Different gases released by the microbiota at the level of the intestine have been detected, and the three most abundant are carbon dioxide (C02), hydrogen (H2), and methane (CH4). Those gases were initially thought to be inert, but Ignarro et al.1 found that nitic oxide (NO), another gas produced by endothelial cells, controls smooth muscle tone, increasing the relevance of the effect of those gases produced at the intestinal level on intestinal function.

Carbohydrate fermentation at the intestinal level results in the production of different gases: H2, CO2, hydrogen sulfide (H2S), and CH4. A portion of H2 and CH4 is absorbed in the colon, transported to the lungs, and excreted in exhaled air,2 through breath, and their remaining portions are expelled through flatus.3 Those small molecules can cross the plasma membranes, and move into tissues by concentration gradients. Thus, they can have local access to the components of the intestinal wall, including the neuromuscular pathway, and consequently, can modulate intestinal function.4

CH4 can be measured through breath tests and exhaled levels correlate well with CH4 obtained by stool-based qPCR testing.5 In a study conducted by Rezaie et al.6 that evaluated 12,183 subjects, utilizing a baseline methane cutoff value ≥ 5 ppm, they obtained 96% sensitivity, 99.7% specificity, 98.5% PPV, and 99.3% NPV for detecting a methanogenic population. A subject is considered “methanogenic” if his/her baseline exhaled levels are above 3-5 ppm,7 with an increase ≥ 10 ppm over the baseline value, after the administration of a carbohydrate as a substrate.8

CH4 production in humans takes place exclusively in the intestine, carried out by colonic anaerobes. The primary microorganisms are Methanobrevibacter smithii,9Methanobrevibacter oralis, Methanobacterium ruminantium,10 and Methanosphaera stadtmaniae. Multiple animal models and studies on humans have shown the effect of methane on intestinal and colonic motility. For example, in studies on animals, the infusion of CH4 has been shown to increase the noncontractile activity of the small bowel and delay intestinal transit.11 Cloarec et al.,12 utilizing lactulose breath tests in humans, found that intestinal transit time in methanogenic patients, compared with non-methane producers, was much higher (111 min versus 68 min). The presence of intestinal CH4 has been associated with different intestinal diseases, such as chronic functional constipation, irritable bowel syndrome (IBS) that is constipation-predominant, obesity, constipation, and colon cancer, among others. Attaluri et al.13 evaluated patients with chronic constipation that had slow transit and those that had normal transit, through colonic markers and the comparison with healthy controls, and found a significantly higher prevalence of methanogens in the group with constipation, as well as a higher prevalence of methanogens in the slow transit group, compared with the normal transit group.

The prevalence of methanogens in open populations varies from 36 to 63%. It is higher in Africa, reaching 80%, and lower in the United States14 and Europe, with reports of 35-40%. In the Mexican population, the prevalence of methanogens and their associated factors is unknown. Therefore, the aim of our study was to evaluate the prevalence of CH4 producers and associated factors in a group of patients with IBS and healthy controls, in a Mexican population.

The prevalence of methanogenic subjects worldwide varies and depends on the geographic region, as illustrated by the wide gap between the prevalence reported in the African countries of Nigeria (77%) or South Africa (87%) and the prevalence reported in Norway (34%) or the United States (40%).15 The causes of said differences are not yet completely established, but they are thought to depend on genetic and/or dietary factors that influence the specific microbiota of each population.

Our study is the first to explore the prevalence of CH4-producing subjects in Mexico. We found an overall prevalence of CH4-producing subjects of 38%, in an open population. Those findings are consistent with that reported in other populations of North America. Nevertheless, in the analysis by groups, the prevalence of asymptomatic healthy controls producing CH4 was greater (non-significantly), when compared with IBS patients (41 vs 31%), i.e., more than one-third of the healthy controls were methanogenic.

