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Glycogen Utilization during Running: Intensity, Sex, and Muscle-Specific Responses

IMPEY, SAMUEL G.1; JEVONS, EMILY1; MEES, GEORGE1; COCKS, MATT1; STRAUSS, JULIETTE1; CHESTER, NEIL1; LAURIE, IEVA2; TARGET, DARREN3; HODGSON, ADRIAN2; SHEPHERD, SAM O.1; MORTON, JAMES P.1

Author Information
Medicine & Science in Sports & Exercise: September 2020 - Volume 52 - Issue 9 - p 1966-1975
doi: 10.1249/MSS.0000000000002332
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Abstract

Since the introduction of the muscle biopsy technique in the late 1960s (1), the importance of muscle glycogen for augmenting exercise capacity and performance in endurance events has been well documented. In addition to high endogenous carbohydrate (CHO) availability, augmenting exogenous CHO availability (typically via gels, drinks, or bars) is also ergogenic to exercise performance (2), an effect likely mediated by liver (3) and muscle glycogen sparing (4), maintaining plasma glucose and CHO oxidation rates (5,6) and/or via direct effects on the central nervous system (7). When taken together, nutritional guidelines for competitive endurance events recommend sufficient CHO loading (e.g., 7–12 g·kg−1 body mass depending on event duration) to ensure elevated muscle and liver glycogen stores, as well as to consume exogenous CHO when exercise duration is >1 h (8,9).

In contrast to competition, contemporary guidelines for training recognize the need to adjust daily CHO intake according to the goal of enhancing training quality versus creating a metabolic stimulus that may enhance training adaptation (8). In this regard, the emergence of the “train-low” paradigm is based on the premise that periodically completing selected training sessions with reduced CHO availability upregulates acute skeletal muscle cell signaling pathways (10), thereby leading to enhanced oxidative adaptations of the skeletal muscle (11–13) and potentially improved exercise performance and capacity (14,15). From a practical perspective, we recently communicated the concept of CHO periodization according to the principle of fuel for the work required, whereby CHO availability is manipulated day-by-day and meal-by-meal in relation to the demands of the specific training session (16,17).

Despite the theoretical rationale for CHO periodization strategies, practical application is limited by the lack of data quantifying the glycogen cost associated with specific training sessions. Indeed, despite over five decades of research examining glycogen metabolism during exercise, the majority of data are based on laboratory protocols (e.g., fasted exercise undertaken at a fixed relative intensity for a given duration, e.g., 1 h at 70% V˙O2max) that may not always be applicable to the field-based training sessions. For example, the oxygen cost of outdoor running is greater than running on a treadmill (18), and the training intensities prescribed to athletic populations are typically anchored to lactate threshold (as opposed to maximal oxygen uptake) and completed in the fed state. Almost a decade ago, Burke et al. (9) highlighted that CHO guidelines for athletic populations are not underpinned by direct knowledge of the glycogen cost of real-life exercise programs, sentiments that were also communicated in 2016 (8) and 2018 (19). In the former article, the authors suggested that to estimate the substrate requirement of specific workouts, practitioners must rely on guesswork supported by information obtained from consumer-based activity/heart rate (HR) monitors and global positioning systems. In the latter article, the fuel costs, glycogen utilization rates, and associated CHO intake requirements of habitual training sessions were identified as a targeted question for future research (19).

In addition to the specifics of the exercise protocol, differences in muscle group examined (12,20) and sex-specific alterations in substrate metabolism (21) may also affect the absolute glycogen utilization associated with specific training sessions. To our knowledge, however, such comparisons of muscle group, sex, and training intensity have not yet been simultaneously examined within the same study. The paucity of data from female participants was recognized by Devries (22), but is much stronger evidenced in the recent meta-analysis of Areta and Hopkins (23). Indeed, of the 180 studies that assessed glycogen utilization in human skeletal muscle during exercise, less than 5% included female participants.

