Citation: King, K.M.; McKay, T.;
Thrasher, B.J.; Wintergerst, K.A.
Maximal Oxygen Uptake, VO2 Max,
Testing Effect on Blood Glucose Level
in Adolescents with Type 1 Diabetes
Mellitus. Int. J. Environ. Res. Public
Health 2022, 19, 5543. https://
doi.org/10.3390/ijerph19095543
Academic Editors: Juan
Pablo Rey-López and Paul
B. Tchounwou
Received: 30 March 2022
Accepted: 30 April 2022
Published: 3 May 2022
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International  Journal  of
Environmental Research
and Public Health
Article
Maximal Oxygen Uptake, VO2 Max, Testing Effect on Blood
Glucose Level in Adolescents with Type 1 Diabetes Mellitus
Kristi M. King 1,*
, Timothy McKay 2, Bradly J. Thrasher 2,3 and Kupper A. Wintergerst 2,3
1
Department of Health and Sport Sciences, University of Louisville, Louisville, KY 40292, USA
2
Norton Children’s Hospital, Louisville, KY 40202, USA; timothy.mckay@nortonhealthcare.org (T.M.);
bradly.thrasher@louisville.edu (B.J.T.); kupper.wintergerst@louisville.edu (K.A.W.)
3
Wendy Novak Diabetes Center, Pediatric Endocrinology, School of Medicine, University of Louisville,
Louisville, KY 40202, USA
*
Correspondence: kristi.king@louisville.edu
Abstract: Assessing maximal oxygen uptake (VO2 max) is generally considered safe when performed
properly for most adolescents; however, for adolescents with type 1 diabetes mellitus (T1DM),
monitoring glucose levels before and after exercise is critical to maintaining euglycemic ranges.
Limited guidance exists for glucose level recommendations for the pediatric population; there-
fore, the purpose of this retrospective clinical chart review study was to determine the effects of
VO2 max testing on blood glucose levels for adolescents with T1DM. A total of 22 adolescents
(mean age = 15.6 ± 1.8 years; male = 13, 59.1%) with a diagnosis of T1DM participated in a Bruce
protocol for VO2 max from January 2019 through February 2020. A statistically signiﬁcant reduction
in glucose levels between pretest (<30 min, mean = 191.1 mg/dL ± 61.2) and post-test VO2 max
(<5 min, mean = 166.7 mg/dL ± 57.9); t(21) = 2.3, p < 0.05) was detected. The results from this current
study can help guide health and ﬁtness professionals in formulating glycemic management strategies
in preparatory activities prior to exercise testing and during exercise testing.
Keywords: maximal oxygen uptake; VO2 max; blood glucose; type 1 diabetes mellitus (T1DM);
adolescents; exercise testing; pediatric; clinical exercise
1. Introduction
One of the tenets of the sports medicine ﬁeld is to advance and integrate scientiﬁc
research to provide educational and practical applications of exercise science and sports
medicine. For individuals engaging in physical activity at any level, whether it is recre-
ational physical activity or competitive sports, there is clear, scientiﬁcally based guidance
regarding exercise testing and prescription for health and ﬁtness professionals to employ
with healthy individuals as well as those living with chronic illnesses [1]. One component
of health-related physical ﬁtness is cardiorespiratory ﬁtness (CRF), the body’s ability to
perform large-muscle, dynamic, moderate-to-vigorous-intensity exercise for prolonged
periods of time. Assessing the maximal oxygen uptake (VO2 max) the body is able to use
during exercise is an established exercise test for determining CRF and is more predictive
of long-term survival than is any traditional risk factor or other measured physiologic
parameter [2]. VO2 max testing provides a measurement of the relative amount of oxygen
consumption per an amount of work. For example, an improved VO2 may allow one to
run longer at the same speed or faster with the same relative effort [3].
The graded exercise test used to elicit VO2 max is aggressive in nature to achieve
a maximal response from the participant. Under stress conditions, the hypothalamus
controls many hormone secretions to adjust glucose metabolism and energy production.
