22 Gastroenterology & Hepatology Volume 8, Issue 1 January 2012
Intestinal Methane Production in Obese
Individuals Is Associated with a Higher
Body Mass Index
Robert J. Basseri, MD, Benjamin Basseri, MD, Mark Pimentel, MD,
Kelly Chong, PhD, Adrienne Youdim, MD, Kimberly Low, Laura Hwang,
Edy Soffer, MD, Christopher Chang, MD, PhD, and Ruchi Mathur, MD
Dr. Robert J. Basseri, Dr. Benjamin Basseri,
Dr. Pimentel, Ms. Low, Ms. Hwang,
Dr. Soffer, and Dr. Chang are affiliated
with the GI Motility Program at Cedars-
Sinai Medical Center in Los Angeles,
California. Dr. Chong is affiliated with the
Division of General Internal Medicine
and Health Services Research and the
Department of Medicine at the University
of California–Los Angeles. Dr. Youdim and
Dr. Mathur are affiliated with the Bariatric
Center of Excellence at Cedars-Sinai
Medical Center in Los Angeles, California.
Address correspondence to:
Dr. Ruchi Mathur
Director of Diabetes
Division of Endocrine, Diabetes,
Department of Medicine
Cedars-Sinai Medical Center
131 Becker Building
Los Angeles, CA 90048;
Obesity, nutrition, secretion, absorption, motility,
Abstract: Background: Obesity is an epidemic that affects 1 in 3 indi-
viduals in the United States, and recent evidence suggests that enteric
microbiota may play a significant role in the development of obesity.
This study evaluated the association between methanogenic archaea
and obesity in human subjects. Methods: Subjects with a body mass
index (BMI) of 30 kg/m2 or higher were prospectively recruited from the
weight loss program of a tertiary care medical center. Subjects who met
the study’s inclusion criteria were asked to complete a questionnaire
that included a series of visual analogue scores for bowel symptom
severities. Subjects then provided a single end-expiratory breath sample
to quantitate methane levels. Bivariate and multivariate analyses were
used to determine associations with BMI. Results: A total of 58 patients
qualified for enrollment. Twenty percent of patients (n=12) had breath
test results that were positive for methane (>3 parts per million [ppm]),
with a mean breath methane concentration of 12.2±3.1 ppm. BMI
was significantly higher in methane-positive subjects (45.2±2.3 kg/m2)
than in methane-negative subjects (38.5±0.8 kg/m2; P=.001). Meth-
ane-positive subjects also had a greater severity of constipation than
methane-negative subjects (21.3±6.4 vs 9.5±2.4; P=.043). Multiple
regression analysis illustrated a significant association between BMI and
methane, constipation, and antidepressant use. However, methane
remained an independent predictor of elevated BMI when control-
ling for antidepressant use (P<.001) and when controlling for both
constipation and antidepressant use (6.55 kg/m2 greater BMI; P=.003).
Conclusion: This is the first human study to demonstrate that a higher
concentration of methane detected by breath testing is a predictor of
significantly greater obesity in overweight subjects.
large National Health and Nutrition Examination Survey found a
besity is a complex, multifactorial disease that contrib-
utes significantly to major health problems such as heart
disease, type 2 diabetes, and certain types of cancer.1-3 A
Gastroenterology & Hepatology Volume 8, Issue 1 January 2012 23
I n t e s t I n a l M e t H a n e P r o d u c t I o n I n o b e s e I n d I V I d u a l s
9% increase in overweight individuals (body mass index
[BMI] ≥25 kg/m2), an 8% increase in obese individu-
als (BMI ≥30 kg/m2), and an almost 2-fold increase in
extreme obesity (BMI ≥40 kg/m2) in the United States
between 1994 and 2000.4 Currently, the age-adjusted
prevalence of obesity in the United States is 33.8%, and
the combined prevalence of obese and overweight indi-
viduals is 68%.5 The potential benefits of reducing obesity
levels are considerable, as 6% of the US healthcare budget
is spent on treating obesity.6 Major contributors to the
increasing prevalence of obesity include genetic predis-
position, metabolic disorders, and changes in physical
activity and diet.7
Increasing evidence supports an association between
the composition of gut microflora and the development
of obesity. Indirect evidence for this association comes
from data showing that obese human subjects have
increased breath ethanol concentrations.8 This increase
in breath ethanol is believed to be related to gut micro-
flora, as earlier animal studies revealed higher breath
ethanol concentrations in obese versus lean mice; these
concentrations decreased following administration of
oral antibiotics.9 More recent animal studies have shown
that the composition and quantity of gut microflora are
altered in obese mice.10
