from mast cell stimulation and histamine overload, or essentially the immune system going crazy and resulting in many of peoples symptoms, methane gas is actually a characterized WHIMS gas with some minor yet important safety implications to humans. In addition to being flammable, like hydrogen, methane gas is an asphyxiant, which in higher concentration can displace oxygen resulting in symptoms of suffocation including headaches, dizziness, weakness, nausea, vomiting and loss of coordination.
Clinically on testing, as methane is detected in breath samples, I have occasionally detected very high levels of methane in exhaled air, which can certainly be implicated in these symptoms. I have also observed this in some patients oxygen status, with measured oximetry lower than expected. Further, I have observed symptoms of air hunger, or a feeling of inability to take a complete breath, as associated with high methane SIBO. Most significantly though, I have found that many high methane producers have no obvious symptoms at all and we simply find the high methane levels on in-office testing or as a result of certain lab test values being off.
Aside from being an asphyxiant, likely one of the largest implications of high methane SIBO is heavy metal toxicity. Methanogen are capable of transforming heavy metals or metalloids into volatile methylated derivatives which are known to be more toxic than the original compounds. For example, if someone has mercury amalgams, this inorganic mercury is not actually overly toxic, however in the presence of a methanogen like M. smithii, the inorganic mercury can be converted to methyl-mercury which can be extremely toxic to humans.
The ecology of the gut microbiata is critical for methane forming bacteria to evolve to a level that can become toxic to humans. This is because Methanogens rely on hydrogen production from specific flora and reduced carbon dioxide to create methane. Essentially methane forming organisms cannot exist in large amounts, specifically in the small intestine, without high levels of hydrogen forming organisms living there first. It is these hydrogen former’s that create the environment for high methanogen growth to exist.
Methane based SIBO seems to be one of the main causes of chronic constipation, however I have certainly observed constipation without the presence of methane production. Although it is unclear how methane results in constipation at present the proposed mechanism is though an effect on intestinal serotonin production that is thought to regulate bowel motility or methane associated damage to nerves that slows motility.
Based on current research the major medical diseases for which methanogens are most likely involved are inflammatory bowel disease (or Crohn’s disease), irritable bowel syndrome (IBS-C), colorectal cancer, diverticulosis, and obesity (PMC3021867). It is possible that this is a result of a negative correlation between fecal butyrate concentration and methanogen abundance as butyrate is the main fuel to help repair the gut mucosal lining (PMC3021867). Butyrate levels, along with methanogens, can be quantified by certain stool tests, which is why it is essential for people with colon cancer to assess gut microbiology as part of there treatment approach.
Methane formation has also been associated with eating disorders, such as anorexia nervosa. In one study, the researchers found found a significantly increased number of M. smithii in anorexia nervosa patients compared to the normal weight persons (PMC3021867).
Other functional motility based gastrointestinal disorders likely also have association with high methane as well. I have personally observed cases of gastroparesis, which is a delay or paralysis of stomach emptying, gastroesophageal reflux disease and biliary reflux disease, all connected to high methane production within the gut.