Associations among organochlorine pesticides, Methanobacteriales, and obesity in Korean women.

Abstract

Although Methanobacteriales in the gut has recently been linked to obesity, no study has examined the hypothesis that waist circumference, a marker of visceral obesity, are positively associated with Methanobacteriales in the general population. Since Methanobacteriales increase in a petroleum-contaminated environment to biodegrade petroleum as one way of autopurification, we also hypothesized that high body burden of highly lipophilic petroleum-based chemicals like organochlorine pesticides (OCPs) is associated with higher levels of Methanobacteriales in the gut.
Among 83 Korean women who visited a community health service center for a routine health checkup, quantitative real-time PCR (qPCR) based on 16S rDNA was used to quantify Methanobacteriales in feces. Nine OCPs were measured in both serum and feces of 16 subjects. Methanobacteriales were detected in 32.5% (27/83 women). Both BMI and waist circumference among women with Methanobacteriales were significantly higher than in women without Methanobacteriales (P = 0.04 and P = 0.01, respectively). Also, Methanobacteriales levels in feces were positively associated with BMI and waist circumference (r = +0.23 and P = 0.03 for both). Furthermore, there were significant correlations between feces Methanobacteriales levels and serum concentrations of most OCPs, including with cis-nonachlor (r = +0.53, P<0.05), oxychlordane (r = +0.46, P<0.1), and trans-nonachlor (r = +0.43, P<0.1). Despite high correlations of serum and feces concentrations of most OCPs, feces OCP concentrations were not clearly associated with feces Methanobacteriales levels.
In this cross-sectional study, the levels of Methanobacteriales in the human gut were associated with higher body weight and waist circumference. In addition, serum OCP concentrations were positively correlated with levels of Methanobacteriales. There may be a meaningful link among body burden of OCP, Methanobacteriales in the gut, and obesity in the general population.

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Associations among Organochlorine Pesticides,
Methanobacteriales, and Obesity in Korean Women
Hae-Sook Lee1, Je-Chul Lee2, In-Kyu Lee3, Hyo-Bang Moon4, Yoon-Seok Chang5, David R. Jacobs, Jr.6,7,
Duk-Hee Lee8*
1Department of Public Health Graduate School, Kyungpook National University, Daegu, Korea, 2Department of Microbiology, Kyungpook National University School of
Medicine, Daegu, Korea, 3Department of Endocrinology, Kyungpook National University School of Medicine, Daegu, Korea, 4Department of Environmental Marine
Sciences, College of Science and Technology, Hanyang University, Ansan, Korea, 5School of Environmental Science and Engineering, POSTECH, Pohang, Korea, 6Division
of Epidemiology, School of Public Health, University of Minnesota, Minneapolis, Minnesota, United States of America, 7Department of Nutrition, University of Oslo, Oslo,
Norway, 8Department of Preventative Medicine, School of Medicine, Kyungpook National University, Daegu, Korea
Abstract
Background: Although Methanobacteriales in the gut has recently been linked to obesity, no study has examined the
hypothesis that waist circumference, a marker of visceral obesity, are positively associated with Methanobacteriales in the
general population. Since Methanobacteriales increase in a petroleum-contaminated environment to biodegrade petroleum
as one way of autopurification, we also hypothesized that high body burden of highly lipophilic petroleum-based chemicals
like organochlorine pesticides (OCPs) is associated with higher levels of Methanobacteriales in the gut.
Methodology/Principal Findings: Among 83 Korean women who visited a community health service center for a routine
health checkup, quantitative real-time PCR (qPCR) based on 16S rDNA was used to quantify Methanobacteriales in feces.
Nine OCPs were measured in both serum and feces of 16 subjects. Methanobacteriales were detected in 32.5% (27/83
women). Both BMI and waist circumference among women with Methanobacteriales were significantly higher than in
women without Methanobacteriales (P=0.04 and P=0.01, respectively). Also, Methanobacteriales levels in feces were
positively associated with BMI and waist circumference (r=+0.23 and P=0.03 for both). Furthermore, there were significant
correlations between feces Methanobacteriales levels and serum concentrations of most OCPs, including with cis-nonachlor
(r=+0.53, P,0.05), oxychlordane (r=+0.46, P,0.1), and trans-nonachlor (r=+0.43, P,0.1). Despite high correlations of
serum and feces concentrations of most OCPs, feces OCP concentrations were not clearly associated with feces
Methanobacteriales levels.