One particular alteration in gut microflora that
is associated with increased weight gain in animal
models is the presence of methanogenic archaea, spe-
cifically Methanobrevibacter smithii.11 Methanogens
are common in normal human enteric flora, and
M. smithii is the most common methanogenic colo-
nizer in humans.12,13 Methanogens have been shown
to affect caloric harvest by increasing the capacity of
polysaccharide-eating bacteria to digest polyfructose-
containing glycans, which leads to increased weight
gain in mice.14 Further, previous studies by our group
have demonstrated that methane gas slows proximal
small intestinal transit by 59% in an in vivo model.15
This slowing of proximal small intestinal transit may
contribute to increased weight gain by increasing the
total gut microbiome load or the amount of time dur-
ing which energy is harvested from meals. Given the
associations between methanogens and weight gain in
animal models, coupled with the finding of an associa-
tion between methane and delayed transit, this study
hypothesized that human subjects with increased con-
centrations of methane on breath testing might exhibit
increased levels of obesity compared to individuals
without elevated methane concentrations. To test
this hypothesis, this study tested for associations
between obesity, altered bowel symptoms, and the
presence or absence of methane in breath samples in
Subjects were prospectively recruited from the weight
management program of a tertiary care medical center.
Individuals were eligible to participate if they were between
18 and 65 years of age and had a BMI of at least 30 kg/m2
(which is the clinical definition of obesity) but no more
than 60 kg/m2. Subjects were excluded if they had a history
of a known gastrointestinal motility disorder, gastrointes-
tinal surgery (except for cholecystectomy and appendec-
tomy), clinically significant abdominal adhesions, collagen
vascular disease, HIV infection, uncontrolled hypo/hyper-
thyroidism, or uncontrolled diabetes. Subjects were also
excluded if they had utilized oral antibiotics or medications
that affect gastrointestinal motility (including prokinetics,
antikinetics, narcotics, or metformin) within 2 months.
The study was approved by the Institutional Review Board
at Cedars-Sinai Medical Center.
Informed written consent was obtained from subjects who
met the eligibility criteria for this study. Subjects were then
asked to complete a questionnaire that collected demo-
graphic and bowel symptom information. The presence
and degree of bowel symptoms were determined based
on a visual analogue scale (VAS).16 The VAS scores were
scaled from 0 to 100, with 100 mm denoting maximum
severity. Bowel symptoms included constipation, diarrhea,
bloating, excess gas, incomplete evacuation, abdominal
pain, urgency, straining, and excessive mucous secretion
from the rectum. Height and weight were recorded to
determine the patient’s current BMI. Data were also col-
lected regarding current medications, medical history, and
medical comorbidities (eg, diabetes mellitus, hypertension,
hyperlipidemia, and fatty liver disease).
After completing the questionnaire, subjects were
asked to provide a breath sample that could be assessed
for the presence of methane. Specifically, subjects were
asked to provide an end-expiratory breath sample using
the Quintron dual bag collection system (Quintron
Instrument Company). The breath sample was then
analyzed using a Quintron SC gas chromatograph (Quin-
tron Instrument Company) to determine the presence
of methane. Subjects were considered to be positive for
methane if methane was detected at a level of 3 parts per
million (ppm) or above.17,18
Bivariate and multivariate analyses were utilized to assess
for associations between the presence of methane on
breath testing and BMI. First, methane-positive and
methane-negative groups were compared in terms of
24 Gastroenterology & Hepatology Volume 8, Issue 1 January 2012
b a s s e r I e t a l
demographics and bowel symptom variables. T-tests were
performed to compare the mean methane concentrations
for each of these continuous variables. Second, Pearson
product-moment correlations of continuous demographic
and bowel symptom variables and BMI were calculated
to determine how strongly each of these predictors cor-
related with BMI. A correlation matrix was also produced
to determine how strongly bowel symptom variables and
BMI intercorrelated. Third, independent sample t-tests
were conducted to compare the mean BMI values for
2 independent groups of dichotomous predictor variables
(eg, gender, presence of any diagnoses or conditions, and
use of medication currently or within the last 2 months).
