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Physiological Reports. 2020;8:e14594.
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DOI: 10.14814/phy2.14594
Isotope tracer assessment of exogenous glucose oxidation during
aerobic exercise in hypoxia
Our laboratories have consistently shown that unacclimatized
lowlanders demonstrate a blunted ability to oxidize exog-
enous glucose during relative (O'Hara etal., , 2017, 2019)
or absolute (Margolis et al., 2019; Young et al., 2018) in-
tensity-matched aerobic exercise within hours of hypoxia
exposure. Our data challenge common recommendations to
increase the carbohydrate intake during exercise at high alti-
tude (>2,500m) to fuel exercise metabolism and augment en-
durance capability (Koehle, Cheng, & Sporer,2014). As such,
we read with interest the recent report by Sumi et al. (Sumi,
Hayashi, Yatsutani, & Goto,2020), as their findings, on the
surface, appear to confirm our previous results (Margolis
etal., 2019; O'Hara etal.,2017, 2019; Young etal., 2018).
Their randomized crossover study aimed to investigate the ef-
fects of acute hypoxia on exogenous glucose oxidation in nine
unacclimatized lowlanders performing 30-min of absolute or
relative intensity-matched aerobic exercise. The investiga-
tors’ interpretation of their primary finding was that exoge-
nous glucose oxidation was lower under hypoxic compared to
normoxic conditions when exercise was matched for absolute
rather than relative intensity.
While these data appear confirmatory, there are
methodological limitations in the work by Sumi et al.
(2020) that should be acknowledged. Most noteworthy,
the only glucose provided by Sumi etal. (2020) was the
0.5g of oral 13C-glucose isotope tracer before exercise.
Surprisingly, no additional glucose (i.e., tracee) was pro-
vided. As such, the tracer/tracee (13CO2/12CO2) ratios
measured by Sumi etal. (2020) reflects the oxidation of
a quantitatively trivial amount of exogenous glucose. The
article referenced by Sumi etal. (2020) justifying their
approach for measuring 13C-excertion after only ingest-
ing the tracer noted that the fasting 13C-glucose breath
test was a proposed clinical screening tool designed to re-
flect the efficiency of hepatic energy utilization (Tanaka
etal.,2013). Furthermore, Sumi etal. (2020) do not ap-
pear to calculate exogenous glucose oxidation (Peronnet,
Rheaume, Lavoie, Hillaire-Marcel, & Massicotte,1998).
To calculate exogenous glucose oxidation during exer-
cise, the equations of (Mosora etal., 1981) or Peronnet,
Massicotte, Brisson, & Hillaire-Marcel (1990) should be
used depending upon the study design:
where V̇CO2 is in liters per min, Rexp is the isotopic composi-
tion of expired CO2 after isotope consumption, Rref is the iso-
topic composition of expired CO2 at rest prior to exercise and
isotope ingestion (Mosora etal.,1981) or during exercise with
the ingestion of a placebo (Peronnet etal.,1990), Rexo is the
isotopic composition of the exogenous glucose ingested, and
k is a constant for the volume of CO2 provided by the complete
oxidation of glucose (Peronnet etal.,1998). In one of our pre-
vious studies, we calculated exogenous glucose oxidation after
giving 0.2g oral 13C-glucose diluted in water as a placebo ver-
sus 0.2g oral 13C-glucose diluted with 80g glucose to assess
changes in total and exogenous glucose oxidation during acute
and chronic hypoxia exposure (Young etal.,2018). The negli-
gible amount of glucose provided in tracer form (i.e., our exper-
imental placebo) expectedly yielded 0g of oxidized exogenous
glucose (Young etal.,2018). As such, the amount of exogenous
glucose oxidized by Sumi etal. (2020) should be essentially
0 g. If the authors had used a plasma precursor method they
could have distinguished plasma glucose oxidation from total
carbohydrate oxidation, with the balance between the two rep-
resenting glycogen oxidation, but as stated in the manuscript,
plasma 13C-glucose enrichments were not measured.
