Prerpints.org logo
Preprint
Review

Beyond the Gut: Unveiling Methane’s Role in Broader Physiological Systems

Altmetrics

Downloads

50

Views

67

Comments

0

Submitted:

04 November 2024

Posted:

06 November 2024

You are already at the latest version

Alerts
Abstract

ecent interest in breath methane has demonstrated considerable growth, with a 400% increase in publications per year between 2010 and 2020 compared to the previous decade. This surge has been facilitated by advancements in measurement techniques that have improved both the precision and ease of breath methane analysis. Consequently, there has been a growing appreciation for both the routes of production as well as the physiological effects of methane within the human body, shifting from a perspective of methane as an end-product of GI microbiota to viewing it as a potentially biologically active molecule with exciting potential implications for endogenous processes. The breath methane field stands at a pivotal juncture, where new technologies enable real-time, repeated measures of breath methane for the first time, paving the way for novel insights into personalized methane levels and their interplay with health. This review explores the origins, physiological effects, and measurement techniques of breath methane, highlighting potential pathways for future research.

Keywords: 
Subject: Biology and Life Sciences  -   Biochemistry and Molecular Biology

Introduction

Methane has long been regarded as having significance primarily as a greenhouse gas, however, it also plays crucial roles in human physiology. Traditionally, it was believed that methane is primarily produced by methanogenic archaea in the gastrointestinal tract of certain individuals. This methanogen activity not only influences digestive processes but has been reported to impact broader physiological functions, including immune modulation and oxidative stress responses. Recent studies have unveiled additional potential sources of methane production, challenging these traditional views that solely attribute its origins to gut microbiota. Understanding methane's diverse origins and physiological effects is thus pivotal for unraveling its implications for health and disease.
In this review, the origins, physiological effects, and clinical implications of methane are explored in human physiology. By evaluating current research findings, we aim to better understand the mechanisms underlying methane production as well as the potential impact of these processes on gastrointestinal function, immune modulation, and metabolic pathways.
Sources of methane in human breath

Gut Microbiome

During the typical metabolism of dietary and endogenous components in the large intestine the gut microbiome can produce many volatile compounds. Some of these have seen widespread interest, including the short-chain fatty acids (SCFAs) acetate (C2), propionate (C3) and butyrate (C4). These SCFAs are normally produced at a ratio of around 60:20:20 with around 5-600 mmol produced per day1–3. However, these SCFAs form a relatively minor part of the ~0.2-1.5 L of gas produced per day by the gut microbiota of most healthy people4–6. Instead the gases that make up the majority of this volume are hydrogen (H2), carbon dioxide (CO2), and methane (CH4), contributing more than 99% of the intestinal gas volume7. These SCFAs form part of the final 1%, which also includes sulfur-containing trace gases, such as hydrogen sulfide (H2S), methanethiol (CH3SH), and dimethyl sulfide ((CH3)2S)8.
The gastrointestinal (GI) microbiota, encompassing a diverse community of microorganisms inhabiting the human gut, constitutes the oldest and most extensively studied source of methane production in humans. Products of the gut microbiome are well known to be affected by a range of factors, including both internal such as composition, as well as external such as diet and age9.
At a composition level, methanogenic archaea are recognized as the primary producers of methane through anaerobic metabolism. These methanogens utilize the methylotrophic pathway, reducing CO2 with H2 or formate to form CH4. This process occurs primarily in the colon and to a lesser extent in the small intestine. There is comparatively little diversity regarding specific methanogen species, as illustrated in Table 1. The predominant species (across both healthy and diseased states) is Methanobrevibacter smithii with Methanobrevibacte stadtmanae occurring to a lesser extent10–12. The levels of these archaea can be seen to reflect levels of methane production, with the microbiomes of people classed as high methane emitters (CH4 > 5 ppm) characterized by a 1000-fold increase in M.smithii 13.
Exogenous dietary consumption can also affect methane production. This can occur directly, such as through modulation of Methanobrevibacter levels, with studies demonstrating that Methanobrevibacter levels are negatively correlated with the intake of total fat, saturated fat, and omega-3 fatty acids13. Diet can also affect methane production indirectly by affecting levels of methanogen substrates. This can be observed in the case of vitamin B12 deficiency which can be linked to altered methane production through the modulation of formate availability13.
Once produced, methane is able to diffuse across the gut mucosa into the portal circulation where it undergoes gas transfer in the alveolar space and is subsequently exhaled14. It is estimated that 20-50%15,16 of the methane produced in the gut is excreted via exhaled breath. This allows the measurement of methane in breath samples to serve as a non-invasive method used clinically to assess GI health and diagnose methane-related disorders. Elevated methane levels are used as diagnostic markers for conditions such as intestinal methanogen overgrowth-small intestinal bacterial overgrowth (IMO-SIBO). IMO, unlike traditional SIBO, represents the overgrowth of methanogenic archaea rather than bacteria. This overgrowth can occur in either the colon or the small intestine, often requiring unique antibiotics to treat. Of note, there are reported physiological differences between methane-positive and methane-negative SIBO, with methane-positive SIBO correlating with delayed small bowel transit, and colonic transit compared to methane-negative SIBO17 which will be discussed further in this review.
The threshold for what constitutes a positive IMO result varies. While a level of methane gas greater than 10 ppm is generally considered indicative of IMO18, some classifications consider baseline levels greater than 1-3 ppm as elevated19. A significant challenge for these assessments is the reliance on a single timepoint measurement. Recent developments in the field of breath methane monitoring, such as the OMED device20, allow for longitudinal measurements of breath methane, and the establishment of a personalized baseline to guide further testing.
The prevalence of methanogens in the gut is also susceptible to external factors, such as increases with age, suggesting an age-related shift in the microbial ecosystem, which favors methane production. This trend has been observed in several studies21–23, indicating that age can be a significant factor in the composition and function of the gut microbiota.

