The Effects of Intact Cereal Grain Fibers, Including Wheat Bran on the Gut Microbiota Composition of Healthy Adults: A Systematic Review

Abstract

The human microbiota is increasingly recognized as a major factor influencing health and well-being, with potential benefits as diverse as improved immunity, reduced risk of obesity, Type 2 diabetes, and improved cognition and mood. Bacteria inhabiting the gut are dependent on the provision of fermentable dietary substrates making diet a major factor driving the composition of the human gut microbiota. Dietary fiber may modify microbiota abundance, diversity, and metabolism including short-chain fatty acid production. The majority of research to date has explored isolated fibers, and the influence of habitual fiber consumption is less well-established. The aim of the current article was to systematically review evidence from human intervention studies for the effects of intact cereal fibers, and their active sub-fractions, on gut microbiota composition in healthy adults. Studies published in the past 20 years were identified through the PubMed and Cochrane electronic databases. Inclusion criteria were: healthy adult participants (>18 years), inclusion of at least one intact cereal fiber, or its sub-fraction, and measurement of fecal microbiota related outcomes. As every individual has a unique microbiota many trials utilized a cross-over design where individuals acted as their own control. Outcome measures included change to the microbiota, species diversity, or species abundance, or metabolic indicators of microbiota fermentation such as short chain fatty acids or fecal nitrogen. Two hundred and twenty three publications were identified and 40 included in the final review. In discussing the findings, particular attention has been paid to the effects of wheat fiber, bran, and arabinoxylans (AXOS) as this is the dominant source of fiber in many Western countries. Thirty-nine of the forty-two studies demonstrated an increase in microbiota diversity and/or abundance following intact cereal fiber consumption, with effects apparent from 24 h to 52 weeks. Increases in wheat fiber as low as 6–8 g were sufficient to generate significant effects. Study duration ranged from 1 day to 12 weeks, with a single study over 1 year, and exploration of the stability of the microbiota following long-term dietary change is required. Increasing cereal fiber consumption should be encouraged for overall good health and for gut microbiota diversity.

Full text

SYSTEMATIC REVIEW
published: 29 March 2019
doi: 10.3389/fnut.2019.00033
Frontiers in Nutrition | www.frontiersin.org 1March 2019 | Volume 6 | Article 33
Edited by:
Jean-Pierre Guyot,
Institut de Recherche pour le
Développement (IRD), France
Reviewed by:
Philippe Gérard,
Institut National de la Recherche
Agronomique (INRA), France
Williams Turpin,
University of Toronto, Canada
*Correspondence:
Angie Jefferson
angie@angiejefferson.co.uk
Specialty section:
This article was submitted to
Nutrition and Microbes,
a section of the journal
Frontiers in Nutrition
Received: 05 December 2018
Accepted: 07 March 2019
Published: 29 March 2019
Citation:
Jefferson A and Adolphus K (2019)
The Effects of Intact Cereal Grain
Fibers, Including Wheat Bran on the
Gut Microbiota Composition of
Healthy Adults: A Systematic Review.
Front. Nutr. 6:33.
doi: 10.3389/fnut.2019.00033
The Effects of Intact Cereal Grain
Fibers, Including Wheat Bran on the
Gut Microbiota Composition of
Healthy Adults: A Systematic Review
Angie Jefferson 1
*and Katie Adolphus 2
1Independent Researcher, Bracknell, United Kingdom, 2The Kellogg Company, Manchester, United Kingdom
The human microbiota is increasingly recognized as a major factor influencing health
and well-being, with potential benefits as diverse as improved immunity, reduced risk
of obesity, Type 2 diabetes, and improved cognition and mood. Bacteria inhabiting the
gut are dependent on the provision of fermentable dietary substrates making diet a
major factor driving the composition of the human gut microbiota. Dietary fiber may
modify microbiota abundance, diversity, and metabolism including short-chain fatty
acid production. The majority of research to date has explored isolated fibers, and
the influence of habitual fiber consumption is less well-established. The aim of the
current article was to systematically review evidence from human intervention studies
for the effects of intact cereal fibers, and their active sub-fractions, on gut microbiota
composition in healthy adults. Studies published in the past 20 years were identified
through the PubMed and Cochrane electronic databases. Inclusion criteria were: healthy
adult participants (>18 years), inclusion of at least one intact cereal fiber, or its
sub-fraction, and measurement of fecal microbiota related outcomes. As every individual
has a unique microbiota many trials utilized a cross-over design where individuals acted
as their own control. Outcome measures included change to the microbiota, species
diversity, or species abundance, or metabolic indicators of microbiota fermentation such
as short chain fatty acids or fecal nitrogen. Two hundred and twenty three publications
were identified and 40 included in the final review. In discussing the findings, particular
attention has been paid to the effects of wheat fiber, bran, and arabinoxylans (AXOS) as
this is the dominant source of fiber in many Western countries. Thirty-nine of the forty-two
studies demonstrated an increase in microbiota diversity and/or abundance following
intact cereal fiber consumption, with effects apparent from 24 h to 52 weeks. Increases in
wheat fiber as low as 6–8 g were sufficient to generate significant effects. Study duration
ranged from 1 day to 12 weeks, with a single study over 1 year, and exploration of
the stability of the microbiota following long-term dietary change is required. Increasing
cereal fiber consumption should be encouraged for overall good health and for gut
microbiota diversity.
Keywords: cereal fiber, dietary fiber, gut microbiome, gut microbiota, prebiotic, wheat fiber, wheat bran,
systematic review

Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
INTRODUCTION
Increased consumption of wholegrains is recommended across
the world due to their association with a reduced risk of
cardiovascular disease, overweight/obesity, Type 2 diabetes, and
cancer (1,2). One of the proposed mechanisms behind these
protective effects is the fermentation of prebiotic cereal dietary
fibers by the colonic microbiota (3). The key metabolic outputs of
the gut microbiota are short chain fatty acids (SCFA): principally
acetate; propionate, and butyrate; each of which effects host
health. Acetate is absorbed and metabolized by the brain, muscle,
and body tissues, propionate is cleared by the liver and may
lower hepatic production of cholesterol, and butyrate provides an
energy source to the cells lining the colon and may help to protect
against colon disorders (3,4). In addition, these SCFA also exert
anti-inflammatory effects, and are thought to play a role in the
modulation of glucose and lipid metabolism (3,4).
The composition of the gut microbiota has been shown
to respond to dietary change, determined by competition for
substrates, and by tolerance of the gut conditions (3). However,
the majority of research into the beneficial effects of prebiotic
fibers has focused on isolated prebiotic fibers, and research into
the impact of intact cereal fibers is less well developed. This may
be due in part to the complexity of fiber molecules resulting in a
high degree of variability in fermentation of different fibers by the
colonic microbiota, and also the high variability in baseline gut
microbiota between individual participants, adding complexity
into any research into intact cereals fibers (5). A summary of the
key bacterial phylotypes and the typical variation of phylotype
proportions reported in the microbiota of Western populations
is provided in Table 1. Evidence is emerging supporting the role
of habitual diet in modulating the structure of both the gut
microbial composition and also its metabolism (3,6).
For the purposes of this review, intact cereal fiber included
both the soluble and insoluble non-digestible carbohydrates
found in cereal grains. To our knowledge, previous systematic
reviews (such as 7–9) in this area have included a range of
prebiotic fibers largely from supplemented, isolated fiber types
(7,8), or have explored specific conditions such as IBS (9), and
there has not been a systematic review exploring the impact of
intact cereal fibers consumed in everyday foods (such as breakfast
cereals and breads) on the gut microbiota in healthy participants.
Therefore, the aim of the current review was to systematically
review the evidence from human intervention studies for the
effects of intact cereal fibers and their active sub-fractions on gut
microbiota composition in healthy adults. A subsidiary aim of the
review was to systematically review the effects of wheat bran fiber
on gut microbiota composition in healthy adults, as wheat bran
fiber is the largest contributor to cereal fiber intake in Western
societies. Wheat bran contains high levels of the hemicellulose
arabinoxylan, which can be utilized by the bacteria inhabiting the
microbiota. As the biggest component of fiber across the Western
world, studies examining the role of wheat arabinoxylan and
arabinoxylan-oligosaccharides (AXOS) have also been included.
As this review has not been carried out previously, studies
published over the past 20 years were included. The effects
of intact cereal fibers were evaluated by change in microbiota
abundance, diversity, increase in specific bacterial species, plus
indicators of microbiota fermentation activity such as short chain
fatty acid production and fecal nitrogen.
The current systematic review increases understanding of
the impact of consumption of intact cereals fibers on the gut
microbiota and consequent health.
METHODS
Search Strategy and Search Terms
The review was reported using Preferred Reporting Items for
Systematic Reviews and Meta-Analyses (PRISMA) guidelines.
Studies were identified by searching the PubMed and Cochrane
electronic database from January 1998 to June 2018. A
combination of search terms relevant to this review were
used including “microbiome,” “microbiota,” or “prebiotic” in
combination with “grain fiber or fiber” “digestive health,”
“wheat,” “oat,” “barley,” “rye,” or “rice.” The active sub-fractions of
wheat: “arabinoxylans” and “AXOS,” were also included into the
search. The reference lists of the studies identified were examined
individually to supplement the electronic search. Additional
unpublished data was provided by one author to supplement
published material (Neacsu M). The process for selecting studies
for inclusion in this review is detailed in Figure 1. A total of 42
studies were included in the final review.
Inclusion and Exclusion Criteria
Publications written in the English language were selected for
review. Only original research studies reporting the influence of
manipulating intake of an intact cereal fiber via a food based
intervention or one of it sub-fractions on the gut microbiota
or bacterial fermentation metabolites in human adult (>18
years) participants were included. All experimental designs were
eligible for inclusion. Studies that included only participants
with a chronic health condition (e.g., Irritable Bowel Syndrome,
Ulcerative Colitis etc.) were excluded from this review. Studies
that included overweight and obese participants who were
otherwise healthy and without abnormal clinical parameters (e.g.,
elevated blood pressure) were included. Additional detail on
potential confounders (such as use of antibiotics, pre/probiotics,
and laxative use), was extracted wherever possible, however
studies failing to provide this specific detail (n=5) were not
excluded in order to capture the microbiota response of free
living individuals in the community. There was no restriction
on length of fiber manipulation, or mechanism by which it was
administered. Studies that included any outcome of objectively
measured gut microbiota composition were included. Studies
that used the following outcomes measures were included in the
review: change to the microbiota in terms of total populations
or individual species, or measurement of the metabolites arising
from bacterial fermentation such as the short chain fatty acids
(acetate, butyrate, and propionate), breath hydrogen or fecal
nitrogen. Where significant changes to other parameters were
reported (e.g., blood lipids, bowel habit or glucose, and insulin
response) these have also been noted in Table 2.
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 1 | Major Bacterial Groups in the Human Gut Microbiota and the Main Fermentation Products.
Major phyla Important families in gut
microbiota
Main fermentation substrates &
by-products
Additional notes:
Firmicutes
Spore forming bacteria—include helpful
species e.g., lactobacillus and pathogenic e.g.,
clostridium
typically form 64–78% western
human microbiota
Lactobacillaceae, Clostridiaceae,
Enterococcus, Lachnospiraceae,
Roseburia, Dialister, Eubacteriaceae,
Ruminococcacea, Anaerostipes,
Carbohydrates—producing
SCFA’s-acetate, formate, lactate,
butyrate, succinate, and propionate
Proteins—producing BCFAs, indoles,
sulfides, phenols, amines, NH3, H2,
CO2, CH4
Diverse group of both beneficial and
pathogenic bacteria
Contain a number of bacterial species
adapted for utilizing fiber as a
substrate and producing butyrate
Wide range of positive benefits
including reduce risk of obesity and
Type 2 diabetes, improved glucose
tolerance, protection against
colon cancer
Bacteroidetes
Opportunistic pathogens
Typically form 13–28% of human gut
microbiota in western populations
Bacteroides, Prevotellae, Barnesiella, Able to metabolize both
carbohydrates and proteins.
Important fermenters of dietary fiber
Bacteroides dominant in western
populations with higher protein and
fat intakes. Prevotellae dominate in
those consuming high levels of fiber.
Actinobacteria
Producers of metabolites & antibacterial/virals
Typically form 3–5% of human gut microbiota in
western populations
Bifidobacteriaceae,
Corynebacteriaceae, Atopobium,
Eggerthella, Collinsella
Able to breakdown simple sugars,
cellulose and hemicellulose Producing
lactate, acetate, and formate from
carbohydrates
Wide range activities including
inhibiting growth pathogens,
improvement gut mucosal barrier and
vitamin production
Proteobacteria
Gram negative bacteria including a wide range
of pathogens
Typically no more than 3% of human gut
microbiota in western populations
Enterobacteraceae,
Pseudomonadaceae,
Helicobacteraceae
Able to ferment carbohydrates and
proteins producing lactate, acetate,
succinate, and formate from
carbohydrates, sulfide from sulfate,
H2S, mercaptans from protein
Include a wide range of pathogens
FIGURE 1 | Process of selecting included human studies. Adapted from Moher et al. (10).
