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Research Article
Concurrent Beet Juice and Carbohydrate Ingestion:
Influence on Glucose Tolerance in Obese and Nonobese Adults
Joseph W. Beals, Scott E. Binns, Janelle L. Davis, Gregory R. Giordano,
Anna L. Klochak, Hunter L. Paris, Melani M. Schweder, Garrett L. Peltonen,
Rebecca L. Scalzo, and Christopher Bell
Department of Health and Exercise Science, Colorado State University, Fort Collins, CO 80523-1582, USA
Correspondence should be addressed to Christopher Bell; christopher.bell@colostate.edu
Received July ; Accepted December ; Published January
Academic Editor: A. Venketeshwer Rao
Copyright © Joseph W. Beals et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Insulin resistance and obesity are characterized by low nitric oxide (NO) bioavailability. Insulin sensitivity is improved with
stimulation of NO generating pathways. Consumption of dietary nitrate (NO−) increases NO formation, via NO−reduction to
nitrite (NO−) by oral bacteria. We hypothesized that acute dietary nitrate (beet juice) ingestion improves insulin sensitivity in obese
but not in nonobese adults. nonobese (body mass index: 26.3 ± 0.8 kg/m2(mean ±SE)) and obese adults (34.0 ± 0.8 kg/m2)
ingested beet juice, supplemented with g of glucose (carbohydrate load: g), with and without prior use of antibacterial
mouthwashtoinhibitNO
−reduction to NO−. Blood glucose concentrations aer beet juice and glucose ingestion were greater
in obese compared with nonobese adults at and minutes (𝑃 = 0.004). Insulin sensitivity, as represented by the Matsuda
Index (where higher values reect greater insulin sensitivity), was lower in obese compared with nonobese adults (𝑃 = 0.009).
Antibacterial mouthwash rinsing decreased insulin sensitivity in obese (5.7 ± 0.7 versus 4.9 ± 0.6) but not in nonobese (8.1 ± 1.0
versus 8.9 ± 0.9) adults (𝑃 = 0.048). In conclusion, insulin sensitivity was improved in obese but not in nonobese adults following
coingestion of beet juice and glucose when oral bacteria nitrate reduction was not inhibited. Obese adults may benet from ingestion
of healthy nitrate-rich foods during meals.
1. Introduction
Insulin resistance and obesity, two common comorbidities,
are both characterized by low nitric oxide (NO) bioavailabil-
ity [–]. It is likely that this low NO bioavailability con-
tributes directly to insulin resistance [, ], potentially via dis-
rupted lipid handling, increased fat mass, and/or decreased
glucose delivery [, , ]. For example, compared with healthy
wild-type mice, mice lacking the gene for endothelial nitric
oxide synthase (eNOS) develop insulin resistance and hyper-
tension []. In rats, disruption of NO generation via nitric
oxide synthase inhibition attenuates insulin mediated glucose
uptake []. In humans, the D and IVS + C alleles of
the eNOS gene are more frequently observed in adults with
type diabetes compared with healthy controls [, ]. In light
of these observations, increasing NO bioavailability may be
an eective strategy to increase insulin sensitivity []. In this
regard, ingestion of dietary nitrate may hold promise.
During the previous decade, the remarkable eects of
dietary nitrate, as beet (root) juice, have been described; these
eects include decreased oxygen cost of standardized physi-
cal activity [], increased exercise performance and fatigue
resistance [, ], and improved regulation of blood ow [,
]. Considerably less attention has been given to the poten-
tial inuence of dietary nitrate supplementation on insulin
resistance and metabolic syndrome. In rodent models of
obesity/diabetes, dietary nitrate supplementation improved
insulin signaling and promoted glucose transporter translo-
cation [, ]. Further, in eNOS-decient mice, weeks
of dietary nitrate supplementation normalized blood glucose
tolerance and glycosylated hemoglobin []. Compared with
animal studies, data from human studies are sparse and do
not consistently demonstrate a favorable benet of dietary
nitrate as it pertains to glucose control [–]. Potential rea-
sons for these inconsistent and/or negative outcomes include
Hindawi
Journal of Nutrition and Metabolism
Volume 2017, Article ID 6436783, 7 pages
https://doi.org/10.1155/2017/6436783
Journal of Nutrition and Metabolism
issues pertaining to nitrate dosing and the short half-lives of
NO and circulating nitrate/nitrite following ingestion.