Even if the isotope methodology was carried out cor-
rectly, other limitations remain that bring their results into
question. When oral 13C-glucose is used to study exogenous
glucose oxidation, exercise is typically performed for 80 to
120-min, and at least the first 40-min of exercise are not used
in the calculation. The exclusion of the initial portion of the
exercise is to allow the time for the 13C/12C in expired CO2 to
equilibrate with the 13C/12C produced in tissues. Taking the
delay between 13CO2 production in tissues and at the mouth
into account is necessary to ensure the accurate assessment
of exogenous glucose oxidation (Peronnet etal.,1998). The
30-min exercise bout used by Sumi etal. (2020) was likely
Exogenous Carbohydrate Oxidation (g
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© 2020 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society
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too short for this equilibration to occur. Such computations
have been demonstrated to underestimate exogenous carbo-
hydrate oxidation.
In conclusion, while it appeared that results from
Sumi etal. (2020) confirmed previously published find-
ings from our laboratories (Margolis etal.,2019; O'Hara
etal.,2017, 2019; Young etal.,2018), careful examination
of their methodological approach revealed several limita-
tions that preclude drawing any conclusions regarding the
effects of acute hypoxia exposure on exogenous glucose
oxidation during aerobic exercise. However, it is clear that
exogenous glucose oxidation is lower in unacclimatized
lowlanders performing aerobic exercise matched for rel-
ative (O'Hara et al., 2017, 2019) or absolute (Margolis
etal.,2019; Young etal.,2018) intensities under acute hy-
poxic conditions compared to normoxia. We are certainly
encouraged to see other laboratories reassessing met-
abolic fueling strategies for exercise at high altitude, as
the complex mechanisms contributing to these differences
are likely multifactorial, resulting from lower exogenous
glucose absorption/release from the gut and impaired pe-
ripheral insulin sensitivity and resultant glucose uptake
(Margolis etal., 2019). We hope our letter, which serves
to highlight the complex intricacies associated with iso-
topic assessments of human metabolism, provides the
fundamental methodological basis for studies to employ
when assessing dietary strategies to enhance performance
at high altitude.
None apply.
The authors declare that they have no conflicts of inter-
est relevant to the content of this article. The opinions
or assertions contained herein are the private views of
the authors and are not to be construed as official or as
reflecting the views of the Army or the Department of
This material is based on the work supported by U.S. Army
Medical Research and Development Command.
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Lee M.Margolis1
John P.O’Hara2
Robert W.Wolfe3
Andrew J.Young1
Stefan M.Pasiakos1
1U.S. Army Research Institute of Environmental
Medicine, Natick, MA, USA
2Carnegie School of Sport, Leeds Beckett University,
Leeds, UK
3Department of Geriatrics, Center for Translational
Research in Aging and Longevity, Donald W. Reynolds
Institute on Aging, University of Arkansas for Medical
Sciences, Little Rock, AR, USA
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Lee M. Margolis, Military Nutrition Division, USARIEM,
10 General Greene Avenue, Bldg. 42, Natick, MA 01760,
Lee M. Margolis
John P. O’Hara
How to cite this article: Margolis LM, O’Hara JP,
Griffiths A, Wolfe RW, Young AJ, Pasiakos SM.
Isotope tracer assessment of exogenous glucose
oxidation during aerobic exercise in hypoxia. Physiol.
ResearchGate has not been able to resolve any citations for this publication.
Full-text available
Purpose: Endurance exercise in hypoxia promotes carbohydrate (CHO) metabolism. However, detailed CHO metabolism remains unclear. The purpose of this study was to evaluate the effects of endurance exercise in moderate hypoxia on exogenous glucose oxidation at the same energy expenditure or relative exercise intensity. Methods: Nine active healthy males completed three trials on different days, consisting of 30 min of running at each exercise intensity: (a) exercise at 65% of normoxic maximal oxygen uptake in normoxia [NOR, fraction of inspired oxygen (Fi O2 ) = 20.9%, 10.6 ± 0.3 km/h], (b) exercise at the same relative exercise intensity with NOR in hypoxia (HYPR, Fi O2 = 14.5%, 9.4 ± 0.3 km/h), and (c) exercise at the same absolute exercise intensity with NOR in hypoxia (HYPA, Fi O2 = 14.5%, 10.6 ± 0.3 km/h). The subjects consumed 113 C-labeled glucose immediately before exercise, and expired gas samples were collected during exercise to determine 13 C-excretion (calculated by 13 CO2 /12 CO2 ). Results: The exercise-induced increase in blood lactate was significantly augmented in the HYPA than in the NOR and HYPR (p = .001). HYPA involved a significantly higher respiratory exchange ratio (RER) during exercise compared with the other two trials (p < .0001). In contrast, exogenous glucose oxidation (13 C-excretion) during exercise was significantly lower in the HYPA than in the NOR (p = .03). No significant differences were observed in blood lactate elevation, RER, or exogenous glucose oxidation between NOR and HYPR. Conclusion: Endurance exercise in moderate hypoxia caused a greater exercise-induced blood lactate elevation and RER compared with the running exercise at same absolute exercise intensity in normoxia. However, exogenous glucose oxidation (13 C-excretion) during exercise was attenuated compared with the same exercise in normoxia.