Conclusion/Significance: In this cross-sectional study, the levels of Methanobacteriales in the human gut were associated
with higher body weight and waist circumference. In addition, serum OCP concentrations were positively correlated with
levels of Methanobacteriales. There may be a meaningful link among body burden of OCP, Methanobacteriales in the gut,
and obesity in the general population.
Citation: Lee H-S, Lee J-C, Lee I-K, Moon H-B, Chang Y-S, et al. (2011) Associations among Organochlorine Pesticides, Methanobacteriales, and Obesity in Korean
Women. PLoS ONE 6(11): e27773. doi:10.1371/journal.pone.0027773
Editor: Raul M. Luque, University of Cordoba, Spain
Received June 14, 2011; Accepted October 25, 2011; Published November 17, 2011
Copyright: ? 2011 Lee et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011-
0004692). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: lee_dh@knu.ac.kr
Introduction
Methanogens are microbes that produce methane gas from
various substrates such as H2and CO2, acetate, and methylamine.
They were identified as belonging to the domain archaea in the
1970s [1]. Pathogenicity, such as tissue invasion and toxin release
commonly observed with classic pathogens, has not yet been
described for methanogens [2]. Although causal pathways are not
clear, significant associations have been observed between
methanogens and periodontitis, colon cancer, or diverticulosis [2].
Methanogens were recently linked to obesity [3]. Many dietary
components that are resistant to initial digestion in the small
intestine are subsequently fermented by the microbial community
of the large intestine, producing short-chain fatty acids (SCFAs)
that are absorbed across the colonic mucosa [4]. These SCFAs
have been estimated to provide 10% of daily caloric intake [4]. In
experimental mice, the presence of methanogens in the colon
promotes calorie harvest and adiposis through improved efficiency
of polysaccharide fermentation by the Bacteroidetes and the
Firmicutes, the primary bacterial fermenters in the gut [3].
Fermentation of polysaccharides generates SCFAs, principally
acetate, propionate, and butyrate, as well as other organic acids
and gases like H2and CO2[5]. Accumulation of H2inhibits
bacterial NADH dehydrogenases, thereby reducing the yield of
ATP. Studies in man-made bioreactors have shown that removal
of H2 by methanogens could promote obesity by improving
fermentation efficiency of dietary polysaccharides [3].
Despite the clear experimental evidence [3], however, human
studies on the associations between methanogens and obesity have
shown inconsistent results [6,7,8,9]. One small-scale human study
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with 9 study subjects also observed an association between
methanogens and extreme obesity [7]. However, in the other
studies, methanogens were increased in anorectic patients or lean
subjects [6,8,9]. To our knowledge, no study has explored
associations between gut methanogens and waist circumference,
a marker of visceral obesity, in the general population. Different
from subcutaneous fat underneath the skin, visceral fat surround-
ing vital organs is strongly linked to various obesity-related chronic
diseases [10].
On the other hand, it is unclear which specific environmental
factors determine the levels of methanogens in the human gut,
although both shared and unique environmental factors have been
suggested in a twin study as determinants of the ecology of
methanogens in the gut [11]. In the field of environmental
microbiology, methanogens have been reported to biodegrade
petroleum hydrocarbons in polluted environments [12]. There-
fore, this kind of microorganism tends to increase in places with
petroleum contamination as one way of autopurification and is
artificially used to remove petroleum-based pollutants in environ-
ments [13]. Therefore, we hypothesized that increased body
burden of petroleum-based man-made chemicals would promote
methanogens in human gut.