Human Endogenous Processes

In addition to the relatively well-characterized production of methane by methanogenic archaea in the gut, emerging data has demonstrated across in vitro to in vivo settings supporting evidence that there are additional host derived endogenous sources that may contribute to measurable methane levels, particularly in settings of stress. These studies suggest that elevated levels of reactive oxygen species (ROS) can produce methane as illustrated in Figure 1. This process relies on the Fenton reaction to generate hydroxyl radicals from hydrogen peroxide (H2O2), which can subsequently oxidatively demethylate methylated sulfur or nitrogen compounds (e.g., methionine, dimethyl sulfoxide, or trimethylamine).
Methane production as a result of oxidative stress in vitro has been demonstrated, with methane being produced after the application of 2M H2O2 to a variety of endogenous compounds. Of these compounds choline chloride was the most potent, generating 4-25 µM methane, but methionine and ethanolamine were also capable of producing measurable amounts of methane24,25. Of note, in these settings compounds that generated appreciable concentrations of methane also demonstrated some anti-oxidant activity, with reductions in the generation of ROS25.
These findings can be extended from these in vitro settings to an ex vivo setting as demonstrated through the application of oxidative stress (induced via H2O2 and ascorbic acid) to isolated mitochondria. In this setting, oxidative stress induced methane production, the rate of which was proportional to both the level of oxidative stress as well as to the quantity of mitochondrial protein added. Levels of production at 100 mM H2O2 and pH 7.4 were 0.3 nmole methane in 60 minutes per mg mitochondrial protein25 and the application of catalase prevented these effects, lending further weight to oxidative stress being required for this methane production24. Using the liver as an example organ, and extrapolating rates based on this observation one can obtain approximate estimated rates of methane production between 58-118 µmole from a liver in 60 minutes (taking the average liver weight of between 968-1860g26, of which around 20% is mitochondria27). Making the approximation of negligible loss during this time, and a blood volume of 5L, one could estimate blood concentrations between 11.6-23.6 µM placing values within an order of magnitude of predicted levels in blood of around 2 µM in blood under normal conditions28.
Taking this one step further, moving from isolated mitochondria to a cultured endothelial cell setting, Adamczuk et al. demonstrated that cultured cells produce methane even at baseline (2 nmol/mg), and that this can be increased by exposure to agents associated with elevations in ROS29 such as sodium azide (NaN3) (15 nmol/mg) or 2,4-dinitrophenol (DNP) (23 nmol/mg)25. This phenomenon has been observed both in mammalian cells, as well as plant cells, with grapevine demonstrating a similar increase in methane production following exposure to NaN3 30.
The final piece of the puzzle, translating these in vitro/ex vivo effects to an in vivo setting was first provided by Tuboly et al. who demonstrated an elevation in methane production following NaN3 administration31. This was further supported by evidence from Keppler et al. first identifying the production of methane from leaves (0.3 ng/g dw)32 and then from humans, demonstrating methane release from radiolabeled methionine both in blood and from the skin (headspace)33. Two additional aspects to these experiments stand out as providing potential additional insights into this phenomenon, firstly Tuboly et al. demonstrated that this elevation in methane could be prevented by co-administration of α-glyceryl phosphorylcholine, a protectant against lipid peroxidation, further supporting that methane production is linked to oxidative stress. Secondly, both Tuboly et al. and Keppler et al. took steps to remove microbial-linked methane production, either via the administration of rifaximin or UV irradiation (respectively). There is therefore good evidence that observed effects were due to extra-microbial production of methane.
The relevance of these findings to human physiology had its first indications in 2013, when Tuboly et al. demonstrated that administration of LPS in mice (an acute sepsis setting) corresponded to a 2-3 fold increase in methane production34. These data suggested that infection, and associated elevations in inflammation/oxidative stress, may provide a real-world setting for elevations in non-microbial methane production. Whilst further work validating this finding in sufficiently powered studies is required, there are some preliminary indications that this may hold for human infection as well, with Keppler et al. demonstrating elevations above baseline (on a similar order of magnitude to Tuboly) in response to COVID-1935.

Detection and Measurement of Methane in Breath

Breath Sampling and Analytical Techniques

Breath sampling techniques largely fall into 3 separate categories: direct exhalation into collection bags, breath sampling via tubes, and real-time breath analysis with direct analyzers. These sampling techniques each have advantages and disadvantages36, as summarized in Table 2. In a similar fashion to sampling, there are a number of possible analytical techniques for methane detection, including GC-FID (Gas chromatography-flame ionization detection), IR (infrared spectroscopy) and MOS (metal oxide sensors) as summarized in Table 3. The relative strengths of each collection, and analytical method, are key considerations when designing a study, especially one investigating personalized methane levels as highlighted in Figure 2.

Potential Effects of Methane

With both the potential routes for methane production considered, as well as the value and mechanism for testing methane concentrations on breath, the next section will focus on the local and systemic potential effects of methane, with an emphasis on delineating correlation from causation. The conclusions are summarized in Figure 3.

Local (GI) Effects

Motility and Function

Data from animal studies suggests that methane can have a profound impact on GI transit time. In experimental settings, exogenous methane gas applied ex vivo has been shown to directly inhibit intestinal transit by 59% in dogs37 and decrease peristaltic velocity in guinea pigs38. This can be translated to observations in human populations, with methane levels correlating strongly with slower intestinal transit times39–43. Work from Soares et al. supplemented these findings with additional detail, demonstrating that total colonic transit time averages 80.5 hours in methane producers compared to 61.0 hours in non-methane producers, and providing a breakdown of these transit times. This revealed substantial delays in specific sections of the colon: 17.5 hours versus 10.5 hours in the right colon, 29.5 hours versus 10.5 hours in the left colon, and 31.5 hours versus 27.0 hours in the rectosigmoid region44.
These data universally support a link between elevated methane production and increased GI transit times, however data so far has been limited to correlations. Work from Pimentel et al. extended these findings towards an in vivo model through direct administration of methane (via intestinal fistulae). This model removed other potential confounders (such as dietary effects) from human studies, demonstrating a 59% increase in transit times in the presence of methane (at a concentration equivalent to a breath methane level of 50 ppm)37. The work of Park et al. provided the first potential mechanism to explain these observations. Namely, they demonstrated in studies involving the infusion of methane under electrical field stimulation, that methane increased the amplitude of ileal contractions across all tested frequencies (1-16 Hz)45.

Interaction with Gut Microbiota

The gut microbiome is inherently synergistic, and so it is reasonable to hypothesize that in methane producers, with high methanogenic archaeal levels, there may be other changes to gut microbiota composition, or products. Indeed, data from Kumpitsch et al. identified that high-methane producers (>5 ppm) demonstrate a significantly higher alpha diversity and substantially different microbiome composition compared to low-methane producers13. Methane-emitting microbiomes were significantly associated with Euryarchaeota (Methanobrevibacter) as well as signatures of Christensenellaceae R7 group, Ruminiococcus/Ruminococcacaeae, Holdemanella and the Eubacterium ruminatium groups13, groups which are associated with dietary fiber degradation. These data supports findings that when Christensenella and Methanobrevibacter are co-grown in vitro they form dense flocs whereby the H2 generated by the Christensenella supports CH4 production by Methanobrevibacter. In this setting SCFA production is shifted more towards acetate and away from butyrate46. Supporting this, high methane producers also show increased levels of formate and acetate in the gut, with these metabolites strongly correlated with dietary habits such as vitamin, fat, and fiber intake.
This association has been investigated in vivo by a number of groups, with seemingly conflicting results. Early work in 1984 found no significant difference between the levels of SCFAs in the feces of methane producers compared to non-methane producers47, which was corroborated by serum findings in 199848. Subsequent work however found significant elevations in both fecal and serum SCFA levels (particularly in propionate, formate, and acetate)13,49,50. Finally, Fernandes et al. identified a negative correlation between breath methane levels and fecal SCFA levels in patients51.
Whilst these results at first glance appear conflicting, when the impact of confounders is considered, namely those that may independently correlate SCFA and breath methane levels (e.g. sex, age, or diet) a trend emerges. When these results are considered within the context of age, which is known to correlate with both increased methane production52 and decreased SCFA levels53, we can observe that studies with an age mismatch48,51 demonstrate no change/a decrease in SCFA levels with elevated methane, whilst those that correct for age13,50 clearly demonstrate elevations in SCFA levels with elevated methane levels. It is noted that, whilst considerations of microbiome level implications of methanogen presence must be considered, these findings are supported from a purely biochemical standpoint whereby removal of H2 by methanogens would be expected to modify, and potentially increase SCFA production through end-product removal54,55.