Data Extraction
For the studies selected for inclusion, general study
characteristics (author, year of publication, study design,
and length of study), characteristics of study population (age,
gender, BMI), experimental manipulation (fiber type, amount,
and length of manipulation), study outcomes (e.g., change
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Summary of human studies exploring the impact of intact cereal fibers on the gut microbiota.
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
WHEAT/WHEAT BRAN
Costabile et al.
(11)
UK
31 adults (15 M & 16 F)
Av age 32 y
BMI 20–30
Prebiotics, probiotics,
high bran or whole grain
breakfast cereals, GI
drugs, antibiotics (6 m),
laxatives, substance or
alcohol abuse, major
illness, GI disease
Wholegrain wheat
breakfast cereal vs.
Wheat bran breakfast
cereal
Randomized
crossover
FISH 3 weeks
Washout 2 weeks
➀48 g WG wheat
breakfast cereal (5.7 g
fiber/serve)
➁48 g wheat bran
breakfast cereal
(13 g fiber/serve)
➀&➁sig ↑in bifidobacteria,
lactobacilli, enteroccoci &
aptobium
However, ↑’s were sig greater
with WG compared to WB
cereal
Sig ↑clostridium with ➁
➀&➁sig↑plasma ferulic
acid with both cereals but
greater increase with WB
WB sig ↑stool frequency
WG sig improved stool form
This study demonstrates a
differential impact of prebiotic
action for WB and WG cereal,
with a sig higher increase in
bacterial no’s effect for WG
within the measured time frame
Vitaglioni et al.
(12)
Italy
68 adults
overweight/obese
Av Age 38.5 y
Av BMI 30
Pregnancy/lactation,
medication (3 m), chronic
illness, high fiber diet,
probiotics,
vitamins/minerals
supplements, or
complementary, and
alternative medicines;
fruit and vegetables >3
servings/d, >500 min
exercise/wk
Whole grain wheat
(Shredded Wheat)
vs.
refined wheat
Randomized
parallel
16S rRNA gene
sequencing
8 weeks ➀70 g/d (3 biscuits/d
−8 g fiber)
➁70 g refined wheat
crackers and toast
(2.2 g fiber)
➀Sig↑Bacteroidetes and
Firmicutes & sig ↓Clostridium
➀Sig 4-fold ↑serum
dihydroferulic acid (DHFA)
and 2-fold ↑fecal ferulic
acid (FA) with WG
WG wheat consumption
significantly ↑excreted FA and
circulating DHFA. Bacterial
communities influenced fecal
FA & were modified by WG
wheat consumption.
Freeland et al.
(13)
Canada
40 adults—pre-diabetic
(↑insulin)
Av age 29 y
Av BMI 26
Antibiotics (3 m)
GI, diabetes,
hyperlipidaemia or a
high-fiber diet.
Wheat bran fiber Randomized
parallel
n/a 1 year ➀60 g All Bran Original
(24 g fiber)
➁49 g Rice Krispies
(0.5 g fiber)
Not measured ➀Sig ↑plasma butyrate,
acetate & GLP-1 in
participants between 9 and
12 months.
Sustained ↑in wheat fiber
intake ↑plasma butyrate &
GLP-1 concnin
hyperinsulinaemic participants,
but it takes 9–12 months for
these changes to occur.
Neacsu et al.
(14)
UK
8 healthy adults
Age 18–55 y
BMI 18–30
Prescribed medication,
use of nutritional
supplements, smoking,
antibiotics (3 M)
Wheat Bran breakfast
cereal
Non-randomized
acute
n/a Single test meal
2 week washout
➀40 g All Bran Original
(11 g fiber)
➁120 g All Bran original
(33 g fiber)
Not measured Sig ↑plasma, urine and fecal
SCFA & butyrate from ➀&➁
Sig ↑plasma ferulic acid by
5 h
No sig differences
between treatments
Significant increase in fecal
butyrate after consumption of a
40 g bowl All Bran suggest that
regular consumption of a
wheat bran breakfast cereal
will help support a healthy gut
environment.
Deroover et al.
(15)
Belgium
10 adults
Age 18–65 y
BMI 18–27
Pregnancy/lactation, GI
disease, Anemia,
Antibiotics, prebiotics,
and probiotics (1 M)
Wheat bran effect Randomized
crossover
n/a 1 day each
Washout 4 weeks
10 g labeled inulin plus
➀20 g wheat bran
➁20 g wheat bran ↓
particle size
➂20 g pericarp enriched
wheat bran
Not measured Labeled fermentation
markers appeared in breath
& plasma around 3 h 45 min
after consumption &
continued for 8 h. No effect
of bran particle size
Fermentation of a readily
fermentable substrate
increased plasma SCFA for
about 8 h, suggesting that a
sustained increase in plasma
SCFA concentrations can be
achieved when a moderate
dose of fermentable
carbohydrate is administered
3x per day
McIntosh et al.
(16)
Australia
28 adult males
Age 40–65y
Regular use of drug
therapy, medication, or
supplements that may
interfere with bowel
function, major illness
High fiber intake from
wholegrain wheat vs.
wholegrain rye—bread,
crispbreads, and
breakfast cereals
Randomized
crossover
n/a 4 weeks each
Washout—length
not stated
➀21 g wheat fiber from
cereal foods
➁21 g rye fiber from
cereal foods
➂6 g fiber as refined
cereal foods
Not measured Sig ↑fecal weight with ➀&
➁& small sig ↓fecal pH.
Sig ↑Butyrate with ➁
Sig ↑propionate with ➀
Sig ↓insulin & glucose
response to breakfast meal
with both ➀&➁
Both high-fiber rye and wheat
foods were equally effective in
improving measures of bowel
health.
Rye foods ➁more effective at
↑plasma enterolactone and
fecal butyrate
(Continued)
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Continued
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
BARLEY
Martinez et al.
(17)
USA
28 adults (11 M and 17 F)
Antibiotics (3 M) GI
disorder,
antihypertensives, lipid
lowering or other regular
drug therapy
Barley
vs. brown rice or
combination of the two
Randomized cross
over
16 S rRNA gene
sequencing
Joint Genome
Institute database
used to identify
large-bowel
associated
bacteria with
b-glucanase
encoding activity.
4 weeks each
Washout 2 weeks
➀60 g whole grain barley
flakes (18.7 g fiber)
➁60 g brown rice flakes
(11.5 g fiber) ➂60 g equal
mix of the two (30 g fiber)
All ↑bacterial diversity with Sig
↑Firmicutes (Roseburia,
Dialister, Eubacterium) &
actinobacteria (Bifidobacteria)
& Sig ↓Bacteroidetes
No effect on SCFA
Substantial individual
variation in response
Sig improvement in
inflammatory responses &
glycaemia with WGB &
BR combined
Short-term intake of whole
grains induced compositional
alterations of the gut
microbiota that coincided with
improvements in physiological
measures
related to metabolic
dysfunctions in humans
Nilsson et al. (4) Sweden
20 adults (10 M and 10 F)
Age 19–30 y
Av BMI 22.1
Not reported Barley
vs.
refined wheat
Randomized
crossover
Washout 1 week
n/a 1 evening test meal
followed by
standard white
wheat bread
breakfast
➀WWB +Barley Dietary
Fiber (BDF) (9.8 g fiber)
➁Spaghetti +BDF (9.8 g
fiber)
➂Spaghetti +BDF
(19.6 g fiber)
➃Spaghetti +oat DF
(9.8 g fiber)
➄Barley porridge
(8.5 g fiber)
Not measured Sig higher breath H2after
➁,➂&➃
➂with double BDF
produced sig higher breath
H2cp to ➄or ➁
Plasma SCFA sig higher
after ➀➁➂➃ compared to ➄
barley porridge
Plasm propionate & butyrate
-ve related to glucose response
suggesting that SCFA derived
from colonic fermentation are
likely to be involved in
modulating glucose response.
Nilsson et al.
(18)
Sweden
15 adults (10 M and 5 F)
Av age 25.6 y
Av BMI 22.5
Antibiotic or probiotic
use (2 W)
Barley
vs.
white bread
Randomized
crossover
Washout 1 week
n/a 8 individual meals 8 evening test meals with
kernel based barley
breads providing varying
amounts of dietary fiber
(13.7–30 g fiber)
Not measured Sig ↑plasma butyrate
-measured following
morning with High amylose
barley and high ß-glucan
barley
Sig reduction in blood
glucose response to test
breakfast between 28
and 36%
The results of this study show
that it is possible to increase
the colonic production of SCFA
in a semi-acute perspective
(i.e., from an evening meal to
the following morning) by
choice of cereal foods rich in
barley DF and RS.
Nilsson et al.
(19)
Sweden
20 adults (3 M and 17 F)
Av age 64 y
Av BMI 23.6
Non-smoker, no
metabolic disorder or
illness
Antibiotics (2 w) or
probiotics (2 w)
Barley bread
vs.
white wheat bread
Randomized
crossover
n/a 3 days
Washout 2 weeks
➀233 g white wheat
bread (9.7 g fiber)
➁338 g barley bread
(37.8 g fiber)
Not measured ➁sig ↑gut hormones, sig ↓
glucose & insulin response
to test breakfast
➁sig ↑metabolic markers
of microbiota
activity—breath hydrogen,
plasma SCFA, and acetate
Intake of barley bread for 3 d
markedly increased gut
fermentation activity
suggesting that the metabolic
benefits are related to gut
fermentation of the DF fraction
in Barley bread
RYE
Graston et al.
(20)
Finland
17 adults (8 M and 9 F)
Av age 42
Av BMI 25.3
Not reported Rye bread
vs. white wheat bread
providing 20% energy
intakes
Randomized
crossover
n/a 4 weeks
Washout 4 weeks
➀Rye bread (5.5–6.5
servings, 17.4–22.2 g
fiber)
➁white wheat bread
(4.5–5.5 servings,
3.9–5.1 g fiber)
Not measured Sig ↑fecal weight & sig ↓
transit time on Rye bread
No sig diff in total fecal SCFA
Butyrate in feces higher in
men on Rye bread phase
Consumption of rye bread in
normal amounts improves
bowel function. Effects on
bacterial activity need
evaluation in larger study
population
Vuholm et al.
(21)
Denmark
70 adults (32 M and 38 F)
Av age 51
Av BMI 27.8
Smoking, GI disorders,
diabetes or CVD,
pregnancy or lactation,
antibiotics (3 M) pre- or
probiotics (1 M)
Rye wholegrains
or
wheat wholegrains
vs. refined grain control
Randomized
parallel
16S rRNA gene
sequencing using
The Greengenes
database for
reference
6 weeks ➀Rye (124 g whole
grains)
➁Wheat (145 g whole
grains)
➂Refined grains
(5g wholegrains)
No effect Fecal Butyrtae sig ↑with
both ➀&➁
Regular consumption whole
grain rye or wholegrain wheat
affected fecal butyrate & GI
symptoms in overweight adults
and can be included in the diet
equally to maintain gut health
Lee et al. (22)
Sweden
21 adults
Av. age 38.6 y
Av BMI 24.9
Diabetes,
hyperlipidaemia, thyroid
or metabolic disease,
eating disorders,
pregnancy, lactation,
allergies, smoking
Wholegrain rye porridge
+inulin/wheat gluten
Randomized
crossover
n/a Single test meal ➀40 g rye (7.1g fiber)
➁55 g rye (9.7g fiber)
➂40 g rye +9.3 g
inulin/gluten (15.5 g fiber)
➃40 g rye +6.6 g
inulin/gluten (12.6 g fiber)
➄40 g rye +3.9 g
inulin/gluten (10.4 g fiber)
Not measured Sig higher breath H2with
➁➂➃ compared to wheat
bread or ➄
Whole grain rye suppressed
hunger compared to wheat
bread but there were no
additional effect from adding
inulin or gluten. Large dose
dependent ↑breath H2in
response to fiber
(Continued)
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Continued
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
RICE
Nemoto et al.
(23)
Japan
36 adults (14 M, 22 F)
Age 22–67 years
Food allergy, serious
illness, antibiotics or
agent known to influence
bowel condition
FBRA-Fermented Brown
Rice and rice bran
Randomized
crossover
16S rRNA gene
sequencing
2 weeks each
Washout 12 weeks
➀21 g FBRA (5 g
fiber)−7 g after each m eal
➁21 g control (0.4 g fiber)
Change to total bacterial no’s
not sig diff between group ➀&
➁or after any test period.