In relation to this hypothesis, recent epidemiological studies on
Persistent Organic Pollutants (POPs), very lipophilic man-made
chemicals produced based on petroleum, are highly relevant. For
example, environmental exposure to POPs is strongly linked to a
variety of chronic diseases and obesity in the general population
[14,15,16,17,18,19]. POPs are very slowly eliminated from the
body via feces, following two major mechanisms of biliary and
intestinal excretion [20]. Therefore, persons with a high body
burden of POPs may also have higher levels of POPs in their feces.
Another direct source of POPs in feces is POPs contained in food.
Therefore, it is reasonable that methanogens may increase to
biodegrade POPs in the feces, similar to the autopurification
commonly observed in petroleum-contaminated environments.
This study was primarily performed to explore if there are
relations of Methanobacteriales, the predominant methanogenic
archaea in the human gut [7], with body mass index or waist
circumference in Korean women. Further, we conducted a pilot
study to examine the associations between serum or stool
concentrations of organochlorine pesticides (OCPs), one subclass
of POPs, and Methanobacteriales in feces in a subsample of 16
women.
Methods
Human subjects and survey methods
We studied 83 Korean women who visited a community health
service center for a routine health checkup in Koryung County,
Korea. They did not have current gastrointestinal symptoms or a
history of chronic gastrointestinal problems. None of them lived in
the same household, and none had received any antibiotic or
probiotic agents in the 3-month period before the collection of
fecal samples. This study was conducted with the approval from
the Institutional Review Board at the Kyungpook National
University Hospital. Written informed consent was obtained from
all study subjects.
Height and body weight were measured using standard methods
in light clothes. Body mass index (BMI) was calculated as weight
divided by height squared (kg/m2). Waist circumference was
measured in the supine position midway between the lowest rib
and the iliac crest. Blood samples were obtained by venipuncture
in the morning after overnight fasting. Participants were also asked
to collect the last part of stools when they defecated. After
collection, both blood and stool samples were frozen at 270uC
until the molecular analyses were conducted. Fasting glucose,
triglycerides, and high density lipoprotein (HDL) cholesterol were
determined by enzymatic methods using ADVIA 1650 (Bayer Inc.,
USA).
Quantification of Methanobacteriales in feces
Quantitative real-time PCR (qPCR) was used to quantify
Methanobacteriales. The genomic DNA was extracted from the
frozen stool samples (wet weight 0.2 g) using the QIAamp DNA
Stool Kit (Qiagen) according to the manufacturer’s instructions.
We performed 16S rDNA-targeted qPCR with TaqMan detection
kit (Applied Biosystems). The Methanobacteriales-specific primers
and probe were MBT-NF (59-TCG CAA GAC TGA AAC TTA
AAG GAA-39), MBT-NR (59-CGG CGT TGA ATC CAA TTA
AAC-39), and MBT-N-probe (59-AGC ACC ACA ACG CGT
GGA GCC-39) [7]. Analyses were performed in a total volume of
15 ml containing the following: 7.5 ml of Universal PCR Mater
Mix (TaqMan), 1.5 ml of oligonucleotide mixture (4 mM of each
primer and 1 mM of FAM-labeled probe), and 1 ml of template
DNA. The amplification program consisted of one cycle of 95uC
for 10 min and then 40 cycles of 95uC for 15 s and 60uC for 15 s.
qPCR was performed in an ABI Prism 7500 cycler (Applied
Biosystems). Standard curves were created using serial 10-fold
dilution of the standard plasmid DNA corresponding to 26102to
261011copies/ml. The plasmid DNA standards of target
organisms were prepared from the representative 16S rDNA
clones, which were amplified from stool samples by specific
primers and then cloned into pGEM-T vector (Promega). We
calculated the copy number of the 16S rDNA as follows: grams/
plasmid molecules = (number of base pairs) * (average weight of
double stranded DNA)/Avogadro’s number and copy numbers/
template ml (q) = (grams/ml)/(grams/plasmid molecules). Finally,
copy number per gram of wet stool = q6D6(50 ml)/(0.2 g), where
D is the dilution factor, 50 ml is the elution volume in genomic
DNA extraction, and 0.2 g is the wet weight of stool used for DNA
extraction.