Glucose secretion and uptake are under the control of nervous and hormonal factors
such as catecholamines, cortisol, glucagon, growth hormone, and insulin, and all have
an immediate impact [4]. Even though exercise testing is generally considered safe when
Int. J. Environ. Res. Public Health 2022, 19, 5543. https://doi.org/10.3390/ijerph19095543
https://www.mdpi.com/journal/ijerph
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performed properly for most individuals, maximal- or vigorous-intensity exercise testing
does pose some risk [5–8]. Speciﬁcally, for individuals with type 1 diabetes mellitus
(T1DM), the risk of hyperglycemia in the initial portion of exercise testing and the risk
for hypoglycemia following completion of testing both present themselves. Monitoring
glucose levels before and after physical activity is fundamental to maintaining glucose
levels in euglycemic ranges during and after exercise [9].
Unfortunately, understanding of safety parameters and the effect of CRF testing on
adolescent populations is limited and in need of research [10]. Glucose level recommen-
dations have yet to be established for adolescents diagnosed with T1DM who participate
in VO2 max exercise testing. Although a decrease in glucose may be expected throughout
and immediately after exercise testing, a minimal pretest glucose setpoint has not been
established to reduce the risk of hypoglycemia. Therefore, the purpose of this study was to
examine the impact of VO2 max testing on blood glucose levels for adolescents with T1DM.
2. Materials and Methods
2.1. Study Design and Setting
This cross-sectional, non-interventional, retrospective chart review study was con-
ducted at a nationally certiﬁed pediatric diabetes care and academic medical center located
in the Southeast region of the United States. At this center, pediatric endocrinologists,
registered nurses, registered dieticians, certiﬁed diabetes educators, and clinical exercise
physiologists treat pediatric patients diagnosed with T1DM up to 26 years of age. The
study was approved by the University Institutional Review board. Retrospective clinical
chart reviews were conducted of clinical pediatric sports medicine and physical activity
program patients with a diagnosis of T1DM who participated in a Bruce protocol for VO2
max from January 2019 through February 2020.
2.2. Participant Characteristics
The baseline characteristics of study participants are displayed in Table 1.
Table 1. Characteristics of the Participants, N = 22.
Characteristics
Mean ± SD or n, %
Range
(Minimum–Maximum)
Age, years
15.6 ± 1.8
13–20
Duration of T1DM diagnosis, years
7.1 ± 4.9
>1–16
Height, centimeters
170.9 ± 8.3
157.3–187.4
Weight, kilograms
67.1 ± 13.4
44.6–107.3
BMI percentile, n = 21
67th percentile ± 17.7
26–99
HbA1c level, n = 21
8.9 ± 1.8
6.1–14.9
Gender
Male, n = 13, 59.1%
Female, n = 9, 40.9%
-
Ethnicity
Non-Hispanic, n = 20, 90.1%
-
Race
White, n = 17, 77.3%
Black or African American, n = 3, 13.6%
Unknown or Not Reported, n = 2, 9.1%
-
Treatment Plan
CGM only, n = 2, 9.1%
Insulin pump only, n = 4, 18.2%
Insulin pump integrated with CGM, n = 9, 40.9%
MDI, n = 7, 31.8%
Note. Data are presented as mean ± standard deviation (SD) or number of participants (n), percent (%);
BMI, body mass index percentile; HbA1c, hemoglobin A1c; CGM, continuous glucose monitor; MDI, multi-
ple daily injections.
Int. J. Environ. Res. Public Health 2022, 19, 5543
3 of 7
2.3. Measures
Socio-demographic, anthropometric, diabetes monitoring and treatment plans, and
hemoglobin A1c (HbA1c) levels data were retrieved from patients’ medical records, main-
tained in the clinical database, as part of standard practice at each patient’s appointment in
the diabetes clinic. The socio-demographic and anthropometric characteristics utilized were
participant’s age, date and duration of T1DM diagnosis, ethnicity, race, gender, insurance
type, and body mass index (BMI) percentile. Diabetes monitoring was assessed (includ-
ing whether the participant used a continuous glucose monitor (CGM)), and the type of
treatment plan was recorded. The hemoglobin A1c (HbA1c) level was obtained from that
day’s clinical lab measures at check-in. HbA1c, the most prevalent and accessible measure
in determining glucose control, was used as an indicator of the average blood glucose
levels over the past 3 months. Adolescents managing T1DM should strive for HbA1c levels
less than 7%, as an elevated HbA1c level is known to increase the risk for diabetes-related
complications [11]. Blood glucose levels and VO2 max data were obtained from sports
medicine records collected by clinical exercise physiologists in the sports medicine clinic.