Full-text available
This study investigated how high-altitude (HA, 4300 m) acclimatization affected exogenous glucose oxidation during aerobic exercise. Sea-level (SL) residents (n = 14 men) performed 80-min, metabolically matched exercise ( V ˙ O2 ∼ 1.7 L/min) at SL and at HA < 5 h after arrival (acute HA, AHA) and following 22-d of HA acclimatization (chronic HA, CHA). During HA acclimatization, participants sustained a controlled negative energy balance (-40%) to simulate the "real world" conditions that lowlanders typically experience during HA sojourns. During exercise, participants consumed carbohydrate (CHO, n = 8, 65.25 g fructose + 79.75 g glucose, 1.8 g carbohydrate/min) or placebo (PLA, n = 6). Total carbohydrate oxidation was determined by indirect calorimetry and exogenous glucose oxidation by tracer technique with 13C. Participants lost (P ≤ 0.05, mean ± SD) 7.9 ± 1.9 kg body mass during the HA acclimatization and energy deficit period. In CHO, total exogenous glucose oxidized during the final 40 min of exercise was lower (P < 0.01) at AHA (7.4 ± 3.7 g) than SL (15.3 ± 2.2 g) and CHA (12.4 ± 2.3 g), but there were no differences between SL and CHA. Blood glucose and insulin increased (P ≤ 0.05) during the first 20 min of exercise in CHO, but not PLA. In CHO, glucose declined to pre-exercise concentrations as exercise continued at SL, but remained elevated (P ≤ 0.05) throughout exercise at AHA and CHA. Insulin increased during exercise in CHO, but the increase was greater (P ≤ 0.05) at AHA than at SL and CHA, which did not differ. Thus, while acute hypoxia suppressed exogenous glucose oxidation during steady-state aerobic exercise, that hypoxic suppression is alleviated following altitude acclimatization and concomitant negative energy balance.
Full-text available
This study compared the effects of coingesting glucose and fructose on exogenous and endogenous substrate oxidation during prolonged exercise at altitude and sea level, in men. Seven male British military personnel completed two bouts of cycling at the same relative workload (55% Wmax) for 120 min on acute exposure to altitude (3375 m) and at sea level (~113 m). In each trial, participants ingested 1.2 g·min⁻¹ of glucose (enriched with ¹³C glucose) and 0.6 g·min⁻¹ of fructose (enriched with ¹³C fructose) directly before and every 15 min during exercise. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total and exogenous carbohydrate oxidation, plasma glucose oxidation, and endogenous glucose oxidation derived from liver and muscle glycogen. Total carbohydrate oxidation during the exercise period was lower at altitude (157.7 ± 56.3 g) than sea level (286.5 ± 56.2 g, P = 0.006, ES = 2.28), whereas fat oxidation was higher at altitude (75.5 ± 26.8 g) than sea level (42.5 ± 21.3 g, P = 0.024, ES = 1.23). Peak exogenous carbohydrate oxidation was lower at altitude (1.13 ± 0.2 g·min⁻¹) than sea level (1.42 ± 0.16 g·min⁻¹, P = 0.034, ES = 1.33). There were no differences in rates, or absolute and relative contributions of plasma or liver glucose oxidation between conditions during the second hour of exercise. However, absolute and relative contributions of muscle glycogen during the second hour were lower at altitude (29.3 ± 28.9 g, 16.6 ± 15.2%) than sea level (78.7 ± 5.2 g (P = 0.008, ES = 1.71), 37.7 ± 13.0% (P = 0.016, ES = 1.45). Acute exposure to altitude reduces the reliance on muscle glycogen and increases fat oxidation during prolonged cycling in men compared with sea level.