Accordingly, the aim of the present study was to quantify net glycogen utilization of the vastus lateralis (VL) and gastrocnemius (G) muscles of recreationally active male and female runners during three types of training sessions considered representative of runners’ habitual workouts. We deliberately chose recreationally active runners given that they comprise the largest running population, and hence, our data may have greater practical relevance. To this end, we quantified glycogen use during a 10-mile road run conducted at lactate threshold, an 8 × 800-m track interval session (8 × 800 m) competed at velocity at V˙O2peak, and finally, a 3 × 10-min (3 × 10 min) track interval session undertaken at a velocity corresponding to lactate turnpoint.

METHODOLOGY

Participants

After providing informed written consent, 21 competitive and recreational runners (11 male and 10 female) volunteered to take part in the study. Inclusion criteria consisted of a minimum of 3-yr competitive running experience, habitually training at least three times per week, and 10-km race time ≤45 min for male participants and ≤50 min for female participants. Participants’ anthropometric, training history, and physiological profiles are displayed in Table 1. All procedures conformed to the standards set by the Declaration of Helsinki, and the study was approved by the NHS Research Authority of the United Kingdom (West Midlands, Black Country Research Ethics Committee, REC reference (15)/WM/0428).

TABLE 1
TABLE 1:
Anthropometric profile, training history, and physiological profile of male and female participants.

Design

In a randomized and repeated-measures design, participants completed three sessions considered representative of those undertaken by runners competing in 10-km events (Spillsbury, personal communication, English Institute of Sport). The training sessions consisted of 1) a 10-mile road run (10-mile) undertaken at a velocity corresponding to lactate threshold, 2), an 8 × 800-m track interval session (8 × 800 m) competed at velocity at V˙O2peak, and 3), a 3 × 10-min (3 × 10 min) track interval session undertaken at velocity corresponding to lactate turnpoint. A summary of each training session is also displayed in Table 2. In an attempt to compare with previously published data (24–28) and also considering that glycogen utilization is lower during the luteal phase (24), we deliberately studied female participants (who self-reported) during the midfollicular phase (days 7–10). In this way, male participants had 7–10 d between trials, whereas female participants had 28 d between trials. Female participants were eumenorrheic with a normal cycle length, and inclusion criteria included use of oral contraception (combined pill), diaphragm, or intrauterine device. Muscle biopsies from both the VL and G muscles and venous blood samples were obtained immediately before and after completion of each training session. At 48 h before commencement of each training session, participants completed a standardized training session followed by standardized dietary intakes in an attempt to replicate preexercise muscle glycogen concentration between trials. All food was provided to the participants in pre-prepared packages having been prepared by a registered sports nutritionist (SENr; author 1).

TABLE 2
TABLE 2:
Summary of training protocols.

Baseline Assessments

The running velocity at which each participant completed the three training sessions was determined by completion of a two-part incremental exercise test on a motorized treadmill (HP Cosmos, Nußdorf, Germany) to establish lactate threshold, lactate turnpoint, and peak oxygen uptake (V˙O2peak). Participants reported to the laboratory in a fasted state between 0700 and 0800 h for an initial assessment of body composition via dual-energy x-ray absorptiometry (Hologic QDR Series; Discovery A, Bedford, MA) according to the dual-energy x-ray absorptiometry best-practice protocol (29). Participants were then provided with a standardized breakfast (2 g·kg−1 body mass CHO, 25 g protein, 10 g fat) at 3 h before commencing the incremental exercise test. To replicate outdoor running conditions (18), the test was commenced at 1% incline (at 8 and 10 km·h−1 for female and male participants, respectively) and after a 10-min self-selected warm-up. Oxygen uptake was measured continuously during exercise via breath-by-breath measurement using a CPX Ultima series online gas analysis system (Medgraphics, St. Paul, MN). The treadmill speed was increased by 1 km·h−1 every 3 min and during the final 30 s of each 3-min stage, blood lactate was assessed using capillary blood samples (Lactate Plus; Nova Biomedical, Waltham, MA). Part 1 of the test terminated once both lactate threshold and turnpoint had been visually identified (defined as ≥0.4 and ≥1.0 mmol·L−1 above resting values, respectively) (30). After a 5-min resting period, part 2 of the test commenced at a velocity of 2 km·h−1 below lactate turnpoint, and the treadmill speed was increased by 1 km·h−1 every minute until volitional fatigue or until completion of the 16-km·h1 stage, after which point the treadmill inclined by 1% every minute until volitional fatigue. V˙O2peak was taken as the highest V˙O2 value obtained in any 10-s period matching two of the following criteria: HR within 10 bpm of age-predicted maximum, RER > 1.1, and plateau of oxygen consumption despite increased workload. To calculate velocity at V˙O2peak, the final treadmill speed was used if the velocity was ≤16 km·h−1, and where participants terminated the test during the inclined component at 16 km·h−1, the following equation was used (30):