2.4. Preparatory Activities Prior to Exercise Testing
Upon registration for an exercise testing appointment, participants were instructed to
not eat a heavy meal two hours prior to testing, to maintain their insulin regimen as they
would on a regular day, and to dress in exercise attire (e.g., shorts, t-shirt, athletic shoes).
Upon arrival to the sports medicine clinic, participants’ blood glucose value was screened
by a clinical exercise physiologist. If the blood glucose was >250 mg/dL the clinical exercise
physiologist obtained urinary ketones with the next void. If urinary ketones were moderate
or large, the participant was excluded from participating in VO2 max testing at that time. If
the blood glucose was >300 g/dL and the participant had no or small ketones, the clinical
exercise physiologist instructed the participant to give a conservative insulin correction of
50% their calculated correction dose.
Approximately 30 min prior to VO2 max testing, a pretest blood glucose sample was
taken using a point-of-care glucometer and lancet device. Upon determination that blood
glucose levels were in the safe range for physical activity, the clinical exercise physiologist
conducted the VO2 max test. Upon completion of the VO2 max test, a post-test glucose
level check was conducted within 5 min using the same glucometer for all participants.
2.5. Exercise Testing Procedures
A Bruce protocol [12] for VO2 max tests, a valid and reliable measure for assessing
cardiorespiratory ﬁtness, was performed on a Woodway ELG treadmill while the partic-
ipants were connected to the Parvo Medics metabolic gas exchange analyzer by way of
respiratory mask. Participants walked on a treadmill in 3 min bouts, starting at 1.7 mph
(45.6 m·min−1) and 10% grade. At each stage, the speed was increased by either 0.8
or 0.9 mph (21.4 or 24.1 m·min−1) and the grade was increased by 2%. This test lasted
approximately 10–20 min.
If following the graded exercise test the participant’s blood glucose was found to be
<70 mg/dL on the glucometer, the participant was treated for hypoglycemia with 15 g
of rapid-acting carbohydrate. Blood glucose was then rechecked at 15 min. This process
was repeated until their blood glucose was >70 mg/dL. Blood glucose and VO2 max data
stored within REDCap on a secure server in the sports medicine program data were linked
to the clinical database by the researchers utilizing the patients’ medical record numbers.
All clinical data were retrieved from that same-day appointment for each participant. Once
data were collected and merged, the full dataset was de-identiﬁed for analysis.
2.6. Data Analysis
All statistical analyses were conducted using IBM SPSS 27.0 (IBM Corp., Armonk,
NY, USA). Descriptive statistics and frequencies for socio-demographic, anthropometric,
diabetes monitoring and treatment plans, HbA1c levels, and pre- (<30 min) and post-
Int. J. Environ. Res. Public Health 2022, 19, 5543
4 of 7
test (<5 min) blood glucose levels were calculated. Shapiro–Wilk’s test (p < 0.05) [13,14],
histograms, Norman Q–Q plots, and box plots were employed to test the normality of
the distribution of the data. A paired-samples t-test was employed to detect differences
in blood glucose levels from pretest to post-test. p-Values of <0.05 were considered as
statistically signiﬁcant.
3. Results
Retrospective VO2 max data were analyzed from a total of 22 adolescents (N = 22;
mean age = 15.6 ± 1.8 years; male = 13, 59.1%) (see Table 1). Most of the participants
identiﬁed as non-Hispanic (n = 20, 90.9%), and over three-quarters identiﬁed as White
(n = 17, 77.3%). Continuous glucose monitors were worn by 13 of the 22 participants
(59.1%). Their average HbA1c prior to participating in the VO2 max test was 8.9% ± 1.8.
The average BMI, based on age and sex, was in the 67th percentile ± 17.7. The average VO2
max peak was 43.4 mL/kg/min ± 6.4 (See Table 2).