Measurement of OCPs
For analyses of OCPs in serum and feces, we selected 16 out of
83 subjects based on the results of Methanobacteriales analyses in
feces; 8 from women with Methanobacteriales and 8 from women
without Methanobacteriales. OCPs in serum were analyzed at the
laboratory of the School of Environmental Science and Engineer-
ing, POSTECH (Pohang, Korea) while OCPs in stool were
analyzed at the laboratory of the Department of Environmental
Marine Sciences (Ansan, Korea). Both of them used an isotope
dilution method with GC-HRMS (gas chromatography-high
resolution mass spectrometry). GC-HRMS measurements were
performed on a JMS-800D instrument (JEOL, Japan) interfaced
with a 6890N gas chromatography (Agilent Technologies, USA).
Statistical analyses
First, we compared BMI and waist circumference between
subjects with and without Methanobacteriales. Next, Pearson
correlation coefficients were presented between log transformed
count of Methanobacteriales and BMI or waist circumference.
Methanobacteriales showed a right-skewed distribution. Also, we
presented detection rates of Methanobacteriales according to tertiles
of waist circumference. Finally, we presented Pearson correlation
coefficients between log transformed concentrations of OCPs in
serum or feces with log transformed count of Methanobacteriales. All
data were analyzed using SAS version 9.1.
Pesticides, Methanogens, and Obesity
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Results
Methanobacteriales in feces were detected in 27 out of 83 study
subjects (32.5%). Among 27 women with Methanobacteriales, mean
and standard deviation of number of Methanobacteriales were
17.6645.1 (6107). Table 1 shows demographic and clinical
characteristics of study subjects depending on the presence of
Methanobacteriales. BMI or waist circumference of women with
Methanobacteriales was statistically significantly higher than those of
those without them (P=0.04 and P=0.01, respectively). There
were no significant differences in age, fasting glucose, triglyceride,
total cholesterol, and HDL-cholesterol.
The number of Methanobacteriales had correlation coefficients of
+0.23 (P=0.03) with both BMI and waist circumference, but
lower correlations with other characteristics (Table 2). Figure 1
shows the detection rate of Methanobacteriales by tertile of waist
circumference. Among women with waist circumference ,83 cm,
Methanobacteriales were detected among 18.5% while women with
waist circumference 83,87 cm and $ 88 cm showed detection
rates of 33.3% and 48.2% (P trend=0.03).
Table 3 shows absolute concentrations of OCPs in serum and
feces and their correlations among 16 women selected to balance
presence or absence of methanobacteriales. As concentrations of
OCPs in feces were measured based on dry weight while
concentrations of OCPs in serum were measured based on lipid
weight or wet weight, direct comparison of concentrations
between two specimens was difficult. However, correlation
coefficients between serum concentrations of OCPs and those of
feces were substantial, in particular with b-hexachlorocyclohexane
and OCPs belonging to the Chlordane family. In case of OCPs
belonging to the DDT family, both p,p;-DDE and p,p;-DDD
showed modest positive associations between serum and feces
while p,p’-DDT showed an inverse association between these two
specimens.
Despite the strong correlations between serum and feces,
correlation coefficients with levels of Methanobacteriales were more
strongly observed with serum than with feces concentrations of
OCPs (Table 3). Although most OCPs in serum showed positive
associations with levels of Methanobacteriales, in particular cis-
nonachlor (r=+0.53, P,0.05), oxychlordane (r=+0.46, P,0.1),
and trans-nonachlor (r=+0.43, P,0.1) showed statistically
significant or marginally significant correlations. OCPs belonging
to the DDT family also showed correlations coefficients of around
+ 0.3 to +0.4, although they did not reach statistical significance.
Figure 2 shows a scatter plot beween serum concentrations of the
mixture of organochlorine pesticides belonging to the Chlordane
family (oxychlordane, trans-Nonachlor, cis-Nonachlor, Heptachlor
epoxide) and number of Methanobacteriales among 16 women.