e purpose of the study described herein was to deter-
mine the inuence of coingestion of dietary nitrate (beet
juice: BJ) with glucose on glucose tolerance in obese adults, a
population with presumably low NO bioavailability [, , ],
and a healthy group of nonobese adults. Dietary nitrate sup-
plementation via BJ ingestion is thought to increase NO bio-
availability through a serious of reactions: nitrate (NO−)is
reduced to nitrite (NO−) by commensal bacteria in the oral
cavity; nitrite is then converted to NO through interaction
with a variety of substances in the gut and systemic circula-
tion, including deoxygenated hemoglobin, xanthine oxidase,
polyphenols, and ascorbic acid [, ]. e reduction of
nitrate to nitrite can be inhibited with prior use of antibac-
terial mouthwash [, –], thus abrogating the NO-
mediated benecial eects of BJ consumption and serving
as an experiment control. Accordingly, we hypothesized, in
obese adults, oral glucose tolerance would be superior follow-
ing coingestion of BJ plus glucose (BJ + Gluc) without prior
use of antibacterial mouthwash, compared with ingestion of
BJ + Gluc following prior mouthwash use. Additionally, we
hypothesized that use of mouthwashwould not inuence oral
glucose tolerance in nonobese adults following BJ + Gluc
ingestion. As an additional experimental control, to deter-
mine the independent eects of mouthwash on glucose tole-
rance, water plus glucose was ingested with and without prior
mouthwash use.
2. Methods
2.1. Research Participants. Adult members of the university
campus and local community were invited to participate
in the study. Inclusion criteria included the following: age
within the range – years, sedentary lifestyle (≤ min/day
of exercise, ≤ days/week), and weight stability (± kg)
during the previous months. Exclusion criteria included the
following: current or past use (previous years) of tobacco
products, pregnancy or nursing, use of vitamins, supplements
or medications known to inuence glucose tolerance, history
of diabetes, and concurrent participation in another study.
Routine use of mouthwash was not an exclusion crite-
rion. Participants were classied as obese when their body
mass index (BMI) was ≥ kg/m2. e Institutional Review
Board at Colorado State University approved the experimen-
tal protocol. e nature, purpose, and risks of the study
were explained to each research participant before written
informed consent was obtained.
2.2. Experimental Design. Tw o modie d oral gl ucose t oler-
ance tests were administered, on two separate occasions, in a
random order, separated by a minimum of and a maximum
of days (dictated by research participant availability). A
schematic of the events is presented in Figure . Participants
reported to the laboratory early in the morning following
-hour fast and -hour abstention from vigorous physical
activity. In addition, participants were informed as to the
types of foods known to be rich in nitrates, such as leafy
Glucose
Insulin
Mouthwash
glucose
50 10 20 30 45 60 90 120 min
Beet juice +
−5
3×1min
F : Schematic of experimental visits. Two modied oral glu-
cose tolerance tests were administered, on two separate occasions,
in a random order. At the start of one visit, participants completed
× min mouthwash regimen. Without delay, they then ingested beet
juice supplemented with glucose within minutes. Venous blood
(∼– mL) was sampled repeatedly over minutes and analyzed
for concentrations of glucose and insulin. See text for more details.