Systemic Effects

Much of the work around methane’s potential bioactivity, and the focus of this review so far has been around the potential local effects of methane in the GI system. However, data have emerged largely via the exogenous application of methane, supporting a potential systemic effect. This includes potential activity as an anti-inflammatory, anti-apoptotic, antioxidant, or metabolic regulatory molecule. In this section, we will focus on some of these effects and their context.

Inflammatory Modulation

The most common systemic effect attributed to methane is its potential as a cytoprotective compound. Studies have associated methane with three potential cytoprotective effects.
  • Anti-inflammatory effects, that manifest as reductions in TNFα, IL-6, and IL-1B levels following intraperitoneal (IP) dosing of methane-rich saline (MRS). These effects appear to be mediated via IL-10, and upstream through the PI3K-AKT-GSK-3B pathway56–66.
  • Anti-oxidative effects, presenting as reductions in MDA or 8-OHdG levels, as well as the prevention of loss of antioxidant activity (SOD/CAT levels)58–65,67–70.
  • Anti-apoptotic effects, manifesting as reductions in TUNEL staining, as well as reduced caspase 3/9 activation59–63,67,68,71.
These effects have been observed across a wide range of diseases, including ischemia/reperfusion injury67–69, inflammatory disease56–58,72, neuronal disease65,73 and others.
These studies generally leverage methane-rich saline (MRS) (at 0.99 mM), first used by Zhouheng et al.59, with doses between 0.5-20 ml/kg demonstrating efficacy (with rough end-dosage of around 9 µmol/kg). Assuming total blood volume of a rat at ~64 mL/kg, full displacement of methane into the blood, and minimal methane loss, this would be expected to give ~ 140 µM, or around 70x the levels expected from microbiome production and 14x levels expected from endogenous production during sepsis. Therefore, the dose-dependent observation of effects in these studies brings into question comparisons between observations within these MRS dosing experiments and their impact in a real-world setting.

Metabolic Impacts

There has been a focus on gut dysbiosis within obesity for over 20 years now, and early work from Turnbaugh et al. demonstrated that the gut microbiomes of obese (ob/ob) mice have increased representation of archaea compared to their control weight (ob/+) littermates74. This was attributed to an increased ability to degrade polysaccharides, a phenomenon which was demonstrated to be transmissible, resulting in greater weight gain in lean germ-free mice following fecal microbiome transplant74. Supporting increased energy harvesting driving this phenomenon, data demonstrated that co-colonization of mice with the symbiotic pairing of M.smithii and B.thetaiotaomicron resulted in significantly greater adiposity compared with colonization of either organism alone55.
Given the known, and well-demonstrated association of dysbiosis with metabolic syndromes75, data surrounding correlations between methane and BMI must be approached cautiously. Despite this, there are two effects of methane that could be expected to contribute towards additional weight gain, and therefore provide a rationale for a positive correlation between BMI and methane production. Namely, slowed GI transit time, providing greater time for nutrient absorption across the GI tract, and increased production of SCFAs increasing calorie availability from food (responsible for ~10% of calorie availability in humans76).
Translating this to a real-world setting, the majority of data support a correlation between elevated methanogen presence, and therefore breath methane levels, and a higher BMI. This has been demonstrated at baseline in obese patients, where those with breath CH4 > 3 ppm display a BMI ~7 higher than those without77 as well as in obese compared to lean children78, and, although not reaching statistical significance (potentially due to study power) also by Fernandes et al48. Of note, in addition to baseline levels, correlations have also been observed between elevated methane levels following a lactulose challenge and BMI, firstly by An et al.79 and also by Matur et al. who demonstrated a correlation only if both breath methane and hydrogen were elevated80.
Despite this evidence for a positive correlation between methane and BMI there is some disagreement amongst the field. Ozato et al. found no significant difference between methane and BMI, but demonstrated a lower visceral fat area in methane producers vs non-producers81. Wilder-Smith et al. even found that people who had detectable methane in their breath following a lactose/fructose challenge had a lower BMI compared to non-methane producers82. Of note, a key difference here was that Wilder-Smith et al. were the only group to study specifically patients with a functional gut disorder (irritable bowel syndrome, as diagnosed by Rome III criteria). On balance, the data above suggests that in the general population, higher breath methane levels are associated with a higher BMI, however, in a subset of people with functional gut disorders this may not hold to be true, potentially due to the presence of additional factors that drive methane levels.
Following from this, methane producers also had worse glucose tolerance compared to non-methane producers83, and pharmacologically reducing breath methane (through antibiotic use) in obese patients improved glucose tolerance84. Patients who were positive for both methane and hydrogen also displayed reduced (prorated) percentage changes in BMI following bariatric surgery85. Whilst these data suggest that methane correlates with increased BMI and altered glucose handling, there is also data suggesting an overall potentially cardioprotective effect of methane. Wu et al. found that the transition from pre-diabetes to type 2 diabetes was associated with a downregulation of bacterial methanogenesis86. Ozato et al. also found that higher methane levels were associated with decreased visceral fat area, a key contributor for cardiometabolic risk81. Finally Laverdure et al. found that, in an in vitro setting, GLP-1 secretion could be stimulated by methane87.

Potential Role of the Vagus Nerve and Cholinergic Pathway

There is limited data focused on elucidating the proposed mechanisms by which methane may elicit both these local and systemic effects. An emerging potential mechanism however involves its interaction with the vagus nerve. The vagus nerve, the longest and most extensively distributed autonomic nerve, originates in the brainstem and extends through the neck into the thoracic and abdominal cavities. This nerve carries both motor and sensory fibers, providing innervation to numerous systems and influencing critical aspects of human physiology, including heart rate, blood pressure, sweating, digestion, and even vocalization88.
Evidence supporting the role of methane in modulating vagal nerve/cholinergic pathway activity was first demonstrated by Park et al. who identified that the application of tetrodotoxin or atropine can abolish methane-induced increases in contraction amplitude in guinea pig ileal muscle strips45. It can be noted that whilst not directly investigated further, there is data supporting that this interaction may occur indirectly, via associated changes in serotonin production89. Supporting the implication of the vagus nerve, the effects of methane appear to correlate closely with the outcomes associated with vagal nerve and cholinergic pathway activation. This relationship is evident in the shared anti-inflammatory effects56–66, alterations in heart rate90,91, modifications in gastrointestinal transit time37–45, and the secretion of pancreatic polypeptide following sham feeding92, as summarized in Figure 4 below.