SCFA production also failed to
show any sig differences.
No significant effects. In vitro
testing from 6 participants
showed ↑bifidobacteria & ↑
total SCFA, lactate & acetate
with FBRA
FBRA increased production of
SCFA and bifidobacteria in
vitro, however it remains
unclear as to whether this has
prebiotic effects in the intestinal
environment.
Sheflin et al. (24) USA
7 adults
Av age 42 y
BMI 22–29
No history food allergy,
no cholestertol lowering
medication of NSAID’s,
pregnancy or lactation,
smoking, antibiotics
(3 M), probiotics (3 M)
30 g heat stabilized rice
bran
Randomized
parallel
16S rRNA gene
sequencing
28 days ➀1 study meal & 1 study
snack daily (30 g rice bran)
Sig ↑bifidobacteria,
ruminococcus species and 6
others
Sig ↑branched chain fatty
acids & butyrate
This pilot study supports that
consumption of 30 g rice bran
can positively affect the gut
microbiota & its metabolites
OAT
Connolly et al.
(25)
UK
30 adults (11 M, 19 F)
mild
hyperglycaemia
or
mild hypercholesterolaemia
Av age. 42 y
Av BMI 26.4
Pregnancy/lactation,
food allergy, antibiotics
(6 w), chronic illness, lipid
lowering drugs, GI
disorder, Drugs affecting
GI, substance misuse,
alcoholism
Whole grain Oat (WGO)
Granola
Randomized
crossover
16S rRNA gene
sequencing &
FISH
6 weeks each
Washout 4 weeks
➀45 g of WGO granola
(2.8 g fiber, 1.3 g
ß-glucan)
➁45 g non-whole grain
(NWG) breakfast cereal
(flaked corn,
1.3 g fiber/serve)
➀Sig ↑bifidobacterial,
lactobacilli & total bacterial
count
➁Sig ↓bifidobacteria & total
bacterial count
No Sig effect fecal SCFA
➀Sig ↓total chol, near sig ↓
fasting glucose
➁Sig ↑total & LDL chol
Dietary WGO ingestion had an
appreciable
Impact on the composition of
the human gut microbiota, and
significantly reduces plasma
TC & LDL-C
Valeur et al. (26) Norway
10 adults
22–49 y
Av BMI 23
Pregnancy, Chronic
illness
Gi disease
Antibiotics (1 M)
Oatmeal porridge Non-randomized
single arm
n/a 8 days 60 g oatmeal (8·5g fiber,
including 4·7 g β-glucans)
Not measured
No sig effect of fecal SCFA
Sig ↓fecal levels of
β-galactosidase & urease
suggesting impact on
microbial activity
Ingestion of oatmeal porridge
daily for 1 week
↑some metabolic markers of
increased microbiota activity
however colonic fermentation
capacity & fecal SCFA
were unaltered.
MAIZE
Carvalho-Wells,
(27)
UK
32 adults (11 M, 21 F)
Av. age 32 y
Av BMI 23.3
Pregnancy/lactation,
antibiotics (6 m), GI
drugs or laxatives,
anemia, hyperlipidaemia
Whole grain maize cereal Randomized
crossover
16S rRNA gene
sequencing
3 weeks each
Washout 3 weeks
➀48 g WG maize
breakfast cereal (14.2 g
fiber)
➁48 g maize cereal
(0.8 g fiber)
➀Sig ↑bifidobacteria and
non-sig ↑lactobacilli &
Atopobium levels.
➁Non-sig changes to
bifidobacteria, lactobacilli, and
Atopobium levels
Treatment effects not
sustained following wash out
period. No sig changes to
fecal SCFA, bowel habit
data, fasted lipids/glucose,
and anthropometric
measures
Present study showed a
prebiotic effect from a WG
maize cereal, which resulted in
a beneficial shift in the fecal
microbiota
MIXED WHOLE GRAIN DIETS (PREDOMINATELY WHEAT)
Walker et al.
(28)
UK
14 obese males
Av age 54 y
Av BMI 39.4
No GI disease
Antibiotics (6 M)
No weight loss (4 M)
Resistant Starch vs. NSP
(wheat bran) or High
protein weight loss diet
Randomized
crossover
16S rRNA gene
sequencing and
qPCR
3 weeks
Washout not stated
➀Maintenance (27.7g
NSP, 5g RS)
➁RS III (25.5g RS, 16g
NSP)
➂NSP (41.9g NSP, 2g
RS)
➃HP WL (25g NSP,
2g RS)
➁Sig ↑ruminococcus &
eubacterium
➂No major change in fecal
microbiota
➃sig ↓eubacterium
& collinsella
Fermentation metabolites
not measured.
RS almost totally digested
(96%) in 12 of 14
participants
Soluble NSP 90% digested
Insoluble NSP 66% digested
Increased intake of RS gave
substantial increases in
species in colonic microbiota.
However the lack of change
resulting from NSP may be due
to smaller increase in NSP
intake (1.5x) compared to a
4.8-fold increase in RS intake
on test diets.
Salonen et al.
(29)
UK
14 males metabolic
syndrome
Av age 53 y
Av BMI 39.4
GI disease
Antibiotics (6 m)
Resistant starch vs.
wheat bran
Randomized
crossover
16S rRNA gene
sequencing &
qPCR
3 weeks each
Washout 1 week
➀Control diet 27.7 g fiber
➁High RS
➂High NSP (wheat bran)
➃Low carb
weight control
Diversity of microbiota was sig
lower ➀&➁but ↑sig with ➂
(wheat bran)
Firmicutes fell ∼3-fold on ➃
Changes to bacterial
abundance were Smaller
increase in abundance on NSP
diet but may be due to smaller
↑in fiber intake cp to control
diet (1.5x cp to 4.8 x on RS)
Fecal acetate, Propionate &
butyrate ↑on ➂
Chemical analysis of fecal
samples showed soluble
NSP digestibility to be
around 88–90%
Insoluble NSP digestibility
=66%
NSP and RS, affect distinct
bacteria, and have different
impact on the community
ecology of the human gut. RS
reduced diversity while
increasing specific bacterial
types while wheat bran had a
more modest impact on
bacterial abundance while
increasing diversity of the
microbiota.
(Continued)
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Continued
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
Christensen
et al. (30)
Denmark
79 overweight/obese
post-menopausal women
Age 45–70 y
BMI 27–37
Not reported Whole grain vs. refined
grain on energy
restricted diet
Randomized
parallel
16 S rRNA gene
sequencing
12 weeks
2 week run in
➀105 g of whole grain
products
➁105 g refined
grain products
Sig ↑Bifidobacterium with ➀
whole wheat group
Sig ↓Bacteroides with ➁RW
group
Fermentation metabolites
not measured.
Sig -ve correlation between
bacteroides abundance & %
fat mass & trunk fat
Sig +ve correlation between
Bifidobacterium & % fat
mass & trunk fat
This study, consistent with
other studies, supports the
prebiotic potential of whole
wheat grain products.
Vetrani et al. (5) Italy
54 adults
overweight/obese (23 M,
31 F)
Av age 58 y
Av BMI 32
Diabetes, renal failure,
liver abnormalities,
anemia, chronic disease,
alcohol abuse
Wholegrain products,
e.g., bread, breakfast
cereals, pasta etc.
Mainly wheat some rye
Randomized
parallel
n/a 12 weeks ➀Wholegrain (40g total
fiber, 29g cereal fiber)
➁refined grain (22g total
fiber, 12 g cereal fiber)
Not measured Sig ↑plasma propionate
with ➀which was +vely &
sig associated with cereal
fiber intake
Habitual consumption
wholegrain foods may promote
colonic fermentation of fiber,
and increased propionate
levels may help to modulate
insulin response.
Cooper et al.
(31)
USA
46 adults (21 M and 25 F)
Av age 23.4 y
Av BMI 23.4
Diabetes, GI
disease/IBS, laxatives,
antibiotics (3 M)
smoking, pregnancy,
lactation
Wholegrain products
e.g., bread, breakfast
cereals, pasta etc.
Mainly wheat some corn
& rice
Randomized
parallel
16S rRNA gene
sequencing
6 weeks ➀6 servings wholegrains
(13.7g fiber)
➁6 servings refined
grains (4.2g fiber)
No sig changes but trends
toward ↑Akkermansia &
lactobacillus&
Erysipelotrichales
N.B. Fecal samples analyzed
from only 28 participants
Not measured Microbial analysis lacked
power due to small sample size
and requires further research
Ampatzoglou
et al. (32)
UK
33 adults (12 M, 21 F)
Chronic
illness/medication
Substance/alcohol
misuse
Antibiotics(3 m)
Probiotics (3 m)
Habitual
high fiber/wholegrain
Commercially available
whole grain pasta, rice,
snacks, and breakfast
cereals
Randomized
crossover
FISH 6 weeks
Washout 4 weeks
➀High whole grain (>80
g/d)
➁low whole grain
(<16 g/d)
No sig effect Fermentation metabolites
not measured.
No sig effects
Trends toward ↓BMI, blood
glucose, & ↑fecal weight
with ➀
Little effect of WG
consumption on blood
biochemical markers, body
composition, BP, fecal
measurements, or gut
microbiology. This may be due
to impact commercial
production processes on levels
of undigestible fermentable
carbohydrates
Ross et al. (33) Switzerland
17 adults (6 M and 11 F)
Age 20–50 y
BMI 19–28
Healthy, no medication,
normal blood lipids, no
recent antibiotics,
non-smilers
Commercially available
wholegrain foods inc.
wheat, oats, and brown
rice
Randomized
crossover
Quantitative PCR
2 weeks
Washout
5–7 weeks
➀150 g wholegrain foods
(34 g fiber)
➁150 g refined grain
foods (19 g fiber)
Both 2/3 wheat
+oats & rice
➀sig ↑clostridium leptum
Trend toward ↑enterococcus
Fermentation metabolites
not measured.
➀sig ↑stool frequency &
trend toward ↓LDL
cholesterol
2 weeks too short to see full
effects on gut microbiota.
Small changes in fecal
microbiota after 2 weeks
suggest that longer term
wholegrain diets could have
greater effects on gut
microbiota. This requires
further study in longer trials
with greater no’s participants
Tap et al. (34) France
19 adults (9 M and 10 F)
Age 18–30 y
BMI 18.5–25
Antibiotics (3 M)
Laxatives (3 M)
No history GI problems
or taken GI
medications (3 M)
High vs. low fiber intake
mixed diet
Randomized
crossover
16S rRNA gene
sequencing &
qPCR
5 days
Washout 2 weeks
➀40g fiber ➁10g fiber ➀Sig ↓Escheria coli
Low level of microbial richness
at outset was associated with
sig microbiota change with ➀
Fermentation metabolites
not measured.
Short-term change in dietary
fiber impacts gut microbiota
differently within
participants—sig change seen
in all individuals.
Vanegas et al.
(35)USA
81 adults (49 M and 32 F
postmenopausal)
Age 40–65 y
BMI <35
Supplement use, weight
loss diet,
NB.
probiotics/supplements
stopped 30 days prior to
trial
Alcohol abuse
Antibiotics (3 M)
Medication use
Whole grains vs. refined
grains
Wheat main source WG
Randomized
parallel
16S rRNA gene
sequencing—
Greengenes
reference
database &
USEARCH
program
6 weeks ➀WG diet 40 g fiber/day
(16 g fiber/1,000 kcal)
➁RG diet 21 g fiber/day
(8 g fiber/1,000 kcal)
➀sig ↑Lachnospira & ↓
Enterobacteriaceae
➀↑bowel movement freq &
stool weight
➀Sig ↑in stool total SCFA’s
& acetate
Short-term consumption of
wholegrains improves bowel
function and has modest
positive effects on gut
microbiota, SCFA and innate
immune response. Prolonged
intervention may give more
pronounced changes in
microbiota &inflammatory
markers.
(Continued)
Frontiers in Nutrition | www.frontiersin.org 7March 2019 | Volume 6 | Article 33

Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Continued
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
IMPACT OF REDUCING CARBOHYDRATE/FIBERINTAKE
Lappi et al. (36) Finland
51 adults (25 M and 26 F)
with metabolic syndrome
Av Age 60 y
Av BMI 31
+at least 3 other features
metabolic syndrome
BMI >40
Very high blood lipids
Diabetes, liver, thyroid,
renal disease
Alcohol abuse
IBS.