Experimental Protocol

Day 1

Participants arrived at the laboratory on the evening (1700 h) of day 1 having avoided alcohol and vigorous physical activity for the previous 24 h. Body mass was recorded and an HR monitor (Polar FT1, Kempele, Finland) fitted. Participants then performed an intermittent running protocol on a motorized treadmill (HP, Cosmos) lasting ~90–120 min in an attempt to deplete muscle glycogen and thus allow for exercise–dietary standardization before the outdoor training sessions. This exercise protocol has been used previously in our laboratory (31) and was chosen in an attempt to deplete muscle glycogen in both type I and type II muscle fibers. The activity pattern and total time to exhaustion were recorded, and water was consumed ad libitum throughout exercise. These parameters were repeated exactly during the second and third experimental trials. Within 30 min of completion of the depletion protocol, participants consumed 1.2 g·kg−1 CHO in the form of sports drinks and bars (Lucozade, United Kingdom) and a 25-g whey protein solution (Upbeat Whey, London, United Kingdom). At 2 h after completion of the depletion protocol, participants also consumed a standardized meal containing 2 g·kg−1 CHO, 40 g protein, and 15 g fat.

Day 2

Subjects did not perform any structured training on day 2 and also adhered to a standardized dietary intake of 6 g·kg−1 CHO, 2 g·kg−1 protein, and 1 g·kg−1 fat.

Day 3

After adhering to a further standardized dietary intake of 6 g·kg−1 CHO, 2 g·kg−1 protein, and 1 g·kg−1 fat on day 3 (consumed during the period between 0700 and 1500 h), subjects commenced one of the three training sessions at approximately 1600 h. Both the 8 × 800-m and 3 × 10-min track interval sessions were completed at an outdoor athletics track (Wavertree Athletics track, Liverpool, United Kingdom). The 10-mile road run was commenced on an outdoor course (designed by the first author) that commenced and finished at the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University. Upon arrival at each respective trial location, a resting venous blood sample and muscle biopsies were obtained from the VL and lateral head of the G muscle. Biopsies taken from each muscle group were from opposite legs (i.e., right VL and left G), and subsequent trials sampled the opposite leg to the previous trial. Muscle samples were immediately snap frozen in liquid nitrogen. Participants wore a GPS watch (Garmin Fore Runner 620) and HR monitor (Garmin) during exercise in all trials to verify the correct exercise intensity. During the 10-mile road run, the first author accompanied the participant (via cycling alongside). During the 8 × 800-m track run, capillary blood lactate was also sampled in the 30 s after completion of intervals 2, 4, 6, and 8 (Lactate Plus; Nova Biomedical). During the 3 × 10-min track interval run, capillary blood lactate was also recorded at the end of each 10-min interval. Upon completion of all three exercise trials, postexercise venous blood sample and VL and G muscle biopsies (at 2 cm distal to the preexercise biopsy from the same leg) were also obtained.

Muscle Biopsies

Muscle biopsies were obtained from the VL and lateral head of the G muscle within 5 min prior to commencing and completing each training session (Bard Monopty Disposable Core Biopsy Instrument 12-gauge × 10-cm length; Bard Biopsy Systems, Tempe, AZ). Samples were obtained under local anesthesia (0.5% marcaine) and immediately frozen in liquid nitrogen and stored at −80°C for later analysis.