Clinical Implications and Future Research

Breath methane measurements have received attention in the past as part of their use in the clinical diagnosis of various gastrointestinal conditions, such as SIBO. These tests largely involve fasting (to minimize baseline sample variance) followed by administration of a challenge substrate (e.g. lactulose, glucose, or fructose) and subsequent breath measurements at timed intervals.
These challenge tests help to pinpoint changes in methane levels that specifically originate in the GI tract, allowing breath methane to be used as a test for SIBO without interference from other potential sources of methane. In contrast, using endogenously generated methane as a biomarker is more complex. Here, longitudinal measurements and comparisons to an individual’s baseline could support breath methane as a reliable biomarker by minimizing or controlling for other potential confounders.
This shift has been observed in recent years across medical fields, with tests moving away from static measures towards more dynamic, continuous and comprehensive monitoring. This has been reflected in assays such as blood glucose monitoring, where technological advancements have brought with them a shift from static fingerstick blood glucose readings to the widespread use and adoption of continuous glucose monitoring. The same can be envisaged for breath methane readings, whereby technological advancements, such as the development of the OMED device, can allow for affordable, at-home, real-time monitoring of breath methane levels.
The field is positioned at an exciting time whereby for the first time there is the potential for population-based longitudinal studies on breath methane levels and so the opportunity to better understand the role of breath methane in health beyond the GI system.

Conclusion

Over the past 20 years, research on methane measurement in human physiology has grown rapidly, with 359 publications from 2005-2014 and 948 from 2015 to mid-2024. As the field expands, so does the understanding of methane’s physiological associations.
Methane's roles in human physiology are diverse and complex, spanning from GI health to systemic inflammatory and oxidative responses. As research progresses, the potential of methane as both a diagnostic marker and a therapeutic target becomes increasingly evident. Addressing the current research gaps and standardizing methodologies will be pivotal in harnessing the full clinical potential of breath methane measurements.
The advent of handheld, real-time breath methane monitors has enabled large-scale, longitudinal data collection. This opens the door to population studies on the role of breath methane in infection markers, correlations with lifestyle factors (e.g., alcohol intake, exercise, BMI, and heart rate), and how methane levels change with shifts in the gut microbiome, such as after antibiotics or other GI treatments. Exploring methane’s origins, mechanisms, and health effects promises new insights for diagnostics and treatments.

Declaration of Interests

M.K, M.B, N.N, R.P-L, M.A and B.B are employees of Owlstone Medical Ltd.