Replacement rye bread
with refined wheat bread
Same total grain intake
but different quality
Randomized
parallel
16S rRNA gene
sequencing and
qPCR analysis
12 weeks ➀High fiber Rye bread
(24 g fiber)
➁refined wheat bread
(19 g fiber)
➁Sig 16% ↑Bryantella
Formatexigens
➁37% ↓in
Bacteroidetes—due to removal
of rye breads
A substantial individuality
characterized microbiota
responses—most
unidirectional but some
not responders
Fermentation metabolites
not measured.
Intentional modulation of the
gut microbiota by withdrawal
or supplementation is not
straightforward due to
individual variations in
microbiota. Changing from
high to low wholegrain diet did
not produces difference sin gut
microbiota in individuals with
metabolic syndrome
Duncan et al.
(37)
UK
19 obese but healthy
adults
Av. Age 36.7y
Av BMI 35.4
No history of
gastrointestinal
problems. No antibiotics
or drugs known to
influence microbiota
Reduction in
carbohydrate and fiber
intake
Randomized
crossover
FISH and 16S
sRNA gene
sequencing
4 weeks
Washout 3 days
➀High protein/medium
CHO (164 g CHO & 11.7g
NSP/day) (HPMC)
➁High protein/low CHO
(24 g CHO & 6.1 g
NSP/day) (HPLC)
➂Maintenance (M) (399 g
CHO & 27.9 g NSP/day)
Sig change in bacterial no’s
Roseburia ➂>➀>➁
Eubacterium rectale
➂>➀>➁
Bifidobacteria
➂>➀>➁
Sig ↓in fecal total SCFA
(50%) & butyrate (75%) with
decreasing CHO & fiber
➂>➀>➁
Total fecal SCFA correlated
positively with fiber intake
Butyrate production Is largely
determined by fermentable
carbohydrate in the diet. Long
term consequences of low
SCFA in colon are unknown
however consideration to
adequate supply of fermen
table substrates should be
given if low carb diet to be
followed for long periods
Brinkworth et al.
(38)
Australia
91 overweight or obese
adults
Age 24–64 years
BMI 26–44
Liver, cardiovascular,
peripheral vascular,
respiratory, GI, renal or
hepatic disease or a
malignancy.
Regular use drug
therapy, medication or
supplements such as
laxatives or antibiotics
Comparison high or low
carb energy restricted
diets
Randomized
parallel
Selective plating
for bifidobacteria,
lactobacilli, total
anaerobes, and E.
coli, coliforms and
total aerobes &
visual counting
8 weeks ➀High carb (46% CHO,
32 g fiber)
➁Low carb (4% CHO,
13 g fiber)
Sig ↓fecal bifidobacteria in low
CHO group
Fecal acetate & butyrate sig
↓(30–60% lower) on the LC
diet
Short term consumption of a
low carb diet had a negative
impact on bowel health:
including lower stool mass,
less frequent bowel
movements, reduced
large-bowel fermentation
(↓concn) & excretion of fecal
SCFA inc butyrate, &
unfavorable shift in fecal
microflora composition (↓
bifidobacteria)
WHEAT BRAN EXTRACT—ARABINO-XYLAN-OLIGOSACCHARIDE (AXOS)
Maki et al. (39) USA
55 adults
Age 18–75 y
BMI 18.5–35.0
Lipid lowering
medication
Wheat bran extract
AXOS
Randomized
crossover
FISH 3 weeks
Washout 2 weeks
AXOS at 0 (control), 2.2 g,
or 4.8 g/d as part of
wheat based ready-to-eat
cereal −2×44 g servings
cereal daily
Sig ↑bididobacteria with 4.8 g
AXOS provided as 2 ×2.4 g
doses in a wheat based
breakfast cereal
Bifidobacteria levels ↑in a
dose-dependent manner 4.8
>2.2 g>control
Sig ↑in plasma ferulic acid
with 2.2 and 4.8 g
AXOS—again dose
dependent trend 4.8>2.2 g
No change to Acetic acid or
proprionic acid, butyric acid
↓with increasing AXOS
Sig ↓LDL cholesterol with 4.8
g/d
Johansson Boll
et al. (40)
Sweden
19 adults (9 M and 10 F)
Av age 23
Av BMI 22
Smoking
Antibiotics (2 w)
Probiotics (2 w)
Food allergise
Metabolic disorders
Wheat Bran extract
AXOS
Randomized
crossover
n/a Single test meal
Washout 1 week
➀White wheat bread
(WWB) (1.2 g RS)
➁WWB +AXOS (8.9 g) +
RS (6.6 g)
➂WWB +hiAXOS
(18.9 g), RS (1 g)
➃WWB +RS (15 g RS)
Not measured
Sig dose dependent ↑
breath Hydrogen with AXOS
➁&➂
Sig dose dependent ↑SCFA
& Butyrate with ➁&➂
No sig diff in post prandial
glucose or insulin after
breakfast meal but improved
insulin sensitivity index
with AXOS
An AXOS rich substance has
the potential to influence
overnight glycaemic regulation
and gut fermentation in healthy
young adults.
(Continued)
Frontiers in Nutrition | www.frontiersin.org 8March 2019 | Volume 6 | Article 33

Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 2 | Continued
References Country, recruitment,
sex, baseline age (years)
Adjustment for
confounding variables
Fiber source Design Bacterial
determination
Test period Intervention daily
intake
Changes to bacterial
species
Other outcomes Conclusions
Windey et al.
(41)
Belgium
29 adults
Age 19–44y
BMI 18.7–24.3
Abdominal surgery, Liver
or kidney failure, GI
conditions. Pregnancy,
lactation, Drugs affecting
GI tract (14 days),
Antibiotics (1 M)
Wheat bran extract (75%
AXOS)
Randomized
crossover
DGGE, plus
real-time PCR
GenBank
DNA database
3 weeks
Washout 3 weeks
➀WBE 10 g/day (2 ×5 g
sachet) AXOS =7.5 g
avDP5
➁10 g (2 ×5 g sachet)
maltodextrin (placebo)
Sig ↑bifidobacteria with ➀
WBE
No diff in fecal SCFA
Sig ↓colonic fermentation
protein with ➀WBE
Supplementing the diet with
WBE clearly altered
fermentation in the colon &
selectively stimulated growth of
bifidobacteria.
Cloetens et al.
(42)
Belgium
12 adults (6 M and 6 F)
Av age 24
Av BMI 21.9
No GI disease Antibiotics
(3 M)
Medication affecting GI
tract (3 M)
AXOS avDP 15 in varied
doses +3 stable
isotopes to measure
gastric emptying, transit
time & colonic NH3
metabolism
Randomized
crossover
n/a Single test meal
Washout 1 week
➀AXOS 0.2g,
➁AXOS 0.7g
➂AXOS 2.2g
➃AXOS 4.9g
➄control
Given in single daily
test meal
Not measured Gut motility not affected.
Both ➂&➃(2.2 g and 4.9 g
AXOS) resulted in: Sig ↑
breath hydrogen & sig ↓
urinary nitrogen Tendency to
↑fecal nitrogen
A minimal does of 2.2 g AXOS
favorably modulates colonic
bacterial metabolism with
increases in indicators of
fermentation and bacterial
growth.
Cloetens et al.
(43)
Belgium
22 adults
Not reported AXOS—same dose
2.25 g but different
degree polymerisation
Randomized
parallel
RT PCR—
bifidobacteria,
lactobacteria, and
eubacteria
2 weeks ➀AXOS 2.25 g av DP 9 ➁
AXOS 2.25 g avDP 15
Sig ↑bifidobacteria with ➀
2.25 g AXOS avDP9
Trend to ↑Bifidibacteria with ➁
but not significant change from
baseline
Not measured Bifidogenic properties of AXOS
are affected by the degree of
polymerisation with shorter
molecules being more
bifidogenic.
Cloetens et al.
(44)
Belgium
20 adults (6 M and 14 F)
Av age 24
Av BMI 20.9
GI complaints,
antibiotics (3 M), drugs
influencing GI transit
(3 M), Abdominal
surgery, pregnancy
Wheat bran extract
(AXOS av DP 6) in
orange juice
Randomized
crossover
RT PCR—
Bifidobacterium,
Bifidobacterium
adolescentis, total
bacteria,
Lactobacillus,
Roseburia–
Eubacterium
rectale, and
enterobacteria
3 weeks
Washout 4 weeks
➀AXOS 10g (2 ×7 g
sachet)
➁10 g maltodextrin (2 ×
7 g sachet)
Sig ↑bifidobacteria at 2 & 3
weeks of AXOS intake
Bifidobacteria also ↑on
placebo but AXOS effect was
sig >than placebo effect
Increase most pronounced in
participants with lowest
starting levels of bifidobacteria
Fecal SCFA not measured.
No influence of plasma
folate, Vit A or minerals.
No sig diff blood lipids.
This higher dose of AXOS was
well tolerated and showed
significant prebiotic activity.
Francois et al.
(45)
Belgium
63 adults (33 M and 30 F)
Av age 42 y
Av BMI 23.3
Low energy/extreme diet
(6 W),
Antibiotics (3 M), drugs
or supplements affecting
GI tract (2 W), abdominal
surgery/GI disease,
excess alcohol/smoking,
pregnancy, lactation
Wheat bran extract (79%
AXOS, Av DP 5) in soft
drink
Randomized
crossover
FISH 3 weeks
Washout 2 weeks
➀0 g WBE
➁3 g WBE(2.4g AXOS)
➂10 g WBE (8g AXOS)
(split over 2 doses)
➂Sig (2-fold)↑bifidobacteria
➁trend to ↑bifidobacteria
(1.3-fold) but not sig. (p0·065)
➂↑fecal total SCFA by 8%
(acetic, propionic & butyrate)
➁sig ↑fecal propionic
acid level
An intake of 10 g WBE/day (8 g
AXOS) increased production of
SCFA, reduced protein
fermentation & increased fecal
bifidobacteria levels, and is well
tolerated.
Scarpellini et al.
(46)
Belgium
13 adults (6 M and 7 F)
Av Age 32.2 y
Av BMI 23
GI disease
Antibiotics (3 M)
Pregnancy
Wheat bran extract
(AXOS) in drink
Randomized
crossover
n/a 48 h
Washout 1 week
➀AXOS 4x 9.4 g/day
➁maltodextrin 4 ×10 g
Not measured Sig ↑breath hydrogen
indicating ↑colonic
fermentation on day 1&2
with AXOS
Acute AXOS administration is
associated with increased
colonic fermentation
Hamer et al.
(47)
Belgium
20 adults (6 M, 13 F)
Av age 23 y
Av BMI 23
Not reported AXOS 10 g/day Non-randomized
intervention
n/a 3 weeks AXOS 10 g/day for 3
weeks
Not measured 179 different VOC’s
identified in fecal samples.
With 24 present in >70%
samples. Shift away from
protein fermentation seen.
AXOS has considerable impact
on colonic fermentation mainly
by suppression of proteolytic
activity.
OTHER ISOLATED FIBERS
Abell et al. (48) Australia
46 adults (16 M, 30 F)
Age 25–66 y
Av BMI 26
Antibiotics (3 m)
Smokers
GI disease
NSP (bran fiber) vs. NSP
+Resistant starch
(barley fiber and HiMaize)
given as dietary
supplements
Randomized
crossover
DGGE of 16S
rRNA gene
fragments
4 weeks each
Washout 2 weeks
➀Normal diet
➁NSP (25 g fiber & 1 g
RS)
➂NSP +RS (25 g fiber +
22 g RS)
Sig shift in bacterial DNA in
fecal samples on both NSP
and NSP+RS diets
↑in Ruminococci species
with NSP+RS
Sig ↑fecal total SCFA
acetate, propionate, and
butyrate
NSP+RS >NSP
This study suggests R.
bromii-related organisms to be
an important player in starch
colonization and digestion.
Longer-term dietary
intervention on the colonic
microbiota is needed to elicit a
stable shift in microflora
M, male; F, female; BMI, body mass index; GI, gastrointestinal; WG, wholegrain; WB -wheat bran; NSP, non-starch polysaccharide; AXOS, arabinoxylooligosaccharides; WBE, wheat bran extract; SCFA; short chain fatty acids; CHO,
carbohydrate; RS, resistant starch; DP, degree of polymerization; VOC, volatile organic compound; FISH, fluorescence in situ hybridization; DGGE, denaturing gradient gel electrophoresis; PCR, polymerase chain reaction, real time (RT),
or quantitative (qPCR).
Frontiers in Nutrition | www.frontiersin.org 9March 2019 | Volume 6 | Article 33

Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
to bacterial abundance or species diversity, and fermentation
metabolites), and any other outcomes such as fecal weight,
frequency, change to lipid levels etc. were extracted. A summary
table of the study characteristics and findings is shown in Table 2.
