Randomized Controlled Trial

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To determine the effects of training with low muscle glycogen on exercise performance, substrate metabolism and skeletal muscle adaptation.

• Endurance-trained male cyclists
• No high-intensity interval training in the four weeks before the investigation.

None mentioned.

Design

Randomized controlled trial (high-glycogen or low-glycogen training) with subjects pair-matched for:

• VO_2max
• W_max
• Time trial performance
• Training history.

Intervention

• Three-phase design with baseline measures, a three-week training intervention and post-training measures
• Training consisted of nine aerobic training (AT) and nine HIT sessions spread over three weeks under the supervision of researchers. AT performed on electromagnetically braked cycle ergometers and consisted of 90-minute continuous cycling at approximately 70% VO_2max. HIT on the subject's bike was attached to stationary trainer, fitted with power-measuring SRM cranks and consisted of a 20-minute warm-up followed by eight five-minute all-out efforts with one minute of recovery. 
• High-glycogen subjects trained once daily, alternating between AT on Day One and HIT the following day, with AT first and HIT one hour later
• Low-glycogen trained twice every second day, with AT first and HIT one hour later
• All training sessions were performed in the morning after an overnight fast. Subjects continued to fast until the entire training session was completed. Water was allowed. The nutritional status of subjects was controlled for 24 hours before the experimental measures (muscle biopsy, 60-minute steady state cycle test and time trial) with 67.5% CHO, 13.5% protein and 19% fat. Subjects were asked to maintain high CHO during the three-week training intervention.

Statistical Analysis

All data were analyzed using a two-factor ANOVA, with one between-subject factor (high vs. low group) and one within-subject factor (pre-training vs. post-training time). The level of significance was set at P<.05 and significant interactions followed up with Tukey honestly significant difference post-hoc test. 

Timing of Measurements

• Preliminary testing: 
  • Incremental exercise test to exhaustion to determine VO_2max and maximum power output (W_max)
  • Three to five days later, subjects had a 60-minute rest and 60 minutes cycling at 70% VO_2max with acetate infusion; acetate was then stopped and subjects performed a 60-minute time trial.
• Baseline and post-training measures (each time with identical measures):
  • Muscle biopsy (approximately 80mg) from vastus lateralis using percutaneous needle biopsy technique modified for use with suction; on the morning of an overnight fast and 48 hours after the last exercise bout
  • A 60-minute steady state cycle test [more than 48 hours post-muscle biopsy, overnight fast, 60 minutes at 70% VO_2max with stable isotope infusion and repeated blood sampling occurred by antecubital vein catheter using a glucose (priming dose and continuous rate) and palmitate infusion (continuous rate)]
  • Blood samples and expired breath samples were collected at baseline, end of the resting period and at 15-minute intervals during exercise
  • Time trial: Subjects completed a set amount of work in a cadence-dependent (linear) mode as fast as possible immediately following steady-state cycling.

Dependent Variables

• Self-selected training intensity
• Time trial performance
• VO₂
• RER
• A 13C:12C ratio via expired breath samples
• Whole-body substract metabolism using stoichiometric equations
• Plasma glucose via blood samples
• Plasma palmitate kinetics (FFA extracted from plasma, isolated and derived to their methyl esters)
• Relative contribution of fat:energy ratio expenditure during exercise
• Relative contribution of muscle glycogen:energy ratio expenditure during exercise
• Muscle glycogen oxidation
• Muscle glycogen content via muscle biopsy
• Protein content:
  • COX2, COX5
  • FAT/CD36, beta-HAD
  • GLUT4.

Independent Variables

Training (high-glycogen training vs. low-glycogen training).

Control Variables

• Nutritional status of subjects was controlled for 24 hours before the experimental measures by giving subjects a standard diet consisting of 67.5% CHO (8g per kg^-1 body mass), 13.5% protein and 19% fat
• Throughout the three-week training intervention, subjects were asked to maintain a high CHO diet and were given detailed instructions on how to achieve this. 

• Initial N: 14
• Attrition (final N): 14.

Age

• High-glycogen: 31±6 years
• Low-glycogen: 30±6 years.

Other Relevant Demographics

• Length of training:
  • High-glycogen: 6.4±2.7 years
  • Low-glycogen: 7.6±3.0 years.
• Training volume:
  • High-glycogen: 334±70km per week
  • Low-glycogen: 321±78km per week.
• VO_2max:
  • High-glycogen: 4.93±0.38L per minute
  • Low-glycogen: 4.94±0.34L per minute.
• W_max:
  • High-glycogen: 372±27W
  • Low-glycogen: 381±28W.

Anthropometrics

• No significant differences between groups
• Height:
  • High-glycogen: 176±5cm
  • Low-glycogen: 178±6cm.
• Body mass:
  • High-glycogen: 75.7±6.9kg
  • Low-glycogen: 75.4±9.4kg.

Location

United Kingdom.

 Key Findings

• Self-selected training intensity was higher in the high-glycogen training group [323±9 (87%±2% of W_max)] compared to the low-glycogen training group [297±8 (78%±2% of W_max)]; P<0.05
• Time trial performance (mean power output, W) improved in the high-glycogen training group (271±13 to 298±13) and low-glycogen training group (278±11 to 307±10);  P<0.001 main effect for time
• Time trial performance (time to complete task in minutes) improved in the high-glycogen (62.10±1.49 minutes to 56.37±1.17 minutes) and low-glycogen (61.90±1.12 minutes to 56.12±1.22 minutes) training groups; P<0.001 main effect for time
• CHO oxidation decreased (P<0.01) in the low-glycogen training group only (220±8mmol to 194±10mmol per kg per minute)
• Fat oxidation increased (P<0.01) in the low-glycogen training group only (26±2mmol to 34±2mmol per kg per minute)
• The ratio of fat:energy expenditure was unaffected in the high-glycogen group but increased in the low-glycogen training group (32%±2% to 40%±2%); P<0.05
• Muscle glycogen content increased 18% and 36% after training for high-glycogen and low-glycogen training groups, respectively (P<0.001). There was no difference between groups.
• HIT power output increased throughout the training period (main effect time, P<0.0001)
• VO_2max was unaffected by training
• RER was unaffected by training in high-glycogen group but decreased after training in low-glycogen group (P<0.001)
• Plasma glucose and palmitate kinetics were unaffected by high-glycogen but decreased after training in low-glycogen group (P<0.05)
• Estimated rates of plasma FFA oxidation were unaffected by training
• Oxidation of muscle-derived TG increased after training in the low group only (20%±1% to 28%±2%; P<0.05), which derived the fat:energy expenditure increase in the low-glycogen group
• The ratio of muscle glycogen:energy expenditure during exercise was unaffected by training in high-glycogen but decreased (from 57%±2% to 50%±2%) after training in low-glycogen; P<0.05
• COX2 and COX5 protein content was unaffected by training; FAT/CD36 increased after training (P<0.05) and tended to be more pronounced after training in low-glycogen than in high-glycogen (41.4% vs. 11.5%, P>0.05). Beta-HAD increased by 43% after training in low-glycogen but decreased by 20% after training in high-glycogen (P<0.01); GLUT4 tended not to increase as much in the low-glycogen group (low at 7.1%±8.4%, high at 20.6%±9.7%).

 

Training with low-muscle glycogen reduced training intensity and, in performance, was no more effective than training with high-muscle glycogen. Whole-body fat oxidation during a moderate-intensity exercise was increased after training with low muscle glycogen. The enhanced metabolic adaptations in skeletal muscle may account for the increase in whole-body fat oxidation.

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