Fourth, multivariate regression models were used to identify
the association between each candidate predictor retained
from bivariate analyses (independent variables) and BMI
(the dependent variable), controlling for potential con-
founding variables. As the primary hypothesis was tested
in the multivariate analyses and pairwise comparisons were
used in bivariate analysis only for selection of predictors
to build the regression model, P-values were not adjusted
for multiple comparisons. A Huber-White standard error
estimator was used to obtain a more conservative estimate
of the P-value.19 For all analyses, P<.05 was considered
to be statistically significant.
Fifty-eight obese subjects (43 female and 15 male) were
enrolled in this study. All subjects completed a VAS survey
to describe and rate the severity of their bowel symptoms
and provided an end-expiratory breath sample for meth-
ane breath testing. The average age of the enrolled subjects
was 41.8 years (range, 22–64 years), and the average BMI
was 40.0 kg/m2 (range, 30.3–57.2 kg/m2).
Of the 58 obese subjects, 12 subjects (20.7%) were cat-
egorized as methane-positive and had an average breath
methane concentration of 12.2±3.1 ppm. On bivariate
analysis, methane-positive subjects had a greater average
BMI than methane-negative subjects (6.7 kg/m2; P=.001;
Table 1). Methane-positive subjects also had a significantly
greater average VAS score for constipation compared to
methane-negative subjects (11.79 mm; P=.043).
Table 1. Subject Characteristics Stratified by Presence of Methane
Total group (n=58) Methane not detected (n=46)Methane present (n=12)
Subject characteristicsMean±SE Mean±SEMean±SE
Bowel symptoms (VAS)
*P-value is comparing methane-producing obese subjects to non-methane–producing obese subjects.
BMI=body mass index; SE=standard error; VAS=visual analogue scale.
Gastroenterology & Hepatology Volume 8, Issue 1 January 2012 25
I n t e s t I n a l M e t H a n e P r o d u c t I o n I n o b e s e I n d I V I d u a l s
Pearson correlation coefficients were calculated for
continuous predictor variables and BMI. Incomplete
evacuation (r=0.35), constipation (r=0.34), and strain-
ing (r=0.29) had the highest correlations with BMI
(Table 2). These symptoms also strongly correlated with
each other (r=0.64) in each pairwise comparison (results
not shown). As these bowel symptoms were highly
intercorrelated, constipation was chosen as the proxy to
encompass incomplete evacuation and straining.
T-tests of dichotomous predictor variables indicated
that observed differences in mean BMI were significant
for comorbidity with depression and antidepressant
use (Table 3). As the depression and antidepressant use
variables were highly correlated (r=0.79; P<.001), anti-
depressant use was selected as the proxy for depression,
since antidepressant use is a tangible variable, while the
self-reported diagnosis of depression is more subjective.
Mean BMI was 5.40 kg/m2 lower in subjects who were
currently taking antidepressant medications compared to
subjects who were not taking antidepressants (P=.017).
Multivariate Analysis of Predictors for Body Mass Index
For the multivariate analysis, significant predictors
retained from the bivariate analyses were included to
build the regression model (Table 4). Since methane has
been associated with constipation in existing literature
and because the motor changes induced by methane
could contribute to constipation, one possibility is that
methane and constipation are collinear.18,20-22 Thus, the
regression analysis was conducted using the following
approach: First, when only antidepressant use (binary
variable) and a positive methane breath test result
(binary variable) were entered into the regression model
(Model 1), both variables were significantly associated
with BMI. The expected BMI was 7.45 kg/m2 higher in
subjects who had a positive methane breath test result
than in methane-negative subjects (P=.002); conversely,
the expected BMI was 4.25 kg/m2 lower in subjects who
were currently on antidepressants (P=.009). The overall
model was statistically significant (F=10.76; P<.001).
Interestingly, this association persisted after adjusting
for constipation. After constipation (continuous vari-
able) was added into the model (Model 2), methane and
antidepressant use remained significant correlates of BMI
(Table 4). Further, constipation was not significantly corre-
lated with antidepressant use (r=–0.14). Subjects who had
a positive methane breath test result had a BMI that was
6.55 kg/m2 higher than the BMI of methane-negative sub-
jects (P=.003), and subjects who were currently on antide-
pressant medications had a BMI that was 3.91 kg/m2 lower
than that of subjects who were not taking antidepressants
(P=.009). In this model, constipation was not a statistically
significant correlate of BMI at the P<.05 level; however, the
overall model remained significant (F=6.96; P<.001).
This study is the first to demonstrate a significant associa-
tion between the presence of methane on breath testing
and the degree of obesity. In a bivariate analysis, methane-
positive obese subjects had a BMI that was 6.7 kg/m2
higher than the BMI of methane-negative obese subjects.