The studies were categorized and discussed according to the
fiber manipulation, such as whole diet wholegrain studies, wheat
bran, oat bran, or specific fiber sub-fractions such as AXOS
or arabinoxylans.
RESULTS
Summary of Studies and
Their Characteristics
The flow of included studies is outlined in Figure 1. Details of
the studies, and their characteristics is presented in Table 2. A
summary of the extracted outcomes is reported in Table 3 in
relation to fiber type and direction of change.
The 40 included studies included a total of 1,308 participants.
Of these studies, 26 utilized a randomized crossover design, 11
were parallel randomized trials, and 3 were non-randomized
interventions. Seven studies involved feeding single test meals in
a laboratory setting, and the remainder involved foods consumed
at home. Intervention length ranged from 1 meal to 1 year.
A total of 4 studies compared the effects of more than one
fiber type, with the remaining 36 studies examining a single fiber
source compared with low fiber foods or habitual dietary intake.
Wheat was the most commonly studied fiber with 9 studies on
wholegrain intake (predominantly wheat but some other grain
fiber), 8 exploring wheat fiber or bran exclusively, and a further 9
utilizing wheat bran AXOS. Five studies reported on rye fiber.
While the prebiotic effect of isolated beta-glucans on the gut
microbiota has been studied extensively, only 2 studies were
identified examining the effects of intact oat fiber on the gut
microbiota. Barley, rice and maize accounted for the remaining
8 studies.
Impact of Wheat Fiber or Wheat Bran on
Gut Microbiota
Wheat fiber or bran is the hard outer layers of the wheat kernel.
Wheat bran is particularly rich in dietary fiber and essential fatty
acids, and also contains appreciable quantities of starch, protein,
vitamins, and dietary minerals. Wheat is widely consumed and a
significant contributor to fiber intakes in Western Societies, with
approved health claims for digestive health in many countries
including the European Union (EFSA 2010 j.efsa.2010.1817),
Canada (Health Canada), USA (US FDA Laxative Monograph),
and Australia (Food Standards Australia New Zealand 2014).
It is also the most studied in relation to its impact on the
gut microbiota.
In total, 8 studies examined the impact of manipulating
wheat on the gut microbiota, of which 5 increased fiber at a
breakfast meal (11–15). Three examined the effect of whole
day diet interventions (16,28,29), of which two conducted
different types of analysis on fecal samples from the same
study sample and intervention (28,29). One analysis identified
change in key dominant bacteria phylotypes (28), while the other
more specific analysis identified change to individual bacterial
species (29). Wheat fiber provision ranged from 5.7 g to 21
g/day and wheat bran from 13 g to 28 g/day. Both bacterial
abundance and fermentation metabolites were measured in 4
studies, and the remaining 4 studies measured only change
to the metabolites of bacterial fermentation. Six of the 8
studies (11–14,29) showed significant effects on gut microbiota
from wheat fiber or bran fiber consumption and 2 showed
no effect (15,28). Significant increases were reported both in
terms of phyla: Bacteroidetes (12,29); Firmicutes (12,29); and
Actinobacteria (29), and specific species: Bifidobacteria (11);
Lactobacillus (11); Atopobium (11); Enterococci (11); Clostridia
(11); Lachnospiraceae (29); Eggerthella (29); Collinsella (29);
Corynebacterium (29); Bacteroides (29); and Prevotella (29).
Four interventions utilizing a single daily serving of wheat
bran fiber at breakfast all demonstrated a significant prebiotic
effect. Costabile et al. (11) examined the effect of consuming 48 g
of wholegrain wheat cereal (5.7 g fiber) or a 48 g of wheat bran
rich cereal (13 g fiber) daily for 3 weeks, and reported significant
increases in Bifidobacteria following wholegrain consumption
and significant increases in Lactobacilli and Enterococci
after either cereal vs. baseline. Bacterial response following
the wholegrain cereal was significantly greater compared to
wheat bran, however the wholegrain cereal used had been
specifically chosen compared to similar cereals for its ability
to stimulate microbial growth. Change to bacterial abundance
was enumerated using Fluorescence in situ Hybridization (FISH)
which depends on the pre-selection of probes for specific
bacterial types. Probes were selected to detect change to
dominant members of the gut microbiota, and change to
less dominant species reflecting wider benefits arising from
fiber consumption may have not been captured. Both cereals
significantly increased plasma ferulic acid levels with higher levels
reported following consumption of wheat bran, and this was the
first study to demonstrate in human participants that the regular
consumption of wholegrain or wheat bran is followed by a slow
and continuous release of phenolic acids into the bloodstream.
Dietary fiber is rich in phenolic compounds, particularly ferulic
acid, the majority of which is bound to the arabinoxylan present
in the bran fraction of the wheat kernel and is released by
microbiota action. The appearance of ferulic acid in plasma
or feces can be used as a marker of bacterial fermentation
in the colon, however levels will reflect all sources of dietary
fiber consumed unless these are carefully controlled. Vitaglioni
et al. (12) examined the effect of consuming 70 g of wholegrain
wheat cereal (8 g fiber) compared to 60 g of refined wheat (2.2 g
fiber) daily for 8 weeks and demonstrated significant increases in
Bacteroidetes and Firmicutes, accompanied by a 4-fold increase
in plasma ferulic acid and 2-fold increase in fecal ferulic acid
among the wholegrain group compared to the refined wheat
consumers. Wholegrain wheat was the unique source of ferulic
acid in this study allowing differentiation between the two
intervention groups.
Neacsu et al. (14) fed either a 40 g bowl of All Bran original
cereal (11 g fiber) or a 120 g bowl of All Bran original (33 g) as a
single test meal and then measured fermentation metabolites in
plasma, urine, and feces over a 24 h period. Significant increases
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
TABLE 3 | Summary of findings for the short-term effect of increasing cereal fiber on gut microbiota outcomes in health adults.
Effect on microbiota Total
studies
Significant changes bacterial
abundance
Significant changes to
fermentation metabolites
No effects Notes
SHORT-TERM EFFECTS OF FIBER MANIPULATION
Whole diet mixed grain fiber
(predominantly wheat)
10 6 sig abundance and/or
diversity
5 sig SCFA and/or
metabolic markers of
fermentation
2 no sig effects bacterial
no’s−1 due to small sample
size
2 studies did not measure
bacterial change
5 studies did not measure
fermentation metabolites
Reduction of carbohydrate
and/or fiber in diet
3 3 2 – 1 study did not measure
fermentation metabolites
Wheat fiber/bran 8 3 sig bacterial types and/or
diversity
6 sig 1 no effect on bacterial
abundance
1 no change to SCFA
4 studies did not measure
change to bacterial abundance
Barley fiber 4 1 sig 3 sig 1 no effect on SCFA despite
↑bacterial no’s
3 studies did not measure
bacterial change
Oat fiber 2 1 sig with fiber and sig
refined grain
1 change to metabolic
markers of fermentation
2 no sig effect on fecal
SCFA
1 did not measure bacterial
change
Maize fiber 1 1 sig Not measured 1 no effect fecal SCFA Sig bifidobacteria
Rye fiber 5 1 shifts in bacterial phylotype 2 sig butyrate and breath
H21 sig butyrate in men
only
2 showed no effect on total
microbiota abundance
3 studies did not measure
change to bacterial abundance
1 study did not measure
fermentation metabolites
Rice fiber 3 2 sig 1 sig SCFA 1 no change to bacterial
abundance
2 no effect on fecal SCFA
Wheat AXOS 9 5 sig target species 6 sig breath H2, plasma
ferulic acid or SCFA
1 no effect fecal
SCFA—poss due rapid
fermentation and absorption
4 studies did not measure
change to bacterial abundance
2 studies did not measure
fermentation metabolites
in total short chain fatty acids were measured in plasma, urine
and fecal samples, following consumption of both 40 or 120 g
wheat bran cereal, with no significant differences found between
treatments. Additional unpublished data provided by the author,
shows the largest increase to occur in fecal butyric acid, with a
2-fold increase over a 24 h period.
Freeland et al. (13) was the only study that ran for longer than
3 months. The intervention consisted of daily consumption of
60 g of wheat bran cereal (24 g fiber) for 1 year, compared to
49 g of low fiber cereal (2.2 g fiber). No other dietary restrictions
were required. Metabolic profiling was undertaken at baseline
and 3-monthly intervals for the duration of the intervention.
Compliance was good and 20 g/day increase in fiber was achieved
(38 g total fiber/day) compared to the control group (19 g total
fiber/day). Fecal samples from this study were lost and so
change to the microbiota was limited to plasma SCFA levels
measured over an 8 h period following a test breakfast taken
at baseline, 3, 6, 9, and 12 months post intervention. Plasma
levels of butyrate were significantly higher for the wheat bran
consumers at 9 months post-intervention vs. low fiber cereal
consumers. There were no other significant effects at any other
time points. The authors acknowledge the limitations of SCFA
measurements in plasma, but conclude that sustained butyrate
and Glucagon-like-peptide-1 levels (GLP-1—a peptide hormone
which stimulates production of insulin thus lowering blood
sugar) from 9 months onwards might provide a mechanism for
the reduced levels of diabetes associated with high fiber intakes.
Similar links between gut microbiota fermentation, SCFA, and
modulation of blood glucose and insulin responses have also been
made by acute trials (4,18) lasting 12–14 h. The final breakfast
intervention measured appearance of metabolites from labeled
inulin (15) and whether the addition of wheat bran to inulin
affected its appearance. The conclusion was that wheat bran
had no additional benefit, however the study had a number of
limitations in relation to wheat bran fermentation which are
discussed later.
These studies demonstrate that a relatively low wheat fiber
intake at a single time point (breakfast) can maintain a prebiotic
effect, despite the presence of a mixed habitual diet. All 3
studies increasing wheat fiber over the whole day (16,28,29)
showed substantial effects on species composition within the
gut microbiota, and the results are presented in more detail
in the whole diet section below (section Impact of Mixed
Whole Grains on Gut Microbiota). Salonen et al. (29) compared
effect of wheat bran vs. resistant starch and reported lower
increases in individual bacteria with wheat bran consumption,
but this was accompanied with a marked increase in overall
microbial diversity.
Impact of Barley Fiber on Gut Microbiota
Barley contains a mixture of both soluble (beta-glucans) and
insoluble fibers giving it a diverse range of potential health
benefits, including moderating blood cholesterol and provision
of a food source for the gut microbiota. Similar to wheat, barley
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
fiber also contains essential fatty acids, starch, protein, vitamins,
and dietary minerals.
A total of 4 studies (4,17–19) have investigated the impact
of barley fiber on the human gut microbiota, 3 of which have
been carried out by the same research group in Sweden over
a period of 7 years (4,18,19). Barley was provided either
as a single test evening meal (n=2) or consumed freely as
a barley bread or barley cereal, with fiber amounts varying
from 9.8 to 19.6 g. All studies showed a substantial impact
of barley fiber on markers of gut microbiota: either change
to microbiota population (17) or fermentation metabolites
(4,18,19). Only 1 of the studies (17) examined microbiota
population and found significant increases in Firmicutes and
Actinobacteria (specifically Roseburia, Dialister, Eubacterium,
and Bifidobacterium) and a significant decrease in Bacteroidetes,
The remaining 3 studies (4,18,19) measured markers of
fermentation (breath hydrogen, plasma SCFA) and showed
significant increases in total SCFA, butyrate and acetate,
and significant increases in breath hydrogen following barley
fiber consumption. All 4 studies demonstrated concomitant
improvements in glycemic response attributed to the positive
effects of fermentation metabolites.
Impact of Rye Fiber on Gut Microbiota
Rye is a staple cereal across Central, Eastern, and Northern
Europe, most often consumed as breads and crispbread, and
providing a significant contribution to fiber intakes in these
communities. Rye contains a mixture of soluble and insoluble
fibers plus essential fatty acids, starch, protein, vitamins, and
dietary minerals.
Of the 5 studies into the effect of rye intake on the gut
microbiota, 4 were carried out in Scandinavia where rye breads
and other products are a staple part of the everyday diet (20–
22,36), and the other carried out in Australia (16) compared
the effects of both wheat and rye. Intake of rye fiber varied
from 7.1 to 24 g daily. Only 2 studies measured the effects on
microbiota abundance, both using 16S rRNA gene sequencing,
and showing no effect on total bacterial abundance (21,36).
Lappi et al. (36) however reported significant shifts in bacterial
composition with a significant (37%) decrease in Bacteroidetes on
switching intake of wholegrains, predominantly rye bread, over
to refined wheat products, with a parallel increase in Firmicutes
(Clostridium sp.) and Actinobacteria (Collinsella and Atopobium
sp.). Bacteria within the Bacteroidetes phyla are known to be able
to utilize the arabinoxylan fiber fractions in rye (49), so a decline
in Bacteroidetes when rye breads are removed from the diet is
not surprising.