green vegetables, beets, and cured meats and instructed to
abstain from these foods during the hours prior to each
laboratory visit. No additional attempt was made to control,
record, or reproduce eating behavior. To prevent inadvertent
removal of commensal bacteria in the oral cavity, participants
also abstained from teeth cleaning, ossing, and use of
mouthwash during the hours prior to each test. Within ve
minutes, participants ingested mL of BJ (Biotta, Carmel,
Indiana, USA), estimated to contain approximately mmol
of nitrate and supplemented with g of glucose (total car-
bohydrate load: g), with and without a prior antibacterial
mouthwash procedure: one s rinse with mL of .%
H2O2(Peroxyl; Colgate Oral Pharmaceuticals, Inc., New
York, NY) followed by two s rinses with mL of antibacte-
rial mouthwash (chlorhexidine digluconate; Corsodyl, BCM
Ltd., Nottingham, UK). Venous blood (∼ mL) was sampled
immediately prior to BJ + Gluc ingestion and again aer ,
,,,,,,andminutesandanalyzedimmedi-
ately for concentrations of glucose (automated device:
Stat Plus, Yellow Springs Instruments, Yellow Springs, Ohio).
Venous blood (∼ mL) was also sampled immediately prior
to BJ + Gluc ingestion and again aer , , , and
minutes and analyzed for insulin concentrations (enzyme-
linked immunosorbent assay, ALPCO Diagnostics, Salem,
NH). Insulin sensitivity was estimated via the Matsuda
Index []. If insulin sensitivity was greater without prior
mouthwash, one possible explanation was that mouthwash
blunted glucose tolerance. To explore this potential con-
founding issue, during two additional/separate visits, a subset
of participants who completed the BJ experiments ingested
water, supplemented with g of glucose, with and without
prior mouthwash use.
2.3. Statistical Analysis. Analysis of variance (ANOVA), with
repeated measures when appropriate, was used to compare
insulinsensitivityandcirculatingglucoseandinsulincon-
centrations in nonobese and obese adults. To determine
the independent eects of mouthwash on glucose tolerance,
ANOVA was also used to compare circulating glucose and
Journal of Nutrition and Metabolism
130
120
120
110
100
90
80
70
0 20406080100
Glucose (mg/dL)
Time (minutes)
∗
∗
Nonobese BJ Nonobese BJ+MW
Obese BJ +MW Obese BJ
(a)
Nonobese BJ Nonobese BJ+MW
Obese BJ +MW Obese BJ
1200 20 40 60 80 100
Time (minutes)
Insulin (𝜇IU/mL)
0
20
40
60
80
120
100
(b)
Nonobese
Obese
BJBJ +MW BJBJ +MW
Matsuda Index
0
2
4
6
8
10
12
14
16
18
20
P = 0.009
P = 0.048
(c)
F : Blood glucose concentration aer beet juice plus glucose consumption was greater in the obese compared with the nonobese
adults at and minutes (𝑃 = 0.004 and denoted by ∗). Inhibition of oral bacteria nitrate reductase activity with mouthwash did not
inuence blood glucose or insulin in either group (𝑃 > 0.08). Insulin sensitivity, as represented by the Matsuda Index (where a higher value
is reective of greater insulin sensitivity), was lower in obese adults compared with nonobese adults (𝑃 = 0.009). Inhibition of oral bacteria
nitrate reductase activity with mouthwash decreased insulin sensitivity in obese adults but not in nonobese adults (𝑃 = 0.048). Glucose and
insulin: data are mean and standard error. Matsuda Index: lines represent individual responses; stand-alone circles represent mean values.
insulin concentrations and insulin sensitivity. Multiple com-
parisons of factor means were performed using Newman-
Keuls test. e level of statistical signicance was set at 𝑃<
0.05. Data are reported as mean ±SE.
3. Results
nonobese ( males and females; BMI: 26.3 ± 0.8 kg/m2;
age: 25 ± 3 years) and obese adults ( males and
females; BMI: 34.0± 0.8 kg/m2,age:43 ±4 years) were tested.