In multivariate analysis, methane status remained signifi-
cant after controlling for constipation and other variables.
Obesity is a growing epidemic in the United States;
currently, 1 in 3 Americans over the age of 20 years are
obese, and 2 in 3 Americans are overweight.23,24 The
healthcare burden of obesity is extremely high, as obesity
is associated with type 2 diabetes mellitus, coronary artery
disease, hypertension, cerebral vascular accidents, numer-
ous malignancies, and other diseases that lead to consid-
erable morbidity and mortality.25,26 The economic cost of
these comorbidities is threatening an already inundated
healthcare system.1-3 During the past 3 decades, caloric
consumption has significantly increased in concert with a
considerable reduction in physical activity, which together
have contributed greatly to the high prevalence of obesity.27
The human gut is an intricate microbial ecosystem pop-
ulated by approximately 1014 bacteria, alterations to which
may contribute to obesity through increasing dietary energy
harvest and adipose deposition.28 Researchers’ understanding
of the microbial composition of the gut is improving as newer
technologies enable better identification and classification of
Table 2. Bivariate Correlations with Body Mass Index for
26 Gastroenterology & Hepatology Volume 8, Issue 1 January 2012
b a s s e r I e t a l
enteric flora.29-31 For example, the metagenome of the gut
microbiome has recently been cataloged.32 An individual’s
indigenous gut flora is established within the first year of
life and is progressively modified throughout adulthood by
endogenous and exogenous factors, including dietary intake
and genetic predisposition.33-38
While obesity generally results from an imbalance
between energy consumption (eating) and energy expen-
diture (physical activity and catabolism), an increase in
the efficiency with which an individual’s gut flora can
extract energy from food may also contribute to obesity.39
Bäckhed and colleagues showed that germ-free mice
weighed significantly less than mice with normal gut
flora, illustrating the significant role of gut microbiota in
nutrient metabolism.40 Further, colonization of the distal
gut of germ-free mice with flora from their convention-
ally raised, obese counterparts resulted in excessive weight
gain. Germ-free lean mice colonized with the microbiome
of obese mice experienced significant increases in body fat
compared to mice colonized with a conventional micro-
Table 3. Observed Differences in Mean Body Mass Index (BMI) for Dichotomous Predictor Variables
Predictor variablesN PercentGroup differences in BMI (kg/m2)
Female gender43 74.1–2.56.216
Prior diagnosis and conditions
Irritable bowel syndrome46.9–2.52 .483
Diabetes8 13.81.14 .665
Hypertension 2339.7 –1.40.454
Cholesterol 1932.80.96 .621
Fatty liver disease813.8–0.49.852
Thyroid disease9 15.5–1.27.613
Bowel surgery23.4–1.49 .766
Other medical problems21 36.2–2.06 .274
Narcotics2 3.44.76 .347
Medications within the last 2 months
Acid reflux medications915.5–0.21 .932
Table 4. Regression Coefficients Relating Body Mass Index to Predictor Variables
R2=0.300 (F=10.76; P<.001)R2=0.335 (F=6.96; P<.001)
*Methane and antidepressant use are binary variables. Constipation is a continuous variable.
Gastroenterology & Hepatology Volume 8, Issue 1 January 2012 27
I n t e s t I n a l M e t H a n e P r o d u c t I o n I n o b e s e I n d I V I d u a l s
biome.14 These data demonstrate that gut flora can play a
significant role in the development of obesity.
In humans, methane-producing archaea (methano-
gens) produce methane through anaerobic fermentation;
the most common methanogen in the human gut is
M. smithii, which is found in 70% of human subjects.30
Analysis of expiratory methane by lactulose breath test-
ing can serve as an indirect measure of methane produc-
tion.17,41,42 A minority of subjects (15%) produce large
quantities of methane early in the breath test, suggesting
a greater methane potential, and increased methane
production as measured by breath testing correlates with
increased levels of M. smithii in stool, as determined by
quantitative polymerase chain reaction assay.13,43,44
Methanogens remove hydrogen atoms and accelerate
the fermentation of polysaccharides and carbohydrates,
thus increasing the production of short-chain fatty acids
that are subsequently absorbed in the intestines and serve
as an additional source of energy for the human host.45
This more efficient energy extraction may lead to weight
gain and may ultimately contribute to obesity.46 A study
by Zhang and colleagues that utilized a different modal-
ity for methane measurement (stool assays) also demon-
strated a promising association between methane and
obesity in human subjects.47
Besides alterations in luminal metabolic processing,
methane gas itself may influence motility. Recently, our
group demonstrated that infusion of methane gas into the
small intestine resulted in a slowing of small intestinal transit
by 59% in an in vivo animal model.15 The slowing effects
of methane on intestinal transit could have 2 possible con-
sequences: First, slowing of intestinal transit could increase
the duration of nutrient absorption in the postprandial state.