In contrast, 3 of the 5 studies (16,21,22) reported an
increase in the metabolites of bacterial fermentation with
significant increases in fecal butyrate (16,21) or breath hydrogen
(22). One study did not measure metabolites (36) and the
other reported no change in fecal butyrate for women, but a
significant increase in men (20). During this study the men
consumed significantly larger quantities of the rye bread and
as a consequence significantly more rye fiber (19.1 g fiber from
the test bread compared to 13.5 g consumed by women). The
authors determined that from this study it was not possible
to conclude whether the differences in response to rye bread
between women and men were due to different amounts of food
consumed or to differences in fiber intake from breads and that
further exploration with larger participant numbers is required.
Impact of Rice Fiber on Gut Microbiota
Like other cereals, the rice grain is enclosed in an outer bran layer,
rich in fiber minerals and vitamins and antioxidants. Rice bran
contains a higher levels of oils compared to other cereal brans,
and so is often removed from the grain to reduce risk of rancidity
and improve storage longevity, as a result reducing the range of
fiber rich rice products commonly available for consumption.
Only 3 studies were identified exploring the links between
rice fiber and the gut microbiota. One was carried out in Japan
(23), and the other 2 in the USA (17,24), however it should
be noted that one of these was a pilot trial involving just
7 participants (24). The Japanese study (23) used Fermented
Brown Rice by Aspergillus (FBRA) which is a high fiber brown
rice and rice bran mix fermented by the Aspergillus fungus
prior to consumption. This intervention failed to find any
significant effects, with no significant change to fecal metabolites
(total or individual SCFA); total bacterial abundance; and no
measurable increase in two target bacterial genus—bifidobacteria
and enetrobacteriaecea. However, in vitro tests using fecal slurry
from 6 of the study participants showed significant increases to
both bifidobacteria and SCFAs. One limitation of this study is the
use of Terminal Restriction Fragment Length Polymorphism (T-
RFLP) to enumerate bacterial species may over simplify diversity
due to convergence of species. The author concluded that the
status of FBRA as a prebiotic remains unclear. The 2 USA
based studies used brown rice flakes providing 11.5 g fiber (17)
or 30 g rice bran with 6.3 g fiber (24) to be consumed at any
time each day. Both treatments showed significant change to
bacterial abundance (Firmicutes, Bifidobacteria, Ruminococcus,
Methanobrevibacter, Paraprevotella, Dialister, Anaerostipes, and
Barnesiella) compared to baseline, but only rice bran induced
increases in SCFA (24)—however given the small sample in
this pilot study these results now need replication from larger
scale study.
Impact of Oat Fiber on Gut Microbiota
The outer layers of the oat grain contain a mixture of both
insoluble and soluble (beta-glucan) fibers, both of which provide
a food source for the gut microbiota. While soluble oat beta-
glucans have been established to help lower blood cholesterol
levels, little research has been carried out into the effects of oat
fiber on the gut microbiota.
Only two studies examined the impact of intact oat fiber on
the gut microbiota (25,26). Conolly et al. (25) examined the
effects of consuming a whole grain oat granola vs. a refined
grain cereal for breakfast daily in a randomized cross over
study for 6 weeks in participants with mild hyperglycemia
or hypercholesterolemia. Significant increases in total fecal
bacteria, lactobacilli, and bifidobacteria were reported following
consumption of wholegrain oat granola, whereas total bacteria
and bifidobacteria both fell after consumption of the refined
grain cereal. No effects were detected to SCFA. Valeur et al.
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
(26) fed oatmeal porridge (8.5 g fiber) daily for 8 days and also
failed to detect change in SCFA levels, however fecal levels of
β-galactosidase (lactase enzyme) and urease (protein enzyme)
both fell suggesting a rapid adaptation of the microbiota toward
utilization of oat fiber.
Impact of Maize Fiber on Gut Microbiota
Maize has a higher content of starch and a lower bran content
compared to other cereal grains, and maize fiber has been less
researched compared to other cereal grains.
A single acute intervention study (27) was identified that
examined the impact of consuming 48 g of whole grain maize
breakfast cereal (14.2 g fiber) on a single occasion on the gut
microbiota compared to 48 g of a low fiber maize breakfast cereal
(0.8 g fiber). This study was carried out by the same group who
demonstrated a prebiotic effect from a breakfast cereal containing
wholegrain wheat or wheat bran (11) and extends these findings
to wholegrain maize breakfast cereal. As discussed previously use
of FISH and specifically selected probes may have limited the
range of bacterial change identified. After 3 weeks, significant
increases in fecal Bifidobacteria were reported in both the high
fiber and low fiber groups, with the greater increase in the higher
fiber groups just failing to reach significance (p=0.056), and
a non-significant increase in Lactobacillus, Enterococcus, and
Atopobium species. Similar to other studies the participants
who were most responsive in terms of a bifidogenic effect
to the wholegrain cereal had the lowest initial populations of
bifidobacteria; conversely, the individuals with the highest initial
samples had a less marked response. This study demonstrated a
measurable prebiotic effect of a single serve of a wholegrain maize
cereal within the context of a freely chosen mixed diet.
Impact of Mixed Whole Grains on
Gut Microbiota
Food based dietary guidelines frequently encouraged an
increased consumption of wholegrain cereal foods in order
to improve not only fiber intake, but also intake of the wide
variety of vitamins, minerals, and antioxidants typically found
in the bran layers of cereal grains. Wholegrain cereals include
the endosperm, germ, and bran elements of the cereal grain
and so have a different nutrient composition to the bran fiber
fractions of cereal grains, which could influence effects on
the gut microbiota. With no standardized global definition of
wholegrain foods comparison of wholegrain intakes, and their
associated fiber content can be challenging.
The largest number of studies (n=10, participants =357)
have been carried out into the effects of manipulating intact cereal
fiber sources across the whole day, providing wheat fiber as the
bulk of the fiber as bread, breakfast cereals, pasta etc., but also
permitting some whole oats, rye, and brown rice (15–17, 32–38).
The interventions varied between providing a specific amount
of whole grains in the diet: 80 g (32); 105 g (30); 150 g/day (33) or
specified amounts of cereal fiber: 13.7 g (31); 21 g (16); 28 g (28,
29); 29 g (5); or 40 g (34,35). Outcome measurements also varied
with 3 studies assessing both fecal bacteria and fermentation
metabolites (28,32,35), 5 examining only fecal bacteria change
(29–31,33,34) and the remaining 2 studies measuring only
metabolites of fermentation (5,16).
In terms of outcomes, 9 out of the 10 studies reported
significant prebiotic effects from increased consumption of
intact cereal fiber, with significant increases also recorded in
bacterial diversity (28,29,34), Actinobacteria (29), Bifidobacteria
(30), Clostridium (33,40), Lachnospira (29,34) and non-
significant trends to increases in Akkermansia (30) Roseburia
(35), Lactobacilli (30), and Enterococcus (33). Significantly
decreased levels of pro-inflammatory Enterobacteriaceae were
also reported (35). Two studies (29,34) reported that response
to a high fiber intervention is dependent upon the baseline gut
microbial richness—those with a limited microbial richness at
baseline exhibit a greater microbiota change over time due to
dietary fiber increase.
Both Cooper et al. (31) and Ampatzoglou et al. (32) failed
to detect significant change in microbiota or fermentation
metabolites following increased intake of wholegrain foods.
There are a number of potential reasons for this: Cooper
et al. included subjects on the basis of self-reported wholegrain
intake and fiber intake from others sources was not assessed,
in addition the wholegrain foods provided provide just 16% of
daily energy intake and so variability in actual food intake, and
fiber intake achieved was likely to be high. Microbiota analysis
was undertaken on just 28 of the 46 subjects and so the study
lacked power to detect anything other than large changes to
the microbiota, which coupled with high baseline variability
meant that trends were observed, but none were significant. The
failure of Ampatzoglou et al. to report significant changes might
be down to the use of FISH analysis. FISH relies on selection
of probes for target bacterial groups and subsequent research
suggests that response to wheat fiber may be greatest in bacteria
not targeted by this study. In addition, no account was taken of
individual change to microbiota abundance and so larger change
for those with lower baseline levels may have been lost in the
population averages.
With regard to fermentation metabolites, Vetrani et al. (5)
found a significant increase in plasma propionate following
consumption of wholegrain foods providing 29 g cereal fiber
daily for 12 weeks, and a direct correlation between cereal fiber
intake and propionate levels. Individual responses varied with
those above the median (responders) showing a reduction in
post prandial insulin. No assessment was made of microbiota at
baseline or post-intervention and so while we could project that
the responders were those with lower levels of target bacteria at
baseline we unfortunately do not have the clinical evidence to
support this. Similarly, McIntosh et al. (16) measured propionate
in feces and also reported a significant increase following 21 g
wheat fiber daily for 4 weeks.
Impact of Reducing Carbohydrate and
Fiber Intake on Gut Microbiota
If an increase in fiber intake promotes a bacterial diversity within
the gut microbiota, then it would follow that reducing fiber
intake would reduce some bacterial phylotypes. This has been
demonstrated by 3 research groups who have taken habitually
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
high fiber consumers and reduced their intake of cereal foods
and fiber. Lappi et al. (36) replaced rye bread with refined white
bread (a 5 g decrease in fiber intake) and measured a significant
37% decline in a specific cluster within the phyla Bacteroidetes.
Duncan et al. (37) compared a maintenance diet [28 g Non-
Starch Polysaccharides (NSP) fiber] with a medium carbohydrate
diet (12 g NSP/day) and a low carbohydrate diet (6 g NSP/day)
and found significant reductions in Roseburia, Eubacterium,
and Bifidobacteria with each decrement in NSP intake, and
concomitant reductions in fecal SCFA, and particularly butyrate.
Brinkworth et al. (38) compared a change from a high fiber
intake (32 g) to a low fiber diet (13 g). These results also showed
significant reductions in Bifidobacteria and both fecal acetate and
butyrate with the lower fiber intake.
Impact of Wheat Bran Arabinoxylans
(AXOS) on Gut Microbiota
One of the largest components of wheat bran fiber are
arabinoxylans, which can also be consumed as the isolated
extract AXOS (arabinoxylan-oligosaccharide). Wheat bran fiber
is commonly consumed in foods across the globe, and average
intakes of AXOS in the US population have been estimated to be
around 7.5 g/day (50). A review of studies examining the effects
of consumption of this important wheat bran component on the
microbiota was therefore also included.
We identified 9 studies exploring the role of AXOS on the gut
microbiota (39–47). Measurements included change to bacterial
abundance, breath hydrogen (as a marker of gut fermentation),
plasma ferulic acid (ferulic acid is bound to AXOS so increasing
plasma levels is a marker of AXOS breakdown by gut bacteria),
and SCFA levels. The levels of AXOS provided ranged from 2.2 g
(39) through to 18.8 g (46).
Maki et al. (39) showed a dose dependent effect for
AXOS, with 4.8 g of AXOS (2 ×2.4 g), contained within two
44 g portions of ready-to-eat-cereal (RTEC) daily, stimulating
significantly greater bifidobacteria growth compared to 2.2 g
AXOS (2 ×1.1 g) in RTEC, which in turn stimulated greater
bifidobacterial growth compared to the control condition of
RTEC with no added AXOS. Plasma ferulic acid increased
significantly with both 2.2 and 4.8 g AXOS, again with a
significant dose dependent relationship. Significant increases in
Bifidobacteria were also reported following administration of
2.25 g AXOS (43), two 3.75 g doses of AXOS (41), two 5 g doses
of AXOS (44). A single 8 g dose of AXOS gave a 2-fold increase in
bifidobacteria (45), and a trend toward increased bifidobacteria
(a 1.3-fold increase) was seen following 2.4 g AXOS as a single
dose for 3 weeks.