References

  1. Cummings, J.H.; Pomare, E.W.; Branch, W.J.; Naylor, C.P.; Macfarlane, G.T. Short chain fatty acids in human large intestine, portal, hepatic and venous blood. Gut 1987, 28, 1221–1227. [Google Scholar] [CrossRef] [PubMed]
  2. Bergman, E.N. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol. Rev. 1990, 70, 567–590. [Google Scholar] [CrossRef] [PubMed]
  3. Macfarlane, S.; Macfarlane, G.T. Regulation of short-chain fatty acid production. Proc. Nutr. Soc. 2003, 62, 67–72. [Google Scholar] [CrossRef] [PubMed]
  4. Suarez, F.L.; Springfield, J.; Levitt, M.D. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour. Gut 1998, 43, 100–104. [Google Scholar] [CrossRef]
  5. Mego, M.; Accarino, A.; Malagelada, J.; Guarner, F.; Azpiroz, F. Accumulative effect of food residues on intestinal gas production. Neurogastroenterol. Motil. 2015, 27, 1621–1628. [Google Scholar] [CrossRef]
  6. Serra, J.; Azpiroz, F.; Malagelada, J. Intestinal gas dynamics and tolerance in humans. Gastroenterology 1998, 115, 542–550. [Google Scholar] [CrossRef]
  7. Suarez, F.L.; Levitt, M.D. An understanding of excessive intestinal gas. Curr. Gastroenterol. Rep. 2000, 2, 413–419. [Google Scholar] [CrossRef] [PubMed]
  8. Arulvasan, W.; Chou, H.; Greenwood, J.; Ball, M.L.; Birch, O.; Coplowe, S.; Gordon, P.; Ratiu, A.; Lam, E.; Hatch, A.; et al. High-quality identification of volatile organic compounds (VOCs) originating from breath. Metabolomics 2024, 20, 1–16. [Google Scholar] [CrossRef]
  9. Salazar, N.; González, S.; Nogacka, A.M.; Rios-Covián, D.; Arboleya, S.; Gueimonde, M.; de los Reyes-Gavilán, C.G. Microbiome: Effects of Ageing and Diet. Curr. Issues Mol. Biol. 2020, 36, 33–62. [Google Scholar] [CrossRef]
  10. A Weaver, G.; A Krause, J.; Miller, T.L.; Wolin, M.J. Incidence of methanogenic bacteria in a sigmoidoscopy population: an association of methanogenic bacteria and diverticulosis. Gut 1986, 27, 698–704. [Google Scholar] [CrossRef]
  11. Fricke, W.F.; Seedorf, H.; Henne, A.; Krüer, M.; Liesegang, H.; Hedderich, R.; Gottschalk, G.; Thauer, R.K. The Genome Sequence of Methanosphaera stadtmanae Reveals Why This Human Intestinal Archaeon Is Restricted to Methanol and H 2 for Methane Formation and ATP Synthesis. J. Bacteriol. 2006, 188, 642–658. [Google Scholar] [CrossRef] [PubMed]
  12. Nkamga, V.D.; Henrissat, B.; Drancourt, M. Archaea: Essential inhabitants of the human digestive microbiota. Hum. Microbiome J. 2016, 3, 1–8. [Google Scholar] [CrossRef]
  13. Kumpitsch, C.; Fischmeister, F.P.S.; Mahnert, A.; Lackner, S.; Wilding, M.; Sturm, C.; Springer, A.; Madl, T.; Holasek, S.; Högenauer, C.; et al. Reduced B12 uptake and increased gastrointestinal formate are associated with archaeome-mediated breath methane emission in humans. Microbiome 2021, 9, 1–18. [Google Scholar] [CrossRef]
  14. Tansel, A.; Levinthal, D.J. UNDERSTANDING OUR TESTS: HYDROGEN-METHANE BREATH TESTING TO DIAGNOSE SMALL INTESTINAL BACTERIAL OVERGROWTH. Clin. Transl. Gastroenterol. 2023, 14. [Google Scholar] [CrossRef]
  15. Florin, T.H.; Zhu, G.; Kirk, K.M.; Martin, N.G. Shared and unique environmental factors determine the ecology of methanogens in humans and rats. Am. J. Gastroenterol. 2000, 95, 2872–2879. [Google Scholar] [CrossRef] [PubMed]
  16. Christl, S.U.; Murgatroyd, P.R.; Gibson, G.R.; Cummings, J.H. Production, metabolism, and excretion of hydrogen in the large intestine. Gastroenterology 1992, 102, 1269–1277. [Google Scholar] [CrossRef]
  17. Suri, J.; Kataria, R.; Malik, Z.; Parkman, H.P.; Schey, R. Elevated methane levels in small intestinal bacterial overgrowth suggests delayed small bowel and colonic transit. Medicine 2018, 97, e10554. [Google Scholar] [CrossRef] [PubMed]
  18. Takakura, W.; Pimentel, M.; Rao, S.; Villanueva-Millan, M.J.; Chang, C.; Morales, W.; Sanchez, M.; Torosyan, J.; Rashid, M.; Hosseini, A.; et al. A Single Fasting Exhaled Methane Level Correlates With Fecal Methanogen Load, Clinical Symptoms and Accurately Detects Intestinal Methanogen Overgrowth. Am. J. Gastroenterol. 2022, 117, 470–477. [Google Scholar] [CrossRef] [PubMed]
  19. Haworth, J. Understanding the Results from Methane Breath CH4ECKTM Test. The Functional Gut Clinic https://thefunctionalgutclinic.com/blog/education/understanding-the-results-from-your-methane-breath-ch4eck-test/ (2021).
  20. Avvisati, R.; Bates, J.; Winter, H.; Ball, M.; Pocock, L.; Davies, H.; Pinto-Lopes, R.; Guagliardo, F.; Nash, M.; Scott, P.; et al. S2217 Development and Validation of a Portable Device for At-Home Hydrogen and Methane Breath Testing. Am. J. Gastroenterol. 2024, 119, S1584–S1585. [Google Scholar] [CrossRef]
  21. Stewart, J.A.; Chadwick, V.S.; Murray, A. Carriage, quantification, and predominance of methanogens and sulfate-reducing bacteria in faecal samples. Lett. Appl. Microbiol. 2006, 43, 58–63. [Google Scholar] [CrossRef]
  22. Rani, S.B.; Balamurugan, R.; Ramakrishna, B.S. Molecular analysis of the human faecal archaea in a southern Indian population. J. Biosci. 2017, 42, 113–119. [Google Scholar] [CrossRef] [PubMed]
  23. Dridi, B.; Raoult, D.; Drancourt, M. Archaea as emerging organisms in complex human microbiomes. Anaerobe 2011, 17, 56–63. [Google Scholar] [CrossRef] [PubMed]
  24. Ghyczy, M.; Torday, C.; Boros, M. Simultaneous generation of methane, carbon dioxide, and carbon monoxide from choline and ascorbic acid: a defensive mechanism against reductive stress? FASEB J. Off. Publ. Fed. Am. Soc. Exp. Biol. 2003, 17, 1124–1126. [Google Scholar] [CrossRef] [PubMed]
  25. Ghyczy, M.; Torday, C.; Kaszaki, J.; Szabó, A.; Czóbel, M.; Boros, M. Hypoxia-Induced Generation of Methane in Mitochondria and Eukaryotic Cells - An Alternative Approach to Methanogenesis. Cell. Physiol. Biochem. 2008, 21, 251–258. [Google Scholar] [CrossRef]
  26. Molina, D.K.; DiMaio, V.J.M. Normal organ weights in men: part II-the brain, lungs, liver, spleen, and kidneys. Am. J. Forensic Med. Pathol. 2012, 33, 368–372. [Google Scholar] [CrossRef]
  27. Alberts, B.; et al. The Mitochondrion. in Molecular Biology of the Cell. 4th edition (Garland Science, 2002).
  28. Szabó, A.; Unterkofler, K.; Mochalski, P.; Jandacka, M.; Ruzsanyi, V.; Szabó, G.; Mohácsi. ; Teschl, S.; Teschl, G.; King, J. Modeling of breath methane concentration profiles during exercise on an ergometer. J. Breath Res. 2016, 10, 017105–017105. [Google Scholar] [CrossRef]
  29. Adamczuk, G.; Humeniuk, E.; Adamczuk, K.; Grabarska, A.; Dudka, J. 2,4-Dinitrophenol as an Uncoupler Augments the Anthracyclines Toxicity against Prostate Cancer Cells. Molecules 2022, 27, 7227. [Google Scholar] [CrossRef]
  30. Wishkerman, A.; Greiner, S.; Ghyczy, M.; Boros, M.; Rausch, T.; Lenhart, K.; Keppler, F. Enhanced formation of methane in plant cell cultures by inhibition of cytochrome c oxidase. Plant, Cell Environ. 2010, 34, 457–464. [Google Scholar] [CrossRef]