Discussion
In this cross-sectional study, we found that Methanobacteriales
were positively associated with BMI and waist circumference
among Korean women who had mean BMI around 25 kg/m2,
consistent with the hypothesis that Methanobacteriales promote
extraction of energy within the human gut. In addition, persons
with high Methanobacteriales in their feces had higher serum
concentrations of OCPs, suggesting that the phenomenon of
increased Methanobacteriales in an oil-contaminated environment
can also be observed in the human gut ecosystem because OCPs
Table 1. General characteristics by presence of
Methanobacteriales (n=83).
Methanobacteriales
Characteristics
Present
(n=27)
Absent
(n=56) P value
Mean 6 standard deviation
Age (years) 58.667.359.367.80.71
Body mass index (kg/m2)26.463.2 24.763.50.04
Waist circumference (cm)88.867.383.669.0 0.01
Fasting glucose (mg/dl)95.6615.4 92.7637.70.27
Triglycerides (mg/dl) 143.2665.4 140.7691.70.58
Total cholesterol (mg/dl)210.6622.7204.1637.5 0.19
HDL-cholesterol (mg/dl)43.969.6 45.668.20.29
Percentages (%)
Body mass index $25 kg/m2
66.7% 46.4%0.08
Cigarette smoker0.0% 1.8%0.48
Alcohol drinker15.4%7.4% 0.27
doi:10.1371/journal.pone.0027773.t001
Figure 1. Detection rate of Methanobacteriales by tertiles of
waist circumference (n=83).
doi:10.1371/journal.pone.0027773.g001
Table 2. Correlation coefficients between levels of
Methanobacteriales with obesity and obesity-related
metabolic variables (n=83).
Characteristics
Correlation
coefficientsP value
Age
20.080.46
Body mass index
+0.23
+0.23
+0.07
+0.04
+0.09
0.03
Waist circumference0.03
Fasting glucose0.55
Triglycerides 0.75
Total cholesterol 0.40
HDL-cholesterol
20.090.42
doi:10.1371/journal.pone.0027773.t002
Pesticides, Methanogens, and Obesity
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are
chemicals.
Even though there was a clear trend between waist circumfer-
ence and the detection rates of Methanobacteriales, the correlation
coefficient of +0.23 between the number of Methanobacteriales and
waist circumference can be interpreted as a weak association.
However, considering the predominant role of many life style-
related factors contributing to obesity, such as excess energy intake
and lack of exercise, the strength of association observed in the
current study cannot be dismissed as weak.
An experimental study reported that methanogens could
promote obesity by improving fermentation efficiency of dietary
polysaccharides [3], but, with the exception of one small scale
study [7], previous human studies did not support the experimen-
typicalhighlylipophilic man-madepetroleum-based
tal evidence [6,8,9]. In one previous human study with 6 study
subjects by Zhang et al, Methanobacteriales was detected in all 3
obese persons with mean BMI of 48.3 kg/m2, but not in 3 persons
with normal weight with mean BMI of 22.7 kg/m [7]. On the
other hand, Armougom et al compared 20 obese subjects with
mean BMI of 47.1 kg/m2, 9 patients with anorexia nervosa with
mean BMI of 12.7 kg/m2, and 20 normal control with mean BMI
of 20.1 kg/m2and reported no difference in the number of
Methanobrevibacter smithii, the predominant type of Methanobacteriales
in humans, between obese and normal weight subjects. However,
they showed that counts of M, smithii were higher among patients
with anorexia nervosa with very low BMI compared with normal
weight subjects [6]. In a followup study by the same research team,
Methanobrevibacter smithii was less frequent and significantly less
abundant in obese persons with mean BMI of 43.6 kg/m2than
controls with mean BMI of 22.1 kg/m2[8]. The depletion of
Methanobrevibacter smithii among obese healthy subjects with BMI $
30 kg/m2compared to lean subjects with BMI ,25 kg/m2also
reported by Schwiertz et al [9].
In contrast to the previous studies, which mostly compared
mean levels of methanogens between extremely obese patients and
normal weight controls, we showed positive associations using
continuous BMI or waist circumference and levels of Methano-
bacteriales using correlation coefficients in the general population.