There is an obesity epidemic in Mexico, with a prevalence of 32% in the adult population.16 In the present study, we found that methane production was higher in patients with overweight and/or obesity, which should be highlighted. As stated above, methane production is associated with obesity in some populations but whether a methanogenic microbiota is a cause, or a consequence, of obesity remains to be clarified. One hypothesis posits that, in the production of methane by M. smithii, hydrogen requirements involve other microorganisms, becoming a symbiotic act, in which their productivity and metabolism is increased, facilitating short-chain fatty acid production and increasing the availability of calories for the organism. In a study by Mathur et al.,17 those authors significantly related the production of methane and hydrogen to an obese population. They were also the first to demonstrate the relation between the presence of methane in breath tests and obesity, reporting that the BMI in the methanogenic obese subjects was 6.7 kg/m2 higher than in the non-methanogenic obese subjects.18

In our study, the methane-producing subjects had a higher BMI than the non-methane-producers, and more than 50% of our total methanogenic population presented with obesity. Obesity in the methanogenic subjects was more frequent in the IBS patients. In particular, we found that the grade of obesity that was more prevalent, overall, in the highest methane producers, was grade I.

Regarding age, neonates and children under 2 years of age have been reported to not excrete methane. Methane production begins at 2 years of age and increases up to 10 years of age, at which time it remains constant up to the eighth decade of life.19 In a Nigerian population, Hudson et al.15 demonstrated a significant difference in the prevalence of methanogens between children and adults. In our study population we found no difference with respect to age in the methanogenic subjects vs the non-methane producers.

Many factors have been identified that appear to increase the possibility of producing methane. Differences in sex appear not to be significant. According to the current literature, there are apparently no significant differences in methane production regarding sex,16 and in our population, we found that age was not a risk factor for increasing or decreasing the prevalence of methanogens. In our overall population, the methanogenic subjects produced 10-times more methane than the non-methanogenic subjects.

The IBS patients with a greater methanogenic propensity were those presenting with the constipation-predominant subtype. Methane was thought to be an inert gas, but there is current experimental and clinical evidence stating that it can alter intestinal transit.6 There is an important relation between methane production and its action on intestinal transit, especially in patients with chronic functional constipation.13 However, there are publications reporting that methane affects intestinal neuromuscular function. In canine models, small bowel transit is reduced, increasing the ileal circular muscle contraction through the mechanical stimulation of the mucosa.9

In an Italian population, a relation of intestinal disorders to methane and hydrogen production was sought, finding that methane was related to IBS-C,20 as was the case in our study population. We found that 71% of the IBS-C patients were methanogenic, a result that concurs with those described in previous studies. We also found that none of our IBS-D patients (albeit there were only 10) had a methanogenic microbiota, making it appear that said microbiota is not associated with the presence of diarrheic stools or intestinal transit. Nevertheless, based on our results, it would appear that methane is related to intestinal disorders, such as IBS, even though its correlation with symptoms is still uncertain. In a recent study by Di Stefano et al.21 on 103 subjects with IBS and 28 controls, they found that despite the fact that 46% of the IBS patients excreted methane in their stools, breath methane was detected in only 26% of them. Those authors concluded that the clinical role of methane is still uncertain and should be re-evaluated, utilizing new technologies, such as microbiota analysis, and especially emphasizing the association of symptoms.

Finally, we believe it is important to underline the fact that 12 subjects (5 controls and 7 IBS patients) produced neither H2 nor CH4, suggesting a microbiota that mainly produces H2S. Nevertheless, there are currently few tools available for evaluating that gas at the intestinal level.

Among the limitations of our study is the fact that it was conducted on a specific population, given that there can be variations in the microbiota that are dependent on local diet and the genetic component. Regarding IBS subtypes, the methanogenic microbiota is described as more prevalent in the constipation-predominant subtype, as confirmed by our study, but that subtype was the most prevalent in our study subjects and we had few cases of the diarrhea-predominant subtype. Despite those limitations, we consider our results relevant, novel, and comparable, regarding those published in other studies. Conducting a multicenter study at different regions of the country is important, to provide conclusions that are more applicable at the national level and to generalize data.

In our study, the prevalence of methanogens in an open population and in patients with IBS was similar to that reported worldwide. Within the methanogenic population, IBS-C was the most prevalent subtype. In addition, obesity was related to methane production, and BMI was higher in the methane producers than in the non-methane producers. The most prevalent obesity grade was grade I.

No financial support was received in relation to this article.

The authors declare that there is no conflict of interest.