Table 2. VO2 max testing measurements, N = 22.
Characteristics
Mean ± SD
Range
(Minimum–Maximum)
VO2 max, mL/kg/min, n = 21
43.4 ± 6.4
29.3–50.5
Peak HR bpm, n = 19
192.3 ± 21.3
119–212
Glucose, mg/dL pretest, n = 22
191.1 ± 61.1
96–296
Glucose, mg/dL post-test, n = 22
166.7 ± 57.9
83–297
Note. Data are presented as mean ± standard deviation (SD); Peak HR bpm, heart rate beats per minute.
Pre- and post-test blood glucose measurements were obtained from 22 participants.
The results of a Shapiro–Wilk’s test indicated that the pre- and post-glucose data were
normally distributed, and a visual inspection of their histograms, Norman Q–Q plots,
and box plots showed that the glucose scores were normally distributed at pretest with
a skewness of 0.107 (SE = 0.49) and a kurtosis of −0.868 (SE = 0.95) and at post-test
with a skewness of 0.657 (SE = 0.49) and a kurtosis of −0.015 (SE = 0.95). A paired-
samples t-test was employed to detect a statistically signiﬁcant reduction in glucose levels
between pretest (<30 min, mean = 191.1 mg/dL ± 61.2) and post-test VO2 max (<5 min,
mean = 166.7 mg/dL ± 57.9); t(21) = 2.3, p < 0.05).
4. Discussion
It is well established that signiﬁcant changes in blood glucose concentration during
physical activity can lead to hypoglycemia or hyperglycemia and, if not prevented or
treated quickly and properly, can lead to a medical emergency [1,9,15–27]. This current
study sought to examine if there was a signiﬁcant drop in blood glucose levels after VO2
max testing, yet it is unique in that it specialized in a pediatric population of adolescents.
Results from a recent retrospective study with adults with T1DM (mean age = 32 years,
SD ± 13; range 18–65 years) who participated in VO2 max exercise testing using a cycle
ergometer did not demonstrate statistically signiﬁcant glucose levels from pretest to post-
test [28], which aligned with similar results from other studies [29,30]. The conﬂicting
results from the present study may be attributed to differences in the age of participants
(and in body composition and hormones) and possibly the modality used during testing.
Given that individuals with T1DM are recommended to participate in daily moderate-
to-vigorous-intensity physical activity [31], and general guidance for glucose targets as
well as nutritional and insulin dose adjustments to protect against exercise-related glucose
excursions are available [9,20,21,26,27], fear of activity-related hypoglycemia has been
regularly cited as a barrier to physical activity [32,33]. Health care providers wishing
to prescribe even modest increases in intensity levels of daily activity, such as walking
and/or jogging, or sport participation that may include moderate-to-vigorous-intensity
Int. J. Environ. Res. Public Health 2022, 19, 5543
5 of 7
activity to their patients with T1DM may consider VO2 max testing as a ﬁrst step in
establishing safety precautions and working toward the adoption and maintenance of
an active lifestyle. For example, participation in sports is touted as a beneﬁcial means
for adolescents to accumulate physical activity [34,35]. However, caution must be taken
if prescribing sport only without the engagement in additional physical activity. This is
because many adolescents who participate in a single sport often do not meet sufﬁcient
physical activity recommendations. A recent study involving 153 children and adolescents
diagnosed with T1DM demonstrated this fact [36]. Although almost two-thirds of the
participants reported playing one or more sports in the previous year, they were only
physically active for at least one hour or more on an average 3.5 days per week, with less
than 8% of the children and adolescents in the study meeting the recommended duration
of one hour and frequency of seven days per week of physical activity.
The results from this current study may help guide health and ﬁtness professionals in
formulating glycemic management strategies in preparatory activities prior to exercise test-
ing and during exercise testing. A pre-exercise glucose level of 90–250 mg/dL is suggested
in order to prevent symptoms of hypoglycemia and to minimize hyperglycemia [9,11,26].