Also, considering the distribution of BMI, our participants were
closer to control subjects of the previous studies, rather than the
extremely obese patients. Therefore, even though at the first
glance the current study seems to be consistent with the results of
Zhang et al. [7], but not with others [6,8,9], our study design is
quite different from that of either of the other studies and is the
only study to address the associations of adiposity and Methano-
bacteriales within the range that is relevant to most people. Thus,
comparison between the current study and the previous studies is
difficult.
The adult intestine contains 10 trillion to 100 trillion microbial
cells and is dominated by members of just two divisions of bacteria,
the Bacteroidetes and the Firmicutes. A role for the intestinal
microbiota in harvesting energy from food [21] and regulating
body fat storage [22] was first proposed in rodents. Germ-free
Table 3. Correlation coefficients among serum concentrations of organochlorine pesticides (OCPs), stool concentrations of OCPs,
and levels of Methanobacteriales (n=16{{).
Mean ±standard deviationCorrelation coefficients
Characteristics
OCPs in serum
(ng/g lipid)
OCPs in feces{
(pg/g){
OCPs in serum
vs. OCPs in feces
OCPs in serum vs.
Methanobacteriales
OCPs in feces vs.
Methanobacteriales
p,p’-DDE114654.314616676
+0.42
+0.24
+0.33
+0.40
+0.41
+0.46*
+0.43*
+0.53**
+0.27
+0.33
20.02
p,p’-DDD 2.161.1 235675.1
20.44*
p,p’-DDT 10.665.92046116
20.48*
20.42
Oxychlordane 4.161.6 49.0636.4
+0.62**
+0.51**
+0.49*
+0.19
+0.15
trans-Nonachlor 6.762.9101651.2
cis-Nonachlor 1.260.629.0614.6
20.06
Heptachlor epoxide{
7.163.2 ---
b-Hexachlorocyclohexane 35.5632.1 217661647
+0.89***
+0.19
+0.23
Hexachlorobenzene6.763.4231662928
20.07
20.13
*:P,0.1, **:P,0.05, ***:P,0.01.
{Heptachlor epoxide was measured only in serum.
{Concentrations of OCPs in feces were presented based on dry weight.
{{8 selected at random among the women with no detectable methanobacteriales, 8 selected at random among the women with detectable methanobacteriales.
doi:10.1371/journal.pone.0027773.t003
Figure 2. Scatter plot beween serum concentrations of mixture
of organochlorine pesticides belonging to Chlordane (oxy-
chlordane, trans-Nonachlor, cis-Nonachlor, Heptachlor epox-
ide) and number of Methanobacteriales (n=16). When one
subjects with the highest number of Methanobacteriales (38.7 6 107)
was excluded, the correlation coeffficent was 0.45 (p=0.09).
doi:10.1371/journal.pone.0027773.g002
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mice colonized by microbiota increase their body fat and develop
insulin resistance despite a 30% decrease in food intake. These
changes were associated with a dysbiosis in obese mice: an
increased representation of the Firmicutes phylum and a reduced
representation of the Bacteroidetes phylum [23]. However, modifi-
cation of the Fermicutes-to-Bacteroidetes ratio is not consistently
observed in human [24,25].
In this study, we focused on methanogenic archaea in the gut,
rather than the distribution of various microbiota, because we
hypothesized that there was a link among POPs, methanogenic
archaea, and obesity. In this study, Methanobacteriales were detected
in 32.5% of study subjects. On the other hand, Methanobacteriales
were not detected in control subjects in Zhang et al. ’s study [7]
while M. smithii were detected in 63.3% ,88.9% in control
subjects in other studies [6,8,9]. As all of these studies quantify the
counts of Methanobacteriales or M. smithii using RT-PCR on the basis
from 16S rRNA sequencing, we do not have any explanation for
why there were the different detection rates among the studies. On
the other hand, when the methane breathing test was used as an
indirect marker of methanogens in gut, it was estimated that
methanogenic archaea were present at high levels in 50–85% of
humans [26,27,28].