Bloodglucoseandplasmainsulinconcentrations,together
with insulin sensitivity, are displayed in Figure . Blood glu-
cose concentration aer BJ + Gluc consumption was greater
inobesecomparedwithnonobeseadultsatandminutes
(𝑃 = 0.004). Inhibition of oral bacteria nitrate reductase
activity with mouthwash did not inuence glucose or insulin
in either group (𝑃 > 0.08). Insulin sensitivity, as represented
by the Matsuda Index (where a higher value reects a greater
insulin sensitivity), was lower in obese compared with non-
obese adults (𝑃 = 0.009). Antibacterial mouthwash rinsing
decreased insulin sensitivity in obese but not in nonobese
adults (𝑃 = 0.048). Although the current study was not
designed with the intention of making meaningful sex com-
parisons, three-way analysis of variance of the Matsuda Index
data revealed no main eect of sex (𝑃 = 0.916) and no inter-
action between sex and obesity status (𝑃 = 0.882), sex and
beet juice with/without mouthwash (𝑃 = 0.965), and sex,
obesity status, and beet juice with/without mouthwash (𝑃=
0.581).
In a subset comprising both nonobese (𝑛=3)and
obese (𝑛=6) adults ( males and females; BMI: 33.7 ±
0.9kg/m2;age:45 ± 4 years), mouthwash did not aect blood
Journal of Nutrition and Metabolism
150
140
130
120
120
110
100
90
80
70
0 20406080100
Glucose (mg/dL)
Time (minutes)
Wat e r +glucose
Wat e r +glucose
+mouthwash
(a)
1200 20 40 60 80 100
Time (minutes)
Wat e r +glucose
Wat e r +glucose
+mouthwash
Insulin (𝜇IU/mL)
0
20
40
60
80
100
(b)
Wat e r Wat e r +MW
Matsuda Index
P = 0.24
0
2
4
6
8
10
(c)
F : Inhibition oforal bacteria nitrate reductase activity with mouthwash did not aect blood glucose or insulin concentrations following
consumption of water plus glucose (𝑃 > 0.83); insulin sensitivity, as represented by the Matsuda Index, was unaected (𝑃 = 0.24). Glucose
and insulin: data are mean and standard error. Matsuda Index: lines represent individual responses; stand-alone circles represent mean values.
glucose or insulin concentrations following consumption of
water and glucose (Figure ; 𝑃 > 0.83); similarly, insulin
sensitivity was unaected (𝑃 = 0.24). Visual inspection did
not reveal any obvious obesity-mediated dierences within
the water/mouthwash data.
4. Discussion
e novel ndings of this study were the following: () insulin
sensitivity was improved in obese but not in nonobese adults
following ingestion of BJ + Gluc when oral bacteria nitrate
reduction was not inhibited and () inhibition of oral bacteria
nitrate reduction prior to ingestion of water plus glucose did
not aect insulin sensitivity. e implication is obese adults
at risk of developing insulin resistance may benet from
ingestion of healthy nitrate-rich foods during meals.
e current study is not the rst to investigate the inu-
ence of dietary nitrate on glucose regulation in adult humans
[–,].Usingavarietyoftechniques,includingthehyper-
insulinemic euglycemic clamp and the oral glucose tolerance
test, none of the previous studies report a favorable eect of
either acute or short-term ( weeks) dietary nitrate glucose
regulation [–]. One critical and discriminatory dierence
between these studies and the present study pertains to the
timing of nitrate ingestion. Many of the techniques for assess-
ing glucose regulation require that the research participant
be in a fasted state, making acute delivery of dietary nitrate
problematic. Accordingly, in most of the prior studies, nitrate
ingestion occurred between . and hours prior to assess-
ment of glucose control; however in the present study the
nitrate ingestion (as beet juice) occurred concurrently with
assessment, that is, the BJ composed part of the carbohydrate
challenge. us, the inference may be that, for maximum
benet to glucose control, obese adults should include dietary
nitrate(s) (e.g., beets and leafy green vegetables) as part of
their meal.