Muscle Glycogen Concentration

Muscle glycogen concentration was determined according to the acid hydrolysis method described by van Loon et al. (32), with glucose concentration quantified using a commercially available kit (GLUC-HK; Randox Laboratories, Antrim, United Kingdom).

Blood Analysis

Venous blood samples were collected in vacutainers containing K2 EDTA, lithium heparin, or serum separation tubes, and stored on ice until centrifugation at 1500g for 15 min at 4°C. Plasma samples were aliquoted and stored at −80°C until analysis. Plasma glucose, lactate, nonesterified fatty acids (NEFAs), and glycerol were analyzed using the Randox Daytona spectrophotometer with commercially available kits (Randox, Crumlin, Ireland), as per the manufacturer’s instructions.

Statistical Analysis

Randomization of training sessions was balanced for both male and female participants by stratifying the randomization by sex. The randomization schedule was generated according to a Williams square for a 3 × 3 crossover study. The planned sample size of 20 participants completing the study had 90% power to detect a glycogen utilization of 85 mmol·kg−1 dry muscle (SE = 34) for each of the exercise protocol sessions (as based on similar running training sessions studied in our laboratory (10,12), at the two-sided 1.7% significance level (the effects of sex were studied as an exploratory analysis). The study outcomes were analyzed using a linear mixed model for parameters that were considered normally distributed, and nonparametric methods (Wilcoxon signed ranks test), otherwise. In the linear model, the dependent variable was the outcome of interest, and the independent variables included main effects for participant, exercise protocol, study period, sex, muscle type, and baseline (where applicable). Interactions between sex and muscle type with exercise protocol were evaluated and were retained in the model if comparisons were to be made within subgroups; otherwise and if nonsignificant (P > 0.05), these interaction terms were dropped from the model. Parameters associated with the anthropometric profile, training profile, and physiological profile were compared between male and female participants using a t-test. P values were not adjusted for multiplicity.

RESULTS

Overview of training workloads

A comparison of workloads (i.e., exercise duration and distance ran) between each training session in male and female participants is displayed in Table 2. The time taken to complete the exercise protocols with set distances (i.e., the 10-mile and 8 × 800-m sessions) was significantly different between sexes such that male participants completed both training sessions faster than female participants (P < 0.001). In male participants, the total exercise duration was different between training protocols such that 10 mile >3 × 10 min > 8 × 800 (P < 0.01 for all comparisons). Similarly, in female participants, the time required to complete the 10-mile road run was slower than both the 8 × 800- and 3 × 10-min session (P < 0.001), although no difference was apparent between the track training sessions. In relation to the training protocol with set duration (i.e., the 3 × 10-min track training session), male participants completed more distance compared with female participants (P < 0.001).

Physiological and metabolic responses to training

Changes in plasma metabolites during exercise are displayed in Figure 1. Plasma glucose did not significantly change during any of the exercise protocols (P = 0.6, 0.9, and 0.8 for 10-mile, 8 × 800-m, and 3 × 10-min, respectively). In contrast, exercise increased both NEFA (P < 0.01 for all exercise protocols) and glycerol (P < 0.01 for all exercise protocols) in all training protocols, whereas exercise only increased plasma lactate (P = 0.002) in the 8 × 800-m protocol. In relation to plasma NEFA, exercise-induced changes in NEFA were greater in the 10-mile session compared with both 8 × 800-m (P = 0.004) and 3 × 10-min protocols (P = 0.003), whereas the 3 × 10-min protocol was also significantly greater than the 8 × 800-m protocol (P = 0.006). Finally, plasma glycerol responses were also significantly greater in both the 10-mile (P = 0.02) and 8 × 800-m (P = 0.02) when compared with the 3 × 10-min protocol. Although female participants displayed significantly greater increases in plasma glycerol than did male participants in the 10-mile run (P = 0.01), there was no difference in plasma metabolite responses between male and female participants in the remaining training protocols (P > 0.05 for all comparisons).