  31. Tuboly, E.; Szabó, A.; Garab, D.; Bartha, G.; Janovszky. ; Ero″S, G.; Szabó, A.; Mohácsi,.; Szabó, G.; Kaszaki, J.; et al. Methane biogenesis during sodium azide-induced chemical hypoxia in rats. Am. J. Physiol. Physiol. 2013, 304, C207–C214. [Google Scholar] [CrossRef]
  32. Keppler, F.; Hamilton, J.T.G.; Braß, M.; Röckmann, T. Methane emissions from terrestrial plants under aerobic conditions. Nature 2006, 439, 187–191. [Google Scholar] [CrossRef]
  33. Keppler, F.; Boros, M.; Polag, D. Radical-Driven Methane Formation in Humans Evidenced by Exogenous Isotope-Labeled DMSO and Methionine. Antioxidants 2023, 12, 1381. [Google Scholar] [CrossRef] [PubMed]
  34. Tuboly, E.; Szabó, A.; Erős, G.; Mohácsi. ; Szabó, G.; Tengölics, R.; Rákhely, G.; Boros, M. Determination of endogenous methane formation by photoacoustic spectroscopy. J. Breath Res. 2013, 7, 046004. [Google Scholar] [CrossRef] [PubMed]
  35. Polag, D.; Keppler, F. COVID19-vaccination affects breath methane dynamics. 2022.07.27.501717 Preprint at. [CrossRef]
  36. Chou, H.; Godbeer, L.; Allsworth, M.; Boyle, B.; Ball, M.L. Progress and challenges of developing volatile metabolites from exhaled breath as a biomarker platform. Metabolomics 2024, 20, 1–25. [Google Scholar] [CrossRef]
  37. Pimentel, M.; Lin, H.C.; Enayati, P.; Burg, B.v.D.; Lee, H.-R.; Chen, J.H.; Park, S.; Kong, Y.; Conklin, J. Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity. Am. J. Physiol. Liver Physiol. 2006, 290, G1089–G1095. [Google Scholar] [CrossRef]
  38. Jahng, J.; Jung, I.S.; Choi, E.J.; Conklin, J.L.; Park, H. The effects of methane and hydrogen gases produced by enteric bacteria on ileal motility and colonic transit time. Neurogastroenterol. Motil. 2011, 24, 185–e92. [Google Scholar] [CrossRef]
  39. Hwang, L.; Low, K.; Khoshini, R.; Melmed, G.; Sahakian, A.; Makhani, M.; Pokkunuri, V.; Pimentel, M. Evaluating Breath Methane as a Diagnostic Test for Constipation-Predominant IBS. Dig. Dis. Sci. 2009, 55, 398–403. [Google Scholar] [CrossRef]
  40. Attaluri, A.; Jackson, M.; Valestin, J.; Rao, S.S. Methanogenic Flora Is Associated With Altered Colonic Transit but Not Stool Characteristics in Constipation Without IBS. Am. J. Gastroenterol. 2010, 105, 1407–1411. [Google Scholar] [CrossRef]
  41. Kunkel, D.; Basseri, R.J.; Makhani, M.D.; Chong, K.; Chang, C.; Pimentel, M. Methane on Breath Testing Is Associated with Constipation: A Systematic Review and Meta-analysis. Dig. Dis. Sci. 2011, 56, 1612–1618. [Google Scholar] [CrossRef]
  42. Chatterjee, S.; Park, S.; Low, K.; Kong, Y.; Pimentel, M. The Degree of Breath Methane Production in IBS Correlates With the Severity of Constipation. Am. J. Gastroenterol. 2007, 102, 837–841. [Google Scholar] [CrossRef]
  43. El Oufir, L.; Flourie, B.; Varannes, S.B.D.; Barry, J.L.; Cloarec, D.; Bornet, F.; Galmiche, J.P. Relations between transit time, fermentation products, and hydrogen consuming flora in healthy humans. Gut 1996, 38, 870–877. [Google Scholar] [CrossRef] [PubMed]
  44. Soares, A.C.F.; Lederman, H.M.; Fagundes-Neto, U.; de Morais, M.B. Breath Methane Associated With Slow Colonic Transit Time in Children With Chronic Constipation. J. Clin. Gastroenterol. 2005, 39, 512–515. [Google Scholar] [CrossRef] [PubMed]
  45. Park, Y.M.; Hussain, Z.; Lee, Y.H.; Park, H. The effects and mechanism of action of methane on ileal motor function. Neurogastroenterol. Motil. 2017, 29. [Google Scholar] [CrossRef] [PubMed]
  46. Ruaud, A.; Esquivel-Elizondo, S.; de la Cuesta-Zuluaga, J.; Waters, J.L.; Angenent, L.T.; Youngblut, N.D.; Ley, R.E. Syntrophy via Interspecies H 2 Transfer between Christensenella and Methanobrevibacter Underlies Their Global Cooccurrence in the Human Gut. mBio 2020, 11. [Google Scholar] [CrossRef]
  47. Høverstad, T.; Fausa, O.; Bjørneklett, A.; Bøhmer, T. Short-Chain Fatty Acids in the Normal Human Feces. Scand. J. Gastroenterol. 1984, 19, 375–381. [Google Scholar] [CrossRef]
  48. Fernandes, J.; Wolever, T.M.; Rao, A.V. Increased Serum Cholesterol in Healthy Human Methane Producers Is Confounded by Age. J. Nutr. 1998, 128, 1349–1354. [Google Scholar] [CrossRef] [PubMed]
  49. Fernandes, J.; Rao, A.V.; Wolever, T.M.S. Different Substrates and Methane Producing Status Affect Short-Chain Fatty Acid Profiles Produced by In Vitro Fermentation of Human Feces. J. Nutr. 2000, 130, 1932–1936. [Google Scholar] [CrossRef]
  50. Wolever, T.M.S.; A Robb, P.; ter Wal, P.; Spadafora, P.G. Interaction between Methane-Producing Status and Diet on Serum Acetate Concentration in Humans. J. Nutr. 1993, 123, 681–688. [Google Scholar] [CrossRef]
  51. Fernandes, J.; et al. Age, Dietary Fiber, Breath Methane, and Fecal Short Chain Fatty Acids Are Interrelated in Archaea-Positive Humans1–3. J. Nutr. 2013, 143, 1269–1275. [Google Scholar] [CrossRef]
  52. Polag, D.; Leiß, O.; Keppler, F. Age dependent breath methane in the German population. Sci. Total. Environ. 2014, 481, 582–587. [Google Scholar] [CrossRef]
  53. Liu, L.; Yi, Y.; Yan, R.; Hu, R.; Sun, W.; Zhou, W.; Zhou, H.; Si, X.; Ye, Y.; Li, W.; et al. Impact of age-related gut microbiota dysbiosis and reduced short-chain fatty acids on the autonomic nervous system and atrial fibrillation in rats. Front. Cardiovasc. Med. 2024, 11, 1394929. [Google Scholar] [CrossRef] [PubMed]
  54. Campbell, A.; Gdanetz, K.; Schmidt, A.W.; Schmidt, T.M. H2 generated by fermentation in the human gut microbiome influences metabolism and competitive fitness of gut butyrate producers. Microbiome 2023, 11, 1–15. [Google Scholar] [CrossRef] [PubMed]
  55. Samuel, B.S.; Gordon, J.I. A humanized gnotobiotic mouse model of host–archaeal–bacterial mutualism. Proc. Natl. Acad. Sci. USA 2006, 103, 10011–10016. [Google Scholar] [CrossRef] [PubMed]
  56. Li, Z.; Jia, Y.; Feng, Y.; Cui, R.; Miao, R.; Zhang, X.; Qu, K.; Liu, C.; Zhang, J. Methane alleviates sepsis-induced injury by inhibiting pyroptosis and apoptosis: in vivo and in vitro experiments. Aging 2019, 11, 1226–1239. [Google Scholar] [CrossRef]
  57. Zhang, X.; et al. Methane limit LPS-induced NF-κB/MAPKs signal in macrophages and suppress immune response in mice by enhancing PI3K/AKT/GSK-3β-mediated IL-10 expression. Sci. Rep. 2016, 6, 29359. [Google Scholar] [CrossRef]
  58. He, R.; Wang, L.; Zhu, J.; Fei, M.; Bao, S.; Meng, Y.; Wang, Y.; Li, J.; Deng, X. Methane-rich saline protects against concanavalin A-induced autoimmune hepatitis in mice through anti-inflammatory and anti-oxidative pathways. Biochem. Biophys. Res. Commun. 2016, 470, 22–28. [Google Scholar] [CrossRef]