In terms of fermentation metabolites, significant increases
in plasma SCFA were recorded in response to 8.9 and 18.9 g
AXOS (40), Maki et al. (39) reported no change to acetic acid
or propionic acid and a surprising decrease in butyric acid with
increasing dose of AXOS, with a significant carry-over for both
AXOS treatments suggesting change to microbiota lingered into
the 2-week washout periods. The authors suggest this may be
due to increased colonocyte activity and uptake of butyric acid
stimulated by increasing levels, but that this requires further
investigation. Windey et al. (41) found significant effect from 2
×3.75 g doses of AXOS on fecal and urinary nitrogen, colonic
protein fermentation, but no effect on fecal SCFA levels, which
may be due to use of a short chain AXOS molecule allowing
fermentation in the proximal colon and rapid absorption of
any SCFA produced. Change to fecal SCFA were also reported
by Francois et al. (45) with total SCFA, acetic, propionic, and
butyric acid all significantly increasing following 8 g AXOS for
3 weeks and propionic increasing significantly with the lower
intake of 2.4 g AXOS. While most studies focused only on change
to limited metabolites, Hamer et al. (47) undertook metabolite
fingerprinting following AXOS consumption (10 g/day for 3
weeks) and identified 179 different volatile organic compounds
(VOCs) in subject fecal samples, with 24 VOCs present in 70% of
participants. The impact of AXOS intake on VOCs was mainly
from the reduction of metabolites from protein fermentation,
indicating a shift away from protein fermenters and their
potentially detrimental metabolites (e.g., phenolic compounds
and sulfur containing compounds).
Two of the studies (40,42) investigated the impact of a single
test meal rich in AXOS on fermentation metabolite production
the following day after a standardized breakfast meal. Cloetens
et al. (42) tested 5 single doses of AXOS (0, 0.2, 0.7, 2.2, and 4.9 g)
in 12 healthy participants and measured markers for colonic
bacterial metabolism (breath hydrogen measured over 10 h, urine
samples collected at 3 time points over 48 h and a stool sample
collected at 72 h). A significant increase in both fecal nitrogen
(a marker of increased bacterial growth and metabolic activity),
and breath hydrogen were observed with AXOS intake of 2.2
and 4.9 g. The second study examined the effect of single higher
doses of AXOS (8.9 and 18.4 g) in combination with resistant
starch on overnight glucose levels (40), using measurement of
SCFA and breath hydrogen as a marker of colonic bacterial
fermentation. Significant, dose dependent increases in breath
hydrogen, plasma SCFA, (acetate and butyrate) occurred with
both AXOS interventions, with no effect from resistant starch.
Significant decreases in glucose and insulin responses were also
reported with increasing effect from increasing dose of AXOS.
A third study gave a higher dose of AXOS (4 ×9.4 g) over a
48 h period (46) and reported significant increases in colonic
fermentation on both days 1 and 2 based on hydrogen breath test,
with no detrimental effects on gastrointestinal tolerance.
Cloetens et al. further developed their work (43,44) by
demonstrating a significant increase in Bifidobacteria after 2
weeks consumption of 2.25 g AXOS daily, and also established
a significant increase in Bifidobacteria and good gastrointestinal
tolerance of a high dose of AXOS for 3 weeks (10 g/day),
with only a mild increase in flatulence to report. Stimulation
of bifidobacteria was most pronounced in participants with
the lowest bifidobacteria at baseline, and significant change
was not seen at 2 weeks, and only achieved after 3 weeks of
consumption (44).
DISCUSSION
Dietary approaches to manipulate the human gut microbiota
have long been used as an approach to improve host health.
The aim of probiotic and prebiotic inclusions into the diet
are to increase beneficial gut bacteria and their activities,
thus generating benefits to human health. These benefits
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
include protection from gastroenteritis by pathogen inhibition,
an improved tolerance to lactose, toxins and cholesterol
reduction, vitamin synthesis, improved mineral bioavailability,
potential protection from bowel cancer, reduced symptoms of
irritable bowel syndrome, improved digestion, gut function,
and immune regulation (51–53). Recent research highlights
a loss of gut microbiota diversity in Western Societies,
particularly bacteria belonging to Lactobacillus, Bifidobacterium,
Bacteroides, Prevotella, Oxalobacter, and other genera that are
essential to our microbial gut community (54). The composition
of a “healthy microbiome” has not been precisely defined,
and may vary from individual to individual, however evidence
suggests that dietary improvements to increasing the amount of
fiber, and diversity of foods consumed, to promote microbial
diversity could help to maintain health today, and improve health
in the future (55).
To date, much of the research into prebiotic effects of dietary
fiber has focused on the effects of isolated individual fiber’s, and
less had been conducted into the potential prebiotic benefits
of consuming intact cereal fiber consumed in everyday foods
as part of habitual daily diet. However, consumption of a
single, highly targeted prebiotic may decrease gut microbiota
diversity as bacteria able to utilize that particular energy source
“bloom,” changing the overall microbiota composition (29) and
conditions within the colon (e.g., a fall on pH due to high levels of
SCFA’s produced). Assuming microbial diversity to be important
in maintenance of good health (55), greater understanding of the
impact of provision of multiple, or complex, fiber substrates from
the consumption of intact cereal fiber on both species within,
and diversity of the gut microbiota is of value. This systematic
review is the first to only look at intact cereal fiber sources adding
important detail to our understanding of dietary manipulation to
promote microbiota diversity.
A summary of the key findings is provided in Table 3.
This systematic review provides evidence that increasing
daily intake of intact cereal fiber can have a prebiotic
effect on gut microbiota composition and activity, helping to
support a diverse bacterial population with an increase in
bacterial types able to utilize complex fiber structures, with
benefits to wide range of bacterial species arising from cross
feeding relationships.
Previous authors (8) have found that short term feeding
studies failed to support an increase in bacterial diversity, in
contrast to observational studies where a habitually high fiber
intake supports a more diverse microbiota (56). A recent review
(57), supported by intervention study evidence (28) suggests that
altering dietary intake over a period of 2–3 days is enough for
enriching not only gut microbiota composition with different
species, but also overall gut microbiota diversity. Long-term
dietary habits may lead to changing states of the gut microbiota
diversity, with Prevotella-dominant gut microbiota reported in
people consuming a plant-based diet and Bacteroides-dominant
gut microbiota in those with higher protein and fat intake
(28,58). However, inter-individual variation in gut microbiota
composition before a dietary intervention may also affect
responses in terms of both gut metabolites and microbiota
composition and must be taken into account.
Individual gut microbiota response to any dietary
intervention varies widely depending on starting levels of
bacterial species within their established gut microbiota. This
was clearly identified in the studies carried out by Carvalho-
Wells et al. (27), Martinez et al. (17), and Cloetens et al. (43)
who all highlighted that the individuals most responsive to
increases in cereal fiber had the lowest starting levels of the
target bacteria, with variations in response documented to be
as high as 10-fold. Salonen et al. (29) divided their subjects into
responders and non-responders to intervention, and identified
their non-responders to have high levels of baseline microbiota
diversity, implying a link between phylogenetic diversity and
ecosystem stability. This suggests that individuals most likely
to benefit from an increase in cereal fiber intake are those who
habitually consume low fiber cereal foods, those who limit intake
of cereal foods (e.g., those following low carbohydrate diets),
or those likely to have decreased bacterial diversity, such as
older people.
The wide variation in individual response also suggests that
this is an exciting area of potential for personalized nutrition
interventions. Studies suggest that the gut microbiota could be
playing a role in long term conditions such as obesity and
Type-2 diabetes (among many others). Identifying the specialist
(keystone) bacterial groups for each condition, and cost effective
approaches to firstly assess individual microbiota populations;
and secondly develop dietary manipulations to support relevant
keystone bacterial groups requires further research in order for
this to become part of mainstream clinical practice. In vitro
studies by Duncan et al. (59) have recently identified bacteria
from the Lachnospiraceae family (Firmicutes) to be keystone
bacteria regarding the utilization of wheat bran. Two studies
included in this review also reported significant increase in
Lachnospiraceae in response to consumption of wheat fiber
(29,34). As we move beyond isolated fiber supplementation
and toward a better understanding of the prebiotic effects of
intact dietary fibers our ability to manipulate the gut microbiota
through dietary advice will become more targeted, and as a
result, more effective.
The short chain fatty acids (SCFA) produced as by-products
of bacterial fermentation are difficult to measure due their rapid
clearance from plasma. Despite this, several research groups have
shown the gut microbiota to be highly responsive, with markers
of fermentation stimulated by a single fiber rich meal measurable
within the first 24 h following consumption of rye, barley, wheat
bran, and wheat bran AXOS (4,14,18,22,40,42).
Both beneficial and pathogenic bacteria produce SCFA
as a by-product of their fermentation, therefore increases in
fecal SCFA may not therefore necessarily be an indicator of
benefit. Rahat-Rosenbloom et al. (60) found increased levels
of fecal SCFA among overweight individual compared to lean,
which was attributed to difference in microbiota populations,
with a 5-fold difference in Firmicutes to Bacteriodetes ratio
between the overweight and lean individuals. A review of
evidence by Lau et al. (61) also supports the role of the
Firmicutes:Bacteriodetes relationship in obesity, and a low
bacterial diversity leading to unwanted weight gain. Recently, de
la Cuesta-Zuluaga et al. (62) demonstrated that in Columbian
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
adults that higher fecal SCFAs are also associated with central
obesity, hypertension, subclinical measures of cardiometabolic
disease (e.g., inflammation, glycemia, and dyslipidemia), as well
as a measure of gut permeability (LPS binding protein). However,
microbial diversity showed association with these outcomes in
the opposite direction. More research is needed to increase
understanding of the relationship between fecal SCFA levels,
plasma SCFA, and metabolic health, however evidence to date
appears to be suggesting that adopting dietary measures to
promote microbiota diversity is likely to be important for long-
term health maintenance.
One of the more frequently reported eating occasions for
cereal fiber manipulation to take place was the breakfast meal.
A simple dietary modification to consume a daily bowl of a
high fiber breakfast appears to have a positive impact on the
gut microbiota for health adults (11–13,27), which can be
measured within the first 3 weeks (11,27) and is still maintained
after 1 year (13). As little as 5.7 g of wheat fiber was shown
to produce significant positive benefits to the gut microbiota,
a finding in line with that of So et al. (8) who suggest that
<5 g fiber may be sufficient to stimulate bacterial growth. A
simple change in eating habits which provides a relatively low,
but important boost to fiber intake at a single time point can
produce a prebiotic effect within a mixed habitual diet. Other
cereal grains reviewed also appear to stimulate gut microbiota at
relatively low levels, with as little as 10 g barley fiber, 7 g rye fiber,
or 2.2 g AXOS providing measurable significant effect. Breakfast
is often shown to provide a significant contribution to daily
fiber intakes of western populations, and is an occasion where
switching to higher fiber foods is more easily accepted by the
consumer. Whether delivering a single bolus of fiber at breakfast
has different stimulatory effects on the gut microbiota compared
to delivery of a steady stream of fiber throughout the day is a
potential area for future research.
One important finding for intact cereal fiber consumption
is the support of a diverse bacterial community, and the more
complex fibers, such as wheat bran appear best placed to promote
this diversity. Bacterial diversity is known to vary depending on
habitual diet consumption, with communities living in agrarian
societies and those consuming diet with high levels of plant based
foods, such as vegans and vegetarians possessing a higher level
of bacterial diversity compared to communities with omnivorous
dietary intakes (53,56,63–65). However, change to bacterial
diversity was only measured in two of the studies reported here
(17,29).
One further consideration when comparing the impact of fiber
sources on the gut microbiota is not only the cereal source of the
fiber, and its complexity, but also the preparation and processing
of the grain in question. Cereal grains differ in composition, with
varying amounts of total dietary fiber, insoluble fibers and soluble
fibers, and processing (e.g., milling, heating, flaking, or extrusion)
of grains has been found to affect in vitro fermentation differently
depending on the grain (66). Grain fibers are not all equal in their
potential for prebiotic effect and the varying effects of preparation
and processing of each grain complicates this further.
Of the 40 studies included into this review, 25 reported
change to bacterial levels (either in terms of bacterial abundance
at genus or species level or population diversity), and 23
out of these 25 studies reported a prebiotic outcomes. In
terms of fermentation metabolites 26 studies reported on these
with 25 showing increased levels of fermentation. Two studies
showed no effect of cereal fiber consumption on gut microbiota
composition, which can be explained in part by methodological
weaknesses. No studies showed any negative implications from
consuming increased levels of intact cereal fibers and their
sub fractions in the metabolic parameters measured (typically
digestive comfort, bowel movements, weight change, blood lipid,
and glucose responses etc.), aside from occasional mild and
transient increases in flatulence.