FIGURE 1
FIGURE 1:
Plasma lactate (A), glucose (B), NEFA (C), and glycerol (D) before and after exercise in the 10-mile road, 8 × 800-m, and 3 × 10-min track runs (panels left to right, respectively). *Significant effect of exercise, P < 0.05. aSignificant difference from 10-mile and 3 × 10-min training sessions, P < 0.05. bSignificant difference from the 3 × 10-min training session, P < 0.05.

Resting muscle glycogen concentration and glycogen utilization during training

Muscle glycogen concentration before and after each training protocol is displayed in Figure 2, where statistical comparisons of training protocol, sex, and muscle group on resting glycogen concentration are visually annotated. When comparing resting muscle glycogen concentration between exercise protocols, no significant differences were evident within each sex and muscle (P > 0.05), with the exception of the G muscle in the female participants where pretraining glycogen concentration was lower in the 8 × 800-m protocol compared with both the 10-mile road run (P = 0.02) and the 3 × 10-min track run (P = 0.01).

FIGURE 2
FIGURE 2:
Muscle glycogen in the G and VL muscles before and after the 10-mile road run (A and B), the 8 × 800-m track run (C and D), and the 3 × 10-min track run (E and F). *Significantly lower resting concentration in the G muscle of female compared with male participants, P < 0.05. aSignificantly lower resting concentration within the G muscle of female participants when compared with the 10-mile and 3 × 10-min trials, P < 0.05. bSignificantly higher resting concentration in the G muscle of male participants compared with the VL muscle, P < 0.05. For statistical comparisons of training protocol, sex, and muscles on glycogen utilization, please see Table 2.

In relation to sex-specific differences in resting muscle glycogen concentration, female participants displayed reduced muscle glycogen concentration in the G muscle when compared with male participants (P < 0.001), although no such differences between sexes were apparent in the VL muscle (P = 0.40). In relation to differences in resting glycogen concentration between muscles, the G muscle displayed higher glycogen concentration than the VL in male participants (P < 0.01), although no such differences were evident in female participants (P = 0.78; Fig. 1).

Total muscle glycogen utilization during exercise (as calculated from pretraining minus posttraining values) is presented in Table 3. Within the G muscle of male participants, there was a significant difference between training protocols such that 10-mile >8 × 800-m > 3 × 10-min (P < 0.05 for all comparisons). Similarly, glycogen utilization within the VL muscle of male participants was greater in the 10-mile compared with both the 8 × 800-m and 3 × 10-min (P < 0.01 for both comparisons), although no differences were apparent between the track running sessions (P = 0.64). In contrast, total glycogen utilization in the female participants was not statistically different between training protocols in both the G and VL muscles (P > 0.05 for all comparisons). When comparing sex-specific responses, total glycogen utilization was greater in male than female participants in both the VL (P = 0.02) and G (P = 0.07) muscle during the 10-mile road run only. With the exception of male participants during the 3 × 10-min protocol (P = 0.28), greater absolute glycogen utilization was observed in the G versus the VL muscle in both male and female participants and during all training protocols (P < 0.05 for all comparisons; Table 3).

TABLE 3
TABLE 3:
Total muscle glycogen utilization in G and VL muscles groups during each training protocol.

Rates of muscle glycogen utilization (as calculated by total glycogen utilization divided by training duration) are presented in Table 4. In male participants within the G muscle, there was a significant difference between training protocols such that 8 × 800-m > 3 × 10-min > 10-mile road run (P < 0.05 for all comparisons). Similarly, rates of glycogen utilization within the VL muscle of male participants were greater in the 8 × 800-m compared with the 10-mile road run (P = 0.003), although no differences were apparent between the track running sessions. In female participants, rates of glycogen utilization were greater in both the 8 × 800-m and 3 × 10-min within the G muscle compared with the 10-mile road run (P < 0.01 for both comparisons), although no differences were apparent between the track running sessions. In contrast, there was no difference in rates of glycogen utilization with the VL muscle of female participants between training sessions (P > 0.05 for all comparisons). When comparing sex-specific responses, rate of glycogen utilization was greater in male than female participants (P < 0.01) in the G muscle during the 8 × 800-m track run only. Finally, there was a significant main effect of muscle group in that higher rates of utilization were typically observed in the G versus the VL muscle in both male and female participants and during all training protocols (P < 0.01).