Background: Exogenous carbohydrate oxidation is lower during steady-state aerobic exercise in native lowlanders sojourning at high altitude (HA) compared to sea level (SL). However, the underlying mechanism contributing to reduction in exogenous carbohydrate oxidation during steady-state aerobic exercise performed at HA have not been explored. Objective: To determine if alterations in glucose rate of appearance (Ra), disappearance (Rd) and metabolic clearance rate (MCR) at HA provide a mechanism for explaining the observation of lower exogenous carbohydrate oxidation compared to during metabolically-matched, steady-state exercise at SL. Methods: Using a randomized, crossover design, native lowlanders (n = 8 males, mean ± SD, age: 23 ± 2 yr, body mass: 87 ± 10 kg, and VO2peak: SL 4.3 ± 0.2 L/min and HA 2.9 ± 0.2 L/min) consumed 145 g (1.8 g/min) of glucose while performing 80-min of metabolically-matched (SL: 1.66 ± 0.14 V̇O2 L/min 329 ± 28 kcal, HA: 1.59 ± 0.10 V̇O2 L/min, 320 ± 19 kcal) treadmill exercise in SL (757 mmHg) and HA (460 mmHg) conditions after a 5-h exposure. Substrate oxidation rates (g/min) and glucose turnover (mg/kg/min) during exercise were determined using indirect calorimetry and dual tracer technique (13C-glucose oral ingestion and [6,6-2H2]-glucose primed, continuous infusion). Results: Total carbohydrate oxidation was higher (P < .05) at HA (2.15 ± 0.32) compared to SL (1.39 ± 0.14). Exogenous glucose oxidation rate was lower (P < .05) at HA (0.35 ± 0.07) than SL (0.44 ± 0.05). Muscle glycogen oxidation was higher at HA (1.67 ± 0.26) compared to SL (0.83 ± 0.13). Total glucose Ra was lower (P < .05) at HA (12.3 ± 1.5) compared to SL (13.8 ± 2.0). Exogenous glucose Ra was lower (P < .05) at HA (8.9 ± 1.3) compared to SL (10.9 ± 2.2). Glucose Rd was lower (P < .05) at HA (12.7 ± 1.7) compared to SL (14.3 ± 2.0). MCR was lower (P < .05) at HA (9.0 ± 1.8) compared to SL (12.1 ± 2.3). Circulating glucose and insulin concentrations were higher in response carbohydrate intake during exercise at HA compared to SL. Conclusion: Novel results from this investigation suggest that reductions in exogenous carbohydrate oxidation at HA may be multifactorial; however, the apparent insensitivity of peripheral tissue to glucose uptake may be a primary determinate.
Purpose: This study compared the co-ingestion of glucose and fructose on exogenous and endogenous substrate oxidation during prolonged exercise at terrestrial high altitude (HA) versus sea level, in women. Method: Five women completed two bouts of cycling at the same relative workload (55% Wmax) for 120 minutes on acute exposure to HA (3375m) and at sea level (~113m). In each trial, participants ingested 1.2 g.min of glucose (enriched with C glucose) and 0.6 g.min of fructose (enriched with C fructose) before and every 15 minutes during exercise. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total and exogenous carbohydrate oxidation, plasma glucose oxidation and endogenous glucose oxidation derived from liver and muscle glycogen. Results: The rates and absolute contribution of exogenous carbohydrate oxidation was significantly lower at HA compared with sea level (ES>0.99, P<0.024), with the relative exogenous carbohydrate contribution approaching significance (32.6±6.1 vs. 36.0±6.1%, ES=0.56, P=0.059) during the second hour of exercise. In comparison, no significant differences were observed between HA and sea level for the relative and absolute contributions of liver glucose (3.2±1.2 vs. 3.1±0.8%, ES=0.09, P=0.635 and 5.1±1.8 vs. 5.4±1.7 grams, ES=0.19, P=0.217), and muscle glycogen (14.4±12.2% vs. 15.8±9.3%, ES=0.11, P=0.934 and 23.1±19.0 vs. 28.7±17.8 grams, ES=0.30, P=0.367). Furthermore, there was no significant difference in total fat oxidation between HA and sea level (66.3±21.4 vs. 59.6±7.7 grams, ES=0.32, P=0.557). Conclusion: In women, acute exposure to HA reduces the reliance on exogenous carbohydrate oxidation during cycling at the same relative exercise intensity.Keys WordsAcute Hypoxia, Carbon Isotope, Exogenous Carbohydrate Oxidation, Liver Glycogen, Muscle Glycogen, Plasma Glucose Oxidation.