One important finding from the current study is the relation
between Methanobacteriales in feces and serum concentrations of
OCPs. To our best knowledge, there has been no previous study
which tested this hypothesis. Based on both findings from
environmental microbiology which shows the role of methanogens
to clear petroleum-contaminated environment by biodegrading
petroleum [11,12] and recent epidemiological findings on POPs
[14,15,16,17,18,19], we first hypothesized that body burden of
POPs would determine the levels of Methanobacteriales in feces. To
mark POPs body burden, we measured OCPs in 16 subjects
because OCPs were most strongly associated with obesity-related
metabolic dysfunctions in previous studies [14,15,19]. Among
OCPs, chemicals belonging to the Chlordane family showed the
strongest associations with the levels of Methanobacteriale in feces.
Interestingly, the Chlordane family was the one which was most
strongly and consistently associated with type 2 diabetes in
previous epidemiological studies [15,17]. If OCPs in the gut are
really important in determining the levels of methanogens in the
gut, the increased levels of Methanobrevibacter smithii among patients
with anorexia nervosa [6] may be explained because it is reported
that weight loss increases serum concentrations of OCPs
[29,30,31]. Increased serum concentrations of OCPs due to severe
weight loss may lead to increased OCP levels and increased
methanogens in the gut because concentrations of POPs in feces
were positively associated with serum concentrations of POPs as
we observed in the current study.
As markers of OCPs in the gut, we measured OCPs in both
serum and feces. As far as we know, there has been no previous
human study which compared concentrations of OCPs in between
serum and feces. In this study, we observed that OCP levels in
these two specimens were highly correlated, but only OCPs levels
in serum showed significant positive associations with the levels of
Methanobacteriales in feces. Intuitively, if OCPs in the gut truly
increase the levels of Methanobacteriales in feces, OCP levels in feces,
rather than OCP levels in serum, should have shown stronger
associations. However, the results were different. This could be
related to poor validity and reliability of OCP levels in feces. To
collect feces samples, the subjects were asked to collect the last part
of stools when they defecated. However, we cannot be sure how
well OCP levels in the collected samples represent OCP levels in
gut. OCP levels in feces can differ substantially depending on the
frequency and quality of defecation as well as recent diet of study
subjects. Also, OCPs would not be homogenously distributed in
the total mass of feces. As we can collect only a small part of feces
to measure OCPs, OCP levels in collected feces likely do not
represent the OCP levels in the whole feces. On the contrary,
serum concentrations of OCPs are considered as a marker of total
body burden of OCPs [32]. As OCPs in serum keep being
excreted through bile acid and large intestine from body [20],
OCP levels in serum may better represent the general pattern of
OPCs in gut.
There are several study limitations. First, we did not study other
diverse bacteria. However, we had a prior hypothesis about
relations the body burden of OCPs, Methanobacteriales in feces, and
obesity. Second, as only women were included in the current
study, we cannot say that the current finding can be applied to
men. Third, OCPs were measured only in a subsample of study
subjects for cost reasons. Even though we did see statistically
meaningful associations, the interpretation should be cautious
because the result was based on only 16 study subjects.
In this study, we tested the hypothesis which links the body
burden of POPs, Methanobacteriales, and obesity. The results are
consistent with our hypotheses that body burden of OCPs may
determine the levels of Methanobacteriales in the human gut, and that
this process can finally lead to increased body weight and waist
circumference. Cautious interpretation should be made about
causality of these associations, given the current cross-sectional
study design. Also, the association between body burden of OCPs
and Methanobacteriales in the human gut needs to be confirmed in
future studies with more study subjects. Future studies on the role
of POPs in the control of methanogens in human gut may be
important to understand a more fundamental role of POPs in
developing the change of gut microbiota and obesity.
Author Contributions
Conceived and designed the experiments: D-HL I-KL. Performed the
experiments: H-SL J-CL H-BM Y-SC. Analyzed the data: H-SL DRJ.
Contributed reagents/materials/analysis tools: J-CL H-BM Y-SC. Wrote
the paper: H-SL J-CL I-KL DRJ D-HL.
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