The studies reviewed here provide some key information and
learnings for future research. Several in vitro studies have shown
that not all Bifidobacterium species can degrade the wheat bran
arabinoxylans, reinforcing the need for accurate identification
of bacteria studied. For example, in vitro fermentation models
have found that, arabinoxylans are completely degraded by
Bifidobacterium adolescentis and Bacteroides vulgatus, partially
degraded by Bifidobacterium longum and Bacteroides ovatus,
and not degraded by Bifidobacterium breve and Bifidobacterium
infantis (67). Cloetens et al. (44) and Windey et al. (41)
both reported on Bifidobacterium adolescentis counts and have
corroborated the in vitro observations, that intake of AXOS
by the healthy human participants stimulated Bifidobacterium
adolescentis. It should be noted that Bifidobacteria do not
produce butyric acid, and studies reporting an increases in
Bifidobacteria accompanied by increases in butyric acid levels
(40,45) therefore suggest a mechanism of cross-feeding with
acetate- or lactate converting bacteria may be involved in
increased colonic butyric acid production. Subsequent work has
confirmed that the ability utilize arabinoxylans is strain specific
within the Bifidobacterium species. For example B. longum subsp.
longum LMG 11047, and B. longum subsp. longum CUETM
193 are both able to fully breakdown complex AXOS molecules,
whereas B. longum subsp. longum NCC2705 is only able to utilize
free arabinose and not more complex AXOS (68). Specificity
not only in terms of bacterial species, but also in terms of
strain is likely to gain precedence as next generation sequencing
techniques become more accessible.
Wheat bran formed a particular interest within this systematic
review due to its contribution to the fiber intake of Western
populations. A number of research papers reviewed here suggest
that wheat bran is a complex fiber, supporting the growth of
distinct specific bacterial populations (14,16,43). Research has
shown that bran particles are colonized by bacteria after 24 h
(no earlier time period was measured) (67), and with normal
colon transit times reported at around 70 h (69). Allowing
sufficient follow-up time for complex cross-feeding relationships
to develop and stabilize is an important learning for research
going forwards. Wheat bran has been ingeniously described as
consisting of a unique combination of fermentable and non-
fermentable fibers, entangled in a porous insoluble network
that could serve as an ideal “dinner table” for micro-organisms
(70). Cellulose and highly branched arabinoxylans are resistant
to fermentation and are proposed to act as a physical surface
or “table” onto which bacteria attach. Fermentable substances
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
in the wheat bran particle (such as starch, proteins and
less substituted arabinoxylans) then provide the “dinner” for
these attached microbes. This concept of a unique microbiota
attaching to wheat bran particles has been explored in vitro
using fecal samples from healthy donors (70). Wheat bran
particles were found to host a distinct microbial community,
compared with the luminal environment. The concept of separate
bacterial colonies co-existing to fully utilize the particulate and
luminal environments of the human colon could partly explain
the variation in response to different fiber types in human
feeding studies.
This to our knowledge is the first review looking solely at the
influence on gut microbiota arising from the consumption of
intact cereal fibers. Comparison of outcomes from this and other
systematic reviews of the prebiotic potential of isolated fibers is
limited by the lack of overlap.
Kellow et al. (7) reviewed the impact of dietary prebiotic
supplementation (e.g., fructans, oligosaccharides, or inulin)
on parameters associated with the development of metabolic
abnormalities such as obesity, glucose intolerance, dyslipidaemia,
non-alcoholic fatty liver disease, and low grade chronic
inflammation. The review found convincing evidence from
short-term high-quality human trials to support the use of
dietary prebiotics as a potential therapeutic intervention for
the regulation of appetite and the reduction of circulating
postprandial glucose and insulin concentrations, however
the shift in microbiota responsible for these benefits was
not elucidated.
The recent review by So et al. (8) included only Randomized
Controlled Trials with either a placebo, low fiber diet or habitual
diet group as comparators. Two outcomes were reported—
diversity and richness of the microbiota population between
groups and change to specified groups of bacteria. Of the 64
included studies, 52 involved supplementation with a prebiotic or
candidate prebiotic fiber and just 12 examined intervention using
whole foods with intact fibers. Prebiotic fiber supplementation
increased specified target bacteria such as Bifidobacterium species
and Lactobacillus species, with as little as 5 g of fiber sufficient to
significantly increase Bifidobacterium species. The considerable
degree of heterogeneity in microbiota between participants was
noted for all analysis sub-groups. Only a small number of studies
reported effect on microbiota diversity and the overall lack
of apparent effect on diversity was noted. Long-term dietary
diversity as opposed to changes in isolated nutrients or foods
over a short period of time may be a stronger driver of microbial
diversity. The review authors made an interesting observation
that microbial diversity was not compromised by any of the
reported interventions, which helps to support the case for
favorable effects of dietary fiber on the gut microbiota.
Sawicki et al. (71) took an Evidence Mapping approach
to explore the influence of dietary fiber on the human gut
microbiota. This mapping exercise highlighted that much of the
current literature has shown positive effects of dietary fiber on
gut function or beneficial bacterial species, or positive effects of
dietary fiber on specific health outcomes, but few seem to be
directly measuring these outcomes together, to provide evidence
of a dietary fiber-modulated gut microbiota and health outcome.
Methodological Issues
One key limitation is the number of studies measuring change
in plasma or fecal SCFA’s. In vivo measurements of SCFA’s
in plasma or feces do not reflect levels reaching the liver or
the colon walls as they are rapidly taken up and utilized by
both sites. Changes measured in either plasma or feces are
therefore likely to provide a gross underestimate of actual levels
of change.
Characterization and measurement of change in the gut
microbiota provides challenges for research. Culture-based
methods of measurement underestimate bacterial diversity, and
as many bacteria cannot been grown in culture these are
lost to measurement using these techniques. Culture-based
methods have been largely superseded over the past decade
by culture-independent methods based on the characterization
of 16S ribosomal ribonucleic acid (rRNA) genes, however
while much improved, 16S rRNA gene sequencing is often
limited to identifying bacteria at genus level, and changes at
species, and strain, level will often not be reported. Using the
example of Bifidobacterium previously explained above, there
will be a prebiotic effect for some strains following cereal fiber
consumption while other strains will not demonstrate a prebiotic
effect—there could be significant compositional change but no
detectable change in total Bifidobacterium abundance which
could lead to conclusion of ‘no effect. In addition, 16S rRNA
analysis depends on a database of reference genes: thus the
assignment of both genus and species may be affected according
to the reference database selected (72).
A large amount of human bacteria still remain to be
characterized, although advances in next-generation sequencing
and metagenomics are rapidly expanding knowledge of the
number and diversity of human bacteria. Many studies target
specific reference bacteria (most commonly bifidobacteria or
lactobacilli), which is likely to underestimate the benefits arising
from fiber intake. Metabolic response of bacteria varies widely
at species level as shown by Van Laere et al. (73) with
Bifidobacterium adolescentis and longum able to ferment wheat
bran arabinoxylans compared to Bifidobacterium breve and
infantis who are unable to utilize the complex structure of wheat
bran fiber. Examination of the gut microbiome both at species
level and with regard to metabolic response is needed to further
expand our knowledge of the effects of intact fibers on microbiota
composition and diversity.
It is estimated that each individual carries an estimated
160 bacterial species (3), and studies reported here
highlighted the wide diversity in species identified in the
fecal samples of participants (4,5,17,30,43). Several
studies identified that individuals with low levels of specific
target bacteria had large responses to an intervention
compared to those with high starting levels for whom
population increase was more limited (17,27,43). Failure
to consider habitual fiber intake, habitual food diversity,
and the composition of gut microbiota at individual level
within a study population could mean that a substantial
change in some subjects be masked by a more negligible
total population response due to low responders within the
population sample.
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
Recent work has indicated that breakdown of complex
cereal fiber structures by gut bacteria is dependent on the
presence of carbohydrate active enzymes (CAZ-enzymes) able
to cleave the bonds between sugar molecules forming the
backbone of fiber molecules and other bioactive compounds (74).
While humans are thought to possess just 17 different CAZ-
enzyme families, limiting us to digestion of relatively simple
carbohydrate molecules, the gut microbiota is thought to possess
over 170 different CAZ-enzyme families (74). Some bacteria
have a small range of CAZ-enzyme limiting their utilization
to relatively few carbohydrates and so are termed “specialists,”
whereas others have a wider array of CAZ-enzymes, are able
to utilize a large number of different carbohydrate structures
and are termed “generalists.” As research continues to develop,
consideration should be given to the specificity of the range
and types of bacterial species measured in order to capture
the most appropriate bacterial responders for the fiber type/s
under examination.
While the gut bacteria respond rapidly to provision of fiber
substrates, cross-feeding relationships are complex and take time
to establish and stabilize. The ideal duration of intervention to
allow a diverse and stabile gut microbiota to establish is not yet
known, however is likely to extend beyond the 2–3 weeks studied
by much of the work published to date. The full potential benefits
that could arise from providing the gut microbiota with a diverse,
high fiber diet every day have therefore yet to be accurately
established and longer term trials of several months in length
are needed.
Many studies controlled for confounders (e.g., use of
antibiotic, probiotics of gastrointestinal disease) in the statistical
analysis, via inclusion of many covariates in the analysis. Whilst
this is an important approach for controlling for confounders,
the benefit of including many covariates into a statistical model
should be balanced with the issue of overfitting, particularly in
those studies with small sample sizes. Therefore, there is a need
for studies with larger sample sizes and careful selection and
included covariates.
Habitual diet, environmental factors, geographical location,
and race all influence gut microbiota composition. All of
the studies included in this review included adults from a
single geographical locale, participants were predominantly
Caucasian, with a single study conducted in Japan. Given the
known diversity of microbial composition between individuals
it cannot be assumed that results found in any or all
of these studies will transfer across regions or between
different ethnicity.
Research Recommendations
Although knowledge has advance substantially in recent years,
much remains to be discovered regarding the gut microbiome
and how to achieve the greatest potential benefits from dietary
manipulations. The wide variation in individuals microbiota
highlighted in a number of studies reported here requires further
research: what is the level of variation between individuals
and what are the factors contributing to this? Can dietary
manipulation of fiber type correct microbiota dysbiosis and over
what time frame? Response to dietary intervention appeared
to stimulate little effect in a sub-group of people lacking
key bacterial species, however this was measured over days
or weeks rather than months. Long-term studies are needed
to establish whether maintaining a high fiber intake could
overcome initial shortfalls in the gut microbiota population.
Could development of cost-effective and reliable mechanisms to
elucidate an individual’s microbiome hold the potential to open
up a new and exciting field of personalized targeted nutrition
recommendations to promote microbiota health? This too needs
to be explored.
Research results in the past may have been limited by
measurement of bacterial species which are not specialist
fermenters of the fiber substrate provided. This may be
particularly relevant for wheat bran where key, highly specialized
bacterial groups have now been identified. It may be both
relevant and appropriate to repeat previous studies attempting to
elucidate the potential prebiotic effect of wheat bran fiber, with
more participants over a longer follow up period, to understand
more clearly the potential benefit (or otherwise) of increasing
wheat bran fiber intake to our gut microbiota.
CONCLUSIONS
The colonic microbiota community must typically be in a state
of continuous change over time, driven by short-term changes
in dietary intake. This review supports a role of intact cereal
fibers in promoting gut microbiota diversity and abundance. The
strongest evidence lies in the role of wheat bran and wholegrain
wheat fiber promoting gut microbiota diversity, as this is the
cereal fiber which demonstrated the most consistent prebiotic
effects on gut microbiota composition both in its intact form
within commonly consumed foods, and in terms of its key
active constituent AXOS, with demonstrable effects arising from
increases in wheat fiber as low as 6 g/day. Individual response
to fiber intervention varied in terms of microbiota response,
however several studies concur that those with the greatest
response were those with the lowest initial target bacterial levels.
Those with the lowest fiber intakes therefore potentially have the
most to gain from increasing fiber intake. Moving forwards it is
important that future studies take account of individual variance
in response and the species within the microbiota responsible for
this to further understanding of potential to personalize dietary
recommendations of fiber intake to fit gut microbiota profile.
With few notable negative side effects reported from
increasing intake of cereal fiber, and evidence accumulating
for a wide array of beneficial health benefits to be gained
from gut microbiota composition, increasing population fiber
intakes remains a key public health goal. As knowledge grows
of our symbiotic relationship with our gut microbes, so does
knowledge of what helps the microbiota composition to increase
in abundance and/or diversity, which at its simplest is to eat
plenty of dietary fiber. Compared to recommended, dietary
intake of fiber remains universally low in Western societies (75).
Continued encouragement of simple dietary changes to increase
intake of intact cereal fibers (for example to choose breakfast
cereal rich in wheat bran, wholegrain wheat or rye breads, and
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Jefferson and Adolphus Cereal Fiber Influences Microbiota Composition
brown rice), should remain a key focus of dietary advice provided
at individual, community, and at national levels.
AUTHOR CONTRIBUTIONS
AJ and KA developed the study premise. Abstraction of data from
articles was undertaken by AJ. AJ developed the initial draft of the
paper and both authors contributed equally to, and approved the
final version of the manuscript.
FUNDING
AJ received a financial grant from the Kellogg Company Europe
to help support the systematic review of the literature reported in
this publication.
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Conflict of Interest Statement: The authors declare that the research was
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