  59. Ye, Z.; Chen, O.; Zhang, R.; Nakao, A.; Fan, D.; Zhang, T.; Gu, Z.; Tao, H.; Sun, X. Methane Attenuates Hepatic Ischemia/Reperfusion Injury in Rats Through Antiapoptotic, Anti-Inflammatory, and Antioxidative Actions. Shock 2015, 44, 181–187. [Google Scholar] [CrossRef]
  60. Protective Effects of Methane-Rich Saline on Rats with Lipopolysaccharide-Induced Acute Lung Injury - PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5434237/.
  61. Wang, W.; Huang, X.; Li, J.; Sun, A.; Yu, J.; Xie, N.; Xi, Y.; Ye, X. Methane Suppresses Microglial Activation Related to Oxidative, Inflammatory, and Apoptotic Injury during Spinal Cord Injury in Rats. Oxidative Med. Cell. Longev. 2017, 2017, 2190897–2190897. [Google Scholar] [CrossRef]
  62. Shen, M.; Fan, D.; Zang, Y.; Chen, Y.; Zhu, K.; Cai, Z.; Liu, Y.; Sun, X.; Liu, J.; Gong, J. Neuroprotective effects of methane-rich saline on experimental acute carbon monoxide toxicity. J. Neurol. Sci. 2016, 369, 361–367. [Google Scholar] [CrossRef]
  63. Wang, L.; Yao, Y.; He, R.; Meng, Y.; Li, N.; Zhang, D.; Xu, J.; Chen, O.; Cui, J.; Bian, J.; et al. Methane ameliorates spinal cord ischemia-reperfusion injury in rats: Antioxidant, anti-inflammatory and anti-apoptotic activity mediated by Nrf2 activation. Free. Radic. Biol. Med. 2017, 103, 69–86. [Google Scholar] [CrossRef]
  64. Xin, L.; Sun, X.; Lou, S. Effects of Methane-Rich Saline on the Capability of One-Time Exhaustive Exercise in Male SD Rats. PLoS ONE 2016, 11, e0150925. [Google Scholar] [CrossRef] [PubMed]
  65. Xie, Q.; Fei, M.; Fa, Z.; Wang, L.; Wang, J.; Zhang, Y.; Wang, J.; Deng, X. Methane-rich saline alleviates cerulein-induced acute pancreatitis by inhibiting inflammatory response, oxidative stress and pancreatic apoptosis in mice. Int. Immunopharmacol. 2017, 51, 17–24. [Google Scholar] [CrossRef] [PubMed]
  66. Protective effects of methane-rich saline on diabetic retinopathy via anti-inflammation in a streptozotocin-induced diabetic rat model - PubMed. https://pubmed.ncbi.nlm.nih.gov/26363454/.
  67. Liu, L.; Sun, Q.; Wang, R.; Chen, Z.; Wu, J.; Xia, F.; Fan, X.-Q. Methane attenuates retinal ischemia/reperfusion injury via anti-oxidative and anti-apoptotic pathways. Brain Res. 2016, 1646, 327–333. [Google Scholar] [CrossRef]
  68. Chen, O.; Ye, Z.; Cao, Z.; Manaenko, A.; Ning, K.; Zhai, X.; Zhang, R.; Zhang, T.; Chen, X.; Liu, W.; et al. Methane attenuates myocardial ischemia injury in rats through anti-oxidative, anti-apoptotic and anti-inflammatory actions. Free. Radic. Biol. Med. 2015, 90, 1–11. [Google Scholar] [CrossRef]
  69. Boros, M.M.; Ghyczy, M.; Érces, D.; Varga, G.; Tőkés, T.; Kupai, K.; Torday, C.; Kaszaki, J. The anti-inflammatory effects of methane*. Crit. Care Med. 2012, 40, 1269–1278. [Google Scholar] [CrossRef]
  70. Wang, R.; Sun, Q.; Xia, F.; Chen, Z.; Wu, J.; Zhang, Y.; Xu, J.; Liu, L. Methane rescues retinal ganglion cells and limits retinal mitochondrial dysfunction following optic nerve crush. Exp. Eye Res. 2017, 159, 49–57. [Google Scholar] [CrossRef] [PubMed]
  71. Song, K.; Zhang, M.; Hu, J.; Liu, Y.; Liu, Y.; Wang, Y.; Ma, X. Methane-rich saline attenuates ischemia/reperfusion injury of abdominal skin flaps in rats via regulating apoptosis level. BMC Surg. 2015, 15, 92. [Google Scholar] [CrossRef]
  72. Jia, Y.; et al. Methane-Rich Saline Ameliorates Sepsis-Induced Acute Kidney Injury through Anti-Inflammation, Antioxidative, and Antiapoptosis Effects by Regulating Endoplasmic Reticulum Stress. Oxid. Med. Cell. Longev. 2018, 2018, 4756846. [Google Scholar] [CrossRef]
  73. Sun, A.; Wang, W.; Ye, X.; Wang, Y.; Yang, X.; Ye, Z.; Sun, X.; Zhang, C. Protective Effects of Methane-Rich Saline on Rats with Lipopolysaccharide-Induced Acute Lung Injury. Oxidative Med. Cell. Longev. 2017, 2017, 7430193. [Google Scholar] [CrossRef]
  74. Turnbaugh, P.J.; Ley, R.E.; Mahowald, M.A.; Magrini, V.; Mardis, E.R.; Gordon, J.I. An Obesity-Associated Gut Microbiome with Increased Capacity for Energy Harvest. Nature 2006, 444, 1027–1031. [Google Scholar] [CrossRef]
  75. Fan, Y.; Pedersen, O. Gut microbiota in human metabolic health and disease. Nat. Rev. Microbiol. 2021, 19, 55–71. [Google Scholar] [CrossRef] [PubMed]
  76. I, M.N. The contribution of the large intestine to energy supplies in man. Am. J. Clin. Nutr. 1984, 39, 338–342. [Google Scholar] [CrossRef]
  77. Basseri, R.J.; Basseri, B.; Pimentel, M.; Chong, K.; Youdim, A.; Low, K.; Hwang, L.; Soffer, E.; Chang, C.; Mathur, R. Intestinal methane production in obese individuals is associated with a higher body mass index. . 2012, 8, 22–8. [Google Scholar] [PubMed]
  78. Murga-Garrido, S.M.; Orbe-Orihuela, Y.C.; Díaz-Benítez, C.E.; Castañeda-Márquez, A.C.; Cornejo-Granados, F.; Ochoa-Leyva, A.; Sanchez-Flores, A.; Cruz, M.; Burguete-García, A.I.; Lagunas-Martínez, A. Alterations of the Gut Microbiome Associated to Methane Metabolism in Mexican Children with Obesity. Children 2022, 9, 148. [Google Scholar] [CrossRef] [PubMed]
  79. An, S.; Cho, E.-Y.; Yang, H.; Hwang, J.; Shin, K.; Jung, S.; Kim, B.-T.; Kim, K.-N.; Lee, W. Methane gas in breath test is associated with non-alcoholic fatty liver disease. J. Breath Res. 2024, 18, 046005. [Google Scholar] [CrossRef]
  80. Mathur, R.; Amichai, M.; Chua, K.S.; Mirocha, J.; Barlow, G.M.; Pimentel, M. Methane and Hydrogen Positivity on Breath Test Is Associated With Greater Body Mass Index and Body Fat. J. Clin. Endocrinol. Metab. 2013, 98, E698–E702. [Google Scholar] [CrossRef]
  81. Ozato, N.; Saito, S.; Yamaguchi, T.; Katashima, M.; Tokuda, I.; Sawada, K.; Katsuragi, Y.; Kakuta, M.; Imoto, S.; Ihara, K.; et al. Association between breath methane concentration and visceral fat area: a population-based cross-sectional study. J. Breath Res. 2020, 14, 026008. [Google Scholar] [CrossRef]
  82. Wilder-Smith, C.H.; Olesen, S.S.; Materna, A.; Drewes, A.M. Breath methane concentrations and markers of obesity in patients with functional gastrointestinal disorders. United Eur. Gastroenterol. J. 2018, 6, 595–603. [Google Scholar] [CrossRef]
  83. Mathur, R.; Goyal, D.; Kim, G.; Barlow, G.M.; Chua, K.S.; Pimentel, M. Methane-producing human subjects have higher serum glucose levels during oral glucose challenge than non-methane producers: a pilot study of the effects of enteric methanogens on glycemic regulation. Res. J. Endocrinol. Metab. 2014, 2, 2. [Google Scholar] [CrossRef]
  84. Mathur, R.; Chua, K.S.; Mamelak, M.; Morales, W.; Barlow, G.M.; Thomas, R.; Stefanovski, D.; Weitsman, S.; Marsh, Z.; Bergman, R.N.; et al. Metabolic effects of eradicating breath methane using antibiotics in prediabetic subjects with obesity. Obesity 2016, 24, 576–582. [Google Scholar] [CrossRef]
  85. Mathur, R.; Mundi, M.S.; Chua, K.S.; Lorentz, P.A.; Barlow, G.M.; Lin, E.; Burch, M.; Youdim, A.; Pimentel, M. Intestinal methane production is associated with decreased weight loss following bariatric surgery. Obes. Res. Clin. Pr. 2016, 10, 728–733. [Google Scholar] [CrossRef] [PubMed]