Considering that the adolescents in this current study experienced a 24.4 mg/dL drop in
glucose levels from pretest to post-test, the implications of these results have clinical and
practical importance. These results can and should be used to help inform patients and
practitioners in clinical care decision making and the formulation of glycemic management
strategies. Similar research ﬁndings suggest that patients and clinical care teams under-
stand the glycemic changes that occur during progressive exercise so that nutritional and
medicinal preparatory routines are safely established [28]. To ensure safe exercise perfor-
mance ahead of exercise testing, practitioners, physiologists, and patients should be aware
of the interindividual responses to VO2 max testing and treat each case accordingly. With
the assistance of a clinical exercise physiologist, physicians can incorporate individualized
recommendations for increasing physical activity and/or exercise prescriptions into their
clinical practices [37]. Physicians and medical care teams can prescribe physical activity
and sport participation when designing treatment plans and refer their patients to qualiﬁed
health and ﬁtness professionals such as athletic trainers, strength and conditioning coaches,
and physical educators who coach or train adolescent athletes diagnosed with T1DM.
These protective measures that are grounded in scientiﬁc evidence [9,11,38] suggest that
adolescent patients diagnosed with T1DM can complete maximal exercise testing without
fear of inducing hypoglycemia if the necessary safety precautions as described in this study
are taken.
Limitations and Future Research
Various limitations have been identiﬁed in this study. Although all participants
were instructed to avoid eating greater than 60 g of carbohydrates prior to exercising,
unless hypoglycemic, the study was not controlled for nutrition. Future studies should
analyze dietary practices leading into exercise testing. In addition, participants were also
instructed to administer insulin per their routine standard of care to create a “real-world”
testing situation for this study. Future studies could beneﬁt from more restrictive insulin
use parameters.
The data in this study were derived from a retrospective chart review of clinical pa-
tients who participated in a pre–post VO2 max test at a newly established (2018) clinic
serving only pediatric patients diagnosed with T1DM up to 26 years of age from January
2019 until February 2020. In March 2020, non-emergency clinical operations were sus-
pended due to COVID-19 safety precautions protocol, and sports medicine programming
and study endeavors resumed in August 2021, which further limited our total sample
size. At the time of the study, there were no matched control group data available. Next,
although the clinic houses the only pediatric endocrinology sports medicine program in
the state, the homogeneity of the participants in race and ethnicity does not make the
ﬁndings generalizable to adolescents in other areas. Future research will beneﬁt from a
Int. J. Environ. Res. Public Health 2022, 19, 5543
6 of 7
more extensive and longitudinal review of the pre- and post-VO2 max testing windows to
further understand what variables inﬂuence blood glucose variability
5. Conclusions
The results from this retrospective VO2 max testing study on blood glucose levels in
adolescents with T1DM can add to the scientiﬁc literature for sports medicine programs
that provide clinical care to individuals and their families through patient-centered and
community education as well as clinical research. Regardless of sport or physical activity,
care is focused on improving the health, safety, and athletic performance of every child
and young adult with T1DM. Knowing that a signiﬁcant drop in glucose levels during
VO2 max testing may occur with their adolescent patients, health and ﬁtness professionals
can discuss and implement preventive glycemic management strategies prior to exercise
testing and during exercise testing.
Author Contributions: Conceptualization, K.M.K., T.M., B.J.T. and K.A.W.; methodology, K.M.K.
and T.M.; formal analysis, K.M.K.; investigation, T.M.; data curation, K.M.K.; writing—original draft
preparation, K.M.K.; writing—review and editing, K.M.K., T.M., B.J.T. and K.A.W.; visualization,
K.M.K. All authors have read and agreed to the published version of the manuscript.
Funding: This study was made possible by generous support from the Christensen Family, the
Norton Children’s Hospital Foundation, and the University of Louisville Foundation.
Institutional Review Board Statement: This study was approved by the University’s Institutional
Review Board on 6-18-2020 (Approval # 20.0506).
Informed Consent Statement: This retrospective research study was considered “exempt” by the
Institutional Review Board of University of Louisville.
Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
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Maximal Oxygen Uptake, VO<sub>2</sub> Max, Testing Effect on Blood Glucose Level in Adolescents with Type 1 Diabetes Mellitus.
King, Kristi M,McKay, Timothy,Thrasher, Bradly J,Wintergerst, Kupper A