TABLE 4
TABLE 4:
Rates of muscle glycogen utilization in G and VL muscles groups during each training protocol.

DISCUSSION

The aim of the present study was to quantify glycogen utilization of the VL and G muscles of recreationally active male and female runners during three types of outdoor training sessions that are considered representative of runners’ habitual workouts. Importantly, this is the first time that the effect of training protocol, sex, and muscle sampled on net muscle glycogen utilization has been simultaneously investigated within the same study. Our data demonstrate that 1) prolonged steady-state running necessitates a higher absolute glycogen requirement than shorter but higher-intensity track running sessions, 2) female runners display evidence of reduced resting muscle glycogen concentration and net muscle glycogen utilization when compared with male runners, and 3) net glycogen utilization is higher in the G muscle compared with the VL. Although the pattern of glycogen utilization observed here is, of course, specific to the training status of the participants and the characteristics of the chosen exercise protocols, our data may help to inform practical guidelines in relation to fuelling strategies to promote both training intensity and metabolic adaptations.

In an attempt to standardize resting muscle glycogen concentration between trials, all runners completed an initial bout of glycogen-depleting exercise followed by 48 h of standardized dietary CHO intake equating to 6 g·kg−1·d−1. In this way, our experimental design allowed us to more accurately assess the effects of exercise protocol, sex, and muscle group on net exercise-induced muscle glycogen utilization. Although we observed resting glycogen concentrations in the VL muscle of male participants (i.e., 400–500 mmol·kg−1 dw) that is consistent with the fitness level (i.e., 50 mL·kg−1 min−1) and dietary CHO intake (i.e., 2 d of 6 g·kg−1) reported in a recent meta-analysis (23), comparison of resting glycogen concentrations between muscles and sex also revealed a number of interesting findings. First, we observed that resting glycogen concentration in the G muscle of male participants was higher than that of the VL. Second, we also observed that female participants displayed reduced resting glycogen concentration in the G muscle compared with male participants. Although it is currently difficult to offer definitive mechanisms underpinning such findings, it is possible that the combination of glycogen-depleting exercise coupled with the lower absolute CHO intake in female participants (6 g·kg−1 body mass equating to 360 g CHO) compared with male participants (6 g·kg−1 body mass equating to 460 g CHO) may have contributed, in part, to these results. Indeed, given that the magnitude of postexercise muscle glycogen resynthesis is well known to be dependent on the extent of prior glycogen depletion (33) and that the G muscle was likely depleted to a greater extent than the VL (as reported by Areta and Hopkins (23) and later verified in Table 2), it is suggested that the elevated resting glycogen concentration in the G muscle in male participants may possibly be a reflection of greater absolute utilization during the depletion and subsequent resynthesis in response to a given exercise stimulus and dietary CHO intake.

In relation to sex-specific differences, a reduced capacity of female participants to store glycogen in the VL muscle (as also assessed in the follicular phase) compared with male participants has also been reported previously by Tarnopolsky et al. (21), as evidenced in response to a 3-d CHO loading protocol consisting of cycling-based exercise and elevated dietary CHO intake (increased CHO intake from 55% to 75% of habitual energy intake). Using this approach, the authors observed an approximate 150 mmol·kg−1 dw difference in glycogen storage between male and female participants. The authors suggested that such differences may be due to the combination of greater prior glycogen depletion in male compared with female participants in addition to a higher absolute CHO intake in male (8 g·kg−1 body mass equating to 610 g CHO) compared with female participants (6 g·kg−1 body mass equating to 370 g CHO). The same group later demonstrated that when female participants complete a 4-d CHO loading protocol where a higher relative (9 g·kg−1 body mass) and absolute CHO intake is consumed (540 g CHO), no differences in glycogen concentration are apparent when compared with male participants who consume a comparable absolute dose (600 g CHO equating to 8 g·kg−1 body mass) (25,26). When considered this way, it is possible that the shorter duration of dietary standardization (i.e., 2 d) utilized here coupled with the lower absolute CHO intake consumed by female participants may have contributed to the present findings. Although the preexercise glycogen availability achieved here was sufficient to fuel the workloads of the present training protocols, our data perhaps add further evidence to the suggestion that female participants require greater relative CHO intakes than male participants in order to achieve comparable absolute CHO intakes and subsequent CHO loading responses (likely to be especially relevant when the training session is more prolonged in nature).