Second, slowing of transit could result in higher levels of
gut microflora. Both of these effects could lead to increased
weight gain and the development of obesity.
The current study demonstrates that humans with
methane detectable via breath testing have a significantly
higher BMI than methane-negative controls. This find-
ing was remarkable because all subjects in this study
were obese, per the study’s inclusion criteria. This result
remained significant when controlling for other factors,
including constipation, which is an indicator of slowed
transit. This result may be due to the collinearity of consti-
pation and BMI. Although it remains unclear why meth-
ane was significant even when controlling for the clinical
manifestation of transit (ie, constipation), the results of
a recent animal study may help to explain this observa-
tion. In a study that has been submitted for publication,
our group found for the first time that colonization of
the rat gut with the methanogen M. smithii is not limited
to the large bowel but rather extends to the small bowel,
including the ileum, jejunum, and duodenum. Therefore,
obese human subjects may have increased numbers of
methanogens in the small bowel, rather than in the colon,
thus exerting slowing effects in the small bowel while
preserving colonic transit.
Another interesting finding in this study was that sub-
jects who were currently taking antidepressant medications
had a BMI that was 3.91 kg/m2 lower than the BMI of sub-
jects who were not taking antidepressants. While specific
antidepressant medications have been shown to produce
weight gain, obesity is also associated with depression, and
overeating can be a sign of depression. Thus, one possible
explanation for the observed data is that depression leads
to a sedentary lifestyle and self-destructive behaviors such
as overeating in some subjects. By treating depression with
antidepressant medications, perhaps the provocation for
these eating behaviors is decreased and the desire to exer-
cise or engage in other physical activities is increased. In
addition, tricyclic antidepressants have anticholiergic side
effects; these medications can, therefore, lead to suppres-
sion of appetite due to delayed gastric emptying. Further
studies with larger numbers of subjects would be required
to test this association.
This study clearly demonstrates a relationship between
intestinal methane production and BMI. However, there
are some limitations to the study’s data. First, this is a
preliminary study that was intended to evaluate a novel
relationship; thus, the sample size was relatively small, and
the study was performed at a single center. The observed
lack of statistical significance for some comparisons
may therefore be related to the small sample size in the
methane-positive group, although the multivariate analysis
found that methane remained an independent predictor
of elevated BMI when controlling for antidepressant use
(P<.001) and when controlling for both constipation and
antidepressant use (6.55 kg/m2 greater BMI; P=.003). Sec-
ond, the subjects in this study were all seeking assistance for
surgical or medical weight loss, and such patients may be
different from obese individuals who are not actively try-
ing to lose weight. Therefore, larger studies will be needed
to confirm our findings. However, our data are supported
by recent findings in gnotobiotic animal studies; Samuel
and coauthors found that Bacteroides thetaiotaomicron–
M. smithii co-colonization produced a significant increase
in host adiposity compared to monoassociated animals or
B. thetaiotaomicron–Desulfovibrio piger biassociated ani-
mals.45 As M. smithii is the most common methanogen
colonizing the human gut, the increased breath methane
concentration associated with greater BMI in this study also
likely results from increased M. smithii colonization.13,48,49
In conclusion, this study demonstrates that the pres-
ence of methane is associated with higher BMI among
obese subjects. This finding further supports the role of gut
flora in obesity. Moreover, this information may expand
28 Gastroenterology & Hepatology Volume 8, Issue 1 January 2012 Download full-text
b a s s e r I e t a l
on the evolving data in animal models, which support a
specific association between methanogenic archaea and
obesity. While the mechanism of this association remains
unknown (slowed transit vs metabolic interactions of gut
microflora), these intriguing results lay the foundation for
further research in this area.
The authors would like to thank Dr. Alexis Peraino and
Dr. Theodore Khalili, Cedars-Sinai Medical Center, Los
Angeles, California, for their assistance in recruiting patients.
In addition, the authors would like to thank the Beatrice and
Samuel A. Seaver Foundation for support of this work.
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