Dietarynitrate,intheabsenceofantibacterialmouthwash
use, improved insulin sensitivity during a glucose challenge
in obese but not in nonobese adults. is observation is con-
sistent with prior reports of lower NO bioavailability in obese
compared with nonobese adults [, ], increased NO bio-
availability following BJ ingestion [, ], and improved
insulin sensitivity following stimulation of NO generating
pathways and/or dietary nitrate administration in animal
Journal of Nutrition and Metabolism
models [, ]. Lower NO bioavailability in obese adults
may be attributed to one or more factors, including increased
oxidative stress [, ], chronic inammation [, ],
and/or limited availability of NO substrates such as tetrahy-
drobiopterin (BH4), citrulline, and/or L-arginine [, , ].
In turn, the lower NO bioavailability of obese adults may
contribute to impaired glucose regulation on account of
the important role of NO in pancreatic beta-cell function
[] and insulin dependent and independent mechanisms
of glucose clearance [, ]. Accordingly, the potential for
favorable outcomes for glucose regulation in obese adults
during increased NO bioavailability, mediated by dietary
nitrate ingestion, is intuitively appealing.
ere are several experimental considerations worthy of
discussion. For example, as described within Methods, aside
from instructions to abstain from high nitrate foodstus prior
to glucose tolerance assessment, no attempt was made to
control, record, or reproduce eating behavior. While it is pos-
sible that this lack of control may have somehow biased our
data and inuenced our nal interpretation, we feel that this
is unlikely on account of the following: () the order of oral
glucose tolerance tests with/without mouthwash was ran-
domized and () there is no reason to suspect that nonobese
subjects would change their eating behavior midway through
the study in a manner that was dierent to how the obese
subjects might change their behavior.
In the context of an exploratory study, we believe our
data are suciently provocative to provide impetus for
further investigation. e rigor of subsequent studies could
be improved through a variety of modications, including
the addition of quantication of NO, circulating concentra-
tions of nitrate and nitrite, and nitrate reductase activity of
commensal bacteria. Inclusion of these measurements would
conrm the inuence of antibacterial mouthwash on the
inhibition of nitrite and NO production from nitrate. Further,
insight into basal dierences between obese and nonobese
adults pertaining to NO bioavailability and concentrations
of nitrate and nitrite would also be provided. An additional
consideration is the use of nitrate-free BJ, as previously
described []. is would alleviate the need for the com-
plicated mouthwash procedure utilized in the current study
and also make the additional studies of sweetened water with
and without mouthwash redundant. In the current study, the
estimated nitrate content of the mL of BJ was mmol.
Relative to previous studies, this represents a large volume
of BJ and a high nitrate load and may account for some
of the reported dierences pertinent to glucose control [–
].Itmightbeofvalueforfollow-upstudiestoexplorethe
potential of a dose-response relationship and begin to iden-
tify an optimum nitrate dose. Further, as an alternative to
BJ, nitrate-rich fruits and vegetables (e.g., spinach and celery)
could also be studied. Finally, from a public health perspec-
tive, we infer that obese adults at risk of developing insulin
resistance may benet from ingestion of healthy nitrate-rich
foodsduringmeals.InthecurrentstudyBJwascoingested
with glucose only. To support/refute our inference, follow-up
studies could examine the inuence of BJ coingestion with an
actual, mixed-substrate meal.
5. Conclusions
In summary, we are the rst to demonstrate that the metabolic
response to beet juice combined with glucose is more favor-
able in obese adults when nitrate reductase activity is not
inhibited. e inference is that beet juice ingestion aug-
mented NO bioavailability and promoted insulin sensitivity.
e implication for public health is postprandial glucose
control in obese adults may be improved if dietary nitrate(s)
areincludedaspartoftheirmeal.
Disclosure
Preliminary data from the study described in the current
manuscript were presented in a free communication at the
th International Sport Sciences Congress, Antalya, Turkey,
in November ; an abstract of the study was published to
support this presentation.
Competing Interests
e authors declare that there is no conict of interests
regarding the publication of this paper.
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