In relation to the glycogen requirement of specific training sessions, we observed that the net glycogen utilization in male participants was greatest in the 10-mile road run (≈70% V˙O2peak) when compared with both the 8 × 800-m (100% V˙O2peak) and 3 × 10-min track runs (≈80% V˙O2peak), a pattern of utilization that was evident in both the G and VL muscles. In addition, net glycogen utilization in G muscle was also greater in the 8 × 800-m training session when compared with the 3 × 10-min session, although such a difference between the track sessions was not evident in the VL muscle. When considering such data in combination with the greater net (and rates of) glycogen utilization observed in the G muscle compared with the VL (Tables 3 and 4), our data extend the classical findings of Costill et al. (20) highlighting that the G muscle is a more suitable muscle (i.e., as reflective of greater muscle fiber recruitment) for which to study glycogen metabolism during running given its sensitivity to detect changes of physiological significance.

The absolute net glycogen utilization induced by a specific training session is, of course, a product of exercise duration and exercise intensity. In accordance with postexercise circulating lactate concentrations (Fig. 1), it is noteworthy that the highest rates of glycogen utilization were also observed in the G muscle during the 8 × 800-m training session. Similarly, the highest rate of glycogen utilization in the VL was also observed during the 8 × 800-m session. In contrast to the male participants, however, no differences in net glycogen use between training protocols were evident in the female participants, despite differences in rates of glycogen utilization between certain training sessions. Although such data may be related, in part, to the fact that relative training intensity in the female participants did not differ between training protocols (i.e., 80%–100% V˙O2max) to the same extent for male participants (i.e., 70%–100% V˙O2max), our data clearly highlight how the interplay between muscle fiber recruitment, relative exercise intensity, and training duration can all modulate the absolute muscle glycogen requirement associated with a specific exercise protocol.

The methodological difficulties of isolating the effects of sex on substrate utilization during exercise have been well documented (22), arising from factors relating to matching of participant characteristics, relative exercise intensity, exercise duration and of course, and overall absolute work done. To this end, we deliberately chose to study the effects of sex on glycogen utilization during three real-world training sessions comprising training at identical relative exercise intensities and distance ran (i.e., both the 10-mile road run and 8 × 800-m track session) as well as a session that was matched for relative training intensity but also in training duration (i.e., 3 × 10-min track session). We observed no statistical differences between absolute or rates of glycogen utilization between male and female participants in either the G or VL muscles during the track-based training sessions (Tables 3 and 4). Such a finding may be related to the fact that these sessions were completed at relative exercise intensities that are already sufficient to activate regulatory enzymes of glycogenolysis and glycolysis while also suppressing NEFA uptake and oxidation by the mitochondria.