  86. Wu, H.; Tremaroli, V.; Schmidt, C.; Lundqvist, A.; Olsson, L.M.; Krämer, M.; Gummesson, A.; Perkins, R.; Bergström, G.; Bäckhed, F. The Gut Microbiota in Prediabetes and Diabetes: A Population-Based Cross-Sectional Study. Cell Metab. 2020, 32, 379–390. [Google Scholar] [CrossRef] [PubMed]
  87. Laverdure, R.; Mezouari, A.; Carson, M.A.; Basiliko, N.; Gagnon, J. A role for methanogens and methane in the regulation of GLP-1. Endocrinol. Diabetes Metab. 2017, 1, e00006. [Google Scholar] [CrossRef]
  88. Kenny, B. J. & Bordoni, B. Neuroanatomy, Cranial Nerve 10 (Vagus Nerve). in StatPearls (StatPearls Publishing, Treasure Island (FL), 2024).
  89. Pimentel, M.; Kong, Y.; Park, S. IBS Subjects with Methane on Lactulose Breath Test Have Lower Postprandial Serotonin Levels Than Subjects with Hydrogen. Dig. Dis. Sci. 2004, 49, 84–87. [Google Scholar] [CrossRef]
  90. Takakura, W.; Chang, C.; Pimentel, M.; Mo, G.; Torosyan, J.; Hosseini, A.; Wang, J.; Kowaleski, E.; Mathur, R.; Chang, B.; et al. Exhaled Methane Is Associated with a Lower Heart Rate. Cardiology 2021, 147, 225–229. [Google Scholar] [CrossRef] [PubMed]
  91. Takakura, W.; Chang, C.; Hosseini, A.; Wang, J.; Kowalewski, E.; Mathur, R.; Rezaie, A.; Pimentel, M. S0462 The Vital Gut Microbe: The Effect of Methane on the Host's Vital Sign. Am. J. Gastroenterol. 2020, 115, S232–S233. [Google Scholar] [CrossRef]
  92. Takakura, W.; Pimentel, M.; Mathur, R.; Rezaie, A. S500 Presence of Methane in the Fasting Breath Is Associated With a Lower Rise in Pancreatic Polypeptide After Modified Sham-Feeding: An Insight into the Connection Between Vagal Dysfunction and Methane. Am. J. Gastroenterol. 2021, 116, S224–S224. [Google Scholar] [CrossRef]
Preprints 138509 g001
Figure 1. Demonstrating the proposed routes for both exogenous and endogenous methane production. Exogenous methane production relies on the generation of methane by prokaryotic species in anaerobic conditions within the GI tract. Endogenous methane production by contrast relies on oxidative demethylation of methane containing moieties by reactive oxygen species.
Figure 1. Demonstrating the proposed routes for both exogenous and endogenous methane production. Exogenous methane production relies on the generation of methane by prokaryotic species in anaerobic conditions within the GI tract. Endogenous methane production by contrast relies on oxidative demethylation of methane containing moieties by reactive oxygen species.
Preprints 138509 g001
Preprints 138509 g002
Figure 2. Summarising the advantages and limitations of the collection and analytical methods used.
Figure 2. Summarising the advantages and limitations of the collection and analytical methods used.
Preprints 138509 g002
Preprints 138509 g003
Figure 3. Summarising both the local and systemic effects proposed for methane within the field. Green icons indicate an effect with positive associations, orange icons indicate an effect with negative associations.
Figure 3. Summarising both the local and systemic effects proposed for methane within the field. Green icons indicate an effect with positive associations, orange icons indicate an effect with negative associations.
Preprints 138509 g003
Preprints 138509 g004
Figure 4. Highlighting the potential role of the vagus nerve/cholinergic pathway in mediating the effects of methane, with focus on potential regulators of this pathway, as well as shared effects.
Figure 4. Highlighting the potential role of the vagus nerve/cholinergic pathway in mediating the effects of methane, with focus on potential regulators of this pathway, as well as shared effects.
Preprints 138509 g004
Table 1. Illustrating the four key archaeal species associated with methane production within the GI tract.
Table 1. Illustrating the four key archaeal species associated with methane production within the GI tract.
Preprints 138509 i001
Table 2. An overview of collection methods for breath methane analysis.
Table 2. An overview of collection methods for breath methane analysis.
Collection bags Tubes Handheld real-time analyzer
Overview One of the most common methods used, where patients exhale directly into a bag. Uses tubes through which the patient exhales. Tubes are then sealed awaiting analysis. Involves the use of portable devices that analyze breath in real-time with no need for separate sample collection/storage
Different forms Mylar bags (made from a type of polyester film impermeable to gases) or Tedlar bags (made from PVF). Vacuum tubes (draw in the breath sample automatically) or glass/plastic tubes. The OMED device.
Procedure Patients take a deep breath and exhale completely into the bag. The bag is subsequently sealed for later analysis. The patient exhales through a mouthpiece connected to the tube, which is then sealed after collection. Patient breathes directly into the analyzer through a mouthpiece. Gas concentrations are fed back in real-time.
Advantages Simple and cost-effective. Easy to use and transport. Provides instant results and allows for simple repeat measures.
Considerations Bags can be challenging to handle post-collection and can lead to sample contamination/loss. Tubes may require specific storage to prevent sample degradation. Device calibration is crucial for accurate readings.
Table 3. An overview of analytical methods for breath methane.
Table 3. An overview of analytical methods for breath methane.
GC-FID IR MOS
Overview GC-FID combines GC for separation of components of a breath sample and flame ionization detection for methane quantification. IR measures the absorption of IR light by methane to determine its concentration. MOS detect gases based on changes in electrical resistances of a metal oxide sensor when it interacts with methane.
Advantages High sensitivity, high specificity and quantitative analysis. Real-time analysis and easier than GC-FID. Cost-effective, durable and provides real-time analyses.
Limitations Complexity and cost, requiring sophisticated equipment and trained personnel. Time-consuming, requiring lab prep. Unlikely to reach the level of GC-FID for low concentrations. Subject to interference from other gases/water vapor if not properly calibrated. Unlikely to reach the level of GC-FID for low concentrations. Requires appropriate calibration.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
Prerpints.org logo

Preprints.org is a free preprint server supported by MDPI in Basel, Switzerland.

facebook logotwitter logolinkedin logo
MDPI Initiatives
Important Links
Subscribe

Disclaimer

Terms of Use

Privacy Policy

© 2024 MDPI (Basel, Switzerland) unless otherwise stated