: Many sports incorporate training at altitude as a key component of their athlete training plan. Furthermore, many sports are required to compete at high altitude venues. Exercise at high altitude provides unique challenges to the athlete and to the sport medicine clinician working with these athletes. These challenges include altitude illness, alterations in training intensity and performance, nutritional and hydration difficulties, and challenges related to the austerity of the environment. Furthermore, many of the strategies that are typically utilized by visitors to altitude may have implications from an anti-doping point of view.This position statement was commissioned and approved by the Canadian Academy of Sport and Exercise Medicine. The purpose of this statement was to provide an evidence-based, best practices summary to assist clinicians with the preparation and management of athletes and individuals travelling to altitude for both competition and training.
Evaluating hepatic insulin resistance (IR) is the key to making a sensitive an accurate diagnosis of glucose intolerance. However, there is currently no suitable method to perform this procedure. This study was conducted to investigate whether the fasting (13)C-glucose breath test (FGBT) is useful as a convenient and highly sensitive clinical test for evaluating hepatic IR. Healthy nonobese subjects and a disease group consisting of patients with mild glucose intolerance were administered 100 mg (13)C-glucose after an overnight fast. A series of breath samples was collected until 360 minutes after ingestion, and the (13)CO2-to-(12)CO2 ratio was measured using an infrared spectrometer and was plotted as a kinetic curve of (13)C excretion. The area under the curve until 360 minutes (AUC360) of the (13)C excretion kinetic curve of the FGBT reflects the efficiency of energy production in the liver. First, we assessed the correlations between the AUC360 (or the (13)C excretion rate at 120 minutes) and the HOMA-IR and HbA1c levels as standard measurements of IR and diabetes mellitus (DM). There were relatively strong correlation coefficients (r = -0.49 to -0.81, r(2) = 0.24-0.66, P < 0.01; n = 35 males, n = 33 females). Second, we compared the AUC360 of healthy subjects and that of the patients with mild glucose intolerance. The AUC360 of the healthy subjects was consistently higher than that of the patients with mild glucose intolerance. The presence of IR or DM in males and females was diagnosed using cutoff values. The FGBT is a novel glucose metabolism test that can be used conveniently and safely to evaluate the balance of glucose metabolism in the liver. This test has excellent sensitivity for diagnosing alterations in hepatic glucose metabolism, particularly hepatic IR.
The purpose of this study is to outline a common mistake made when the rate of oxidation of exogenous substrates during prolonged exercise is computed using 13C naturally labeled substrates. The equation proposed and commonly used in the computation does not take into account that exercise and/or exogenous substrate ingestion modifies the composition of the mixture of endogenous substrates oxidized and, consequently, the isotopic composition of CO2 arising from oxidation of endogenous substrates. The recovery of 13C and the amount of exogenous substrate oxidized are thus overestimated. An adequate procedure for the computation of exogenous substrate oxidation taking into account changes in isotopic composition of CO2 arising from oxidation of endogenous substrates is suggested. Results from a pilot experiment (4 subjects) using this procedure indicate that over 2 h of exercise (66% of maximal O2 uptake), with ingestion of 60 g of glucose, 39 +/- 4 g of glucose were oxidized. Estimates made without taking into account changes in isotopic composition of CO2 arising from oxidation of endogenous substrates range between 70 +/- 8 and 44 +/- 3 g depending on 1) the isotopic composition of exogenous glucose and 2) the isotopic composition of expired CO2 taken as reference (rest or exercise without glucose ingestion). These observations suggest that results from previous studies of exogenous substrate oxidation during exercise using 13C labeling should be used with caution.
Using "naturally labeled 13C-glucose" the conversion to expired air CO2 of 100, 66 and 33 g oral glucose loads was evaluated, during 7 hr, in 6 male healthy volunteers. The total amounts of exogenous glucose converted to expired air CO2 were proportional to the loading doses and averaged, in g/7 hr, 14.6 +/- 2.1 for 33 g, 20.8 +/- 2.1 for 66 g and 31.2 +/- 1.6 for 100 g. In contrast total glucose oxidation, evaluated by indirect calorimetry, was similar in the three groups and was not correlated with the dose of glucose given. The plasma insulin response was significantly correlated with the load of glucose (r = 0.86; p less than 0.001) whereas such correlation was not observed with the blood glucose response. Comparison with data available in the literature on the influence of the size of the oral glucose load on endogenous glucose production and on splanchnic glucose output suggests that the greater oxidation of exogenous glucose seen when the oral load of glucose is increased essentially results from a greater enrichment of the systemic glucose pool with exogenous glucose.