In contrast, we observed sex-specific responses in absolute muscle glycogen utilization in both the VL (P = 0.02) and G muscle (P = 0.07) during the 10-mile road run. Importantly, this run was completed at a running velocity corresponding to lactate threshold (as opposed to a percent of V˙O2peak) given that matching relative exercise intensity according to threshold is considered a more accurate method to assess CHO metabolism within (34,35) and between sexes (36,37). It is, of course, possible that the differences in net glycogen utilization between sexes may be due to the fact that female participants presented with lower resting glycogen concentration as well as actual differences in time taken to complete the 10-mile distance, especially when considering that rates of glycogen utilization in both muscles were not statistically different between male and female participants (although approximate differences of 1 mmol·kg−1 min−1 could be considered of physiological relevance during prolonged exercise). Nonetheless, data do seem consistent with previous observations that female participants exhibit a lower RER during exercise, thus indicative of less reliance on whole-body CHO metabolism to support substrate metabolism during submaximal steady-state exercise (27,28). Although such differences have been demonstrated to be reflective of differences in liver glycogenolysis (27,36), it is noteworthy that our data also seem consistent with the observation that female participants utilize less muscle glycogen during running (21) but not cycling (28). Indeed, the former authors observed that absolute glycogen utilization in the VL was reduced by approximately 25% in female compared with male participants when both groups completed a set running distance of 15.5 km at a relative exercise intensity corresponding to 65% V˙O2max, a similar magnitude of difference and running distance to that studied here. Such observations suggest that running may be a more suitable exercise modality for which to study sex differences in substrate metabolism during exercise, especially when exercise intensity is submaximal and matched according to lactate threshold (37). Nonetheless, we also acknowledge the requirement to study both fiber type–specific differences in glycogen and intramuscular triglyceride metabolism as well as the kinetics of lipid metabolism, as opposed to the limitations of whole muscle homegenate and static measures of postexercise NEFA and glycerol concentrations utilized here. In addition, a comparison of male and female participants at varying stages throughout the menstrual cycle (completing the types of exercise protocols studied here) is also a future research recommendation.

When taken together, our data illustrate how the complex interplay between muscle group, specifics of training protocol, and sex can all modulate the net glycogen requirement associated with a given exercise stress. In addition to informing future research design methodology, our data may be of practical significance in helping to formulate CHO requirements in relation to specific types of training sessions. Indeed, the resting glycogen concentrations achieved by the 2-d dietary CHO intake of 6 g·kg−1 body mass were sufficient to fuel the workloads of the training protocols studied here. In addition, although we observed small differences in substrate storage and metabolism between sexes, it is unlikely that such differences would manifest as sex-specific practical recommendations for the types of training intensities and duration studied here. Finally, it is noteworthy that all subjects were able to sustain the required training intensity during the 10-mile road run in the absence of CHO feeding during exercise. Such data may also be of practical relevance when considering that CHO feeding during exercise may actually attenuate training-induced oxidative adaptations of human skeletal muscle (12), although it is acknowledged that such studies have not yet been performed in female subjects.

In summary, we conclude that 1) prolonged steady-state running necessitates a higher absolute glycogen requirement than shorter but higher-intensity track running sessions, 2) female participants display evidence of reduced resting muscle glycogen concentration and absolute muscle glycogen utilization when compared with male participants, and 3) both absolute and rates of glycogen utilization are higher in the G muscle compared with the VL. Although such observations are specific to the training status of the participants studied here, our data may provide a platform to help better inform CHO periodization strategies for runners and will hopefully stimulate further research.

The authors wish to thank the technical support of Gemma Miller and Dean Morrey.

Funding for this research was provided by Lucozade Ribena Suntory to Professor J. P. M. No conflicts of interest, financial or otherwise, are declared by the authors.

S. G. I., A. H., S. O. S., and J. P. M. contributed to the conception and design of research; S. G. I., E. J., G. M., M. C., J. S., N. C., S. O. S., and J. P. M. performed experiments; S. G. I., D. T., S. O. S., and J. P. M. analyzed data; S. G. I., D. T., S. O. S., and J. P. M. interpreted the results of experiments; S. G. I. and J. P. M. prepared the figures; S. G. I., D. T., S. O. S., and J. P. M. drafted the manuscript; S. G. I., E. J., G. M., M. C., J. S., N. C., I. L., D. T., A. H., S. O. S. and J. P. M. edited and revised the manuscript; S. G. I., E. J., G. M., M. C., J. S., N. C., I. L., D. T., A. H., S. O. S., and J. P. M. approved the final version of the manuscript.

The results of the present study do not constitute endorsement by the ACSM. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

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Keywords:

MUSCLE GLYCOGEN; CARBOHYDRATE; TRAINING; SEX

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