To examine the effect of ingesting a high or low glycemic index (GI) meal during a short-term recovery period on endurance running capacity.

• Male endurance-trained runners
• In order to minimize any variation in nutritional intake, the subjects were requested to consume the same diet in the three days prior to each main trial.

• To exclude any residual effects of previous exercise, the subjects were instructed to refrain from strenuous exertion in the 24 hours prior to each main trial
• They were also asked to avoid the consumption of alcohol or caffeine during this period
• Subjects screened to ensure they had no history of diabetes mellitus.

• Randomized crossover trial, counter-balanced design. The two main trials were separated by at least seven days.
• On two occasions, subjects ran at 70% VO_2max on a level treadmill for 90 minutes (R1), followed by a four-hour recovery (REC) and a further exhaustive run at the same speed (R2)
• Twenty minutes after R1, each subject consumed an isoenergetic meal containing either high GI (HGI, GI=77) or low GI (LGI, GI=37) meals. During REC, subjects also ingested a prescribed volume of water equal to 150% of their BM loss during R1.

Blinding Used

Double blinded.

Intervention

Isocaloric (approximately 2.33MJ for 60kg subject) recovery meals: Provided 1.5g per kg CHO; 65% CHO, 15% PRO, 20% fat; consumed within 15 minutes to 20 minutes and then rested for two hours.

 

Statistical Analysis

The dependent variables, performance time, total substrate oxidation, micronutrient intake for the three days before the main trials, changes in plasma volume, osmolality and urine volume were compared using Student's T-tests for paired data. A factorial (two-way, treatment x time) analysis of variance (ANOVA) with repeated measures was used to analyze differences in capillary blood glucose and lactate concentrations, serum insulin, plasma FFA and glycerol concentrations, and RER, HR, RPE, PT and AD values between the two treatment levels and among phases within the main trials. A Tukey post-hoc test was used to locate where any significant differences occurred. Statistical significance was set at the 0.05 level. The effect size (ES) was estimated for the strength of meaningfulness of the treatment effects.

Timing of Measurements

• Ran at 70% VO_2max on a level treadmill for 90 minutes (R1), followed by a four-hour recovery (REC) and a further exhaustive run at the same speed (R2)
• Post-prandial blood glucose concentrations were analyzed at 15-minute intervals in the first hour following each meal, and every 30 minutes throughout the second hour. The incremental area under the blood glucose response curve was calculated.
• Each subject then began a five-minute warm-up on a treadmill at a running speed corresponding to 60% VO_2max. Immediately following the warm-up, the speed of the treadmill was increased to elicit 70% of the subject’s VO_2max. This pace was maintained for 90 minutes (R1). The run was followed by a four-hour recovery period (REC). Nude body mass was measured before and after each of the runs.
• Heart rate (HR) was monitored continuously throughout the main trials using an HR monitor:
  • Pre-R1; R1: 15 minutes, 30 minutes, 45 minutes, 60 minutes, 75 minutes, 90 minutes
  • Recovery: One hour, two hours, three hours, four hours
  • R2: 15 minutes, 30 minutes
  • Exhaustion.

Dependent Variables

• Performance time
• Total substrate oxidation
• Micronutrient intake for the three days before the main trials
• Changes in plasma volume
• Osmolality
• Urine volume.

Independent Variables

Control variables

• Recovery protocol
• Controlled lab conditions
• Subjects abstained from strenuous exercise 24 hours prior to testing
• Pre-trial diet/weighed food record for three days prior to testing.

• Initial N: Seven males
• Attrition (final N): Seven males
• Age [Mean (±SEM)]: 30.0±2.6 years.

Other Relevant Demographics

• Mean (±SEM) maximum oxygen uptake (VO_2max) and maximum heart rate: 62.1±2.2ml per kg⁻¹ per minute⁻¹ and 191±3 beats per minute⁻¹ , respectively
• At least 50km running distance per week.

Anthropometrics

Body mass [Mean (±SEM)]: 60.7±1.4kg.

Location

Hong Kong.

Key Findings

• In the screening sessions, ingestion of both the HGI and LGI meals resulted in an elevation in mean blood glucose concentrations above the resting values (P<0. 01). Mean blood glucose concentration peaked at 15 minutes in the LGI session and thereafter declined toward pre-meal values (peak vs. pre-meal: 7.2±0.3mmol vs. 4.2±0.1mmol per L^-1, P<0.01). The incremental area under the two-hour blood glucose response curve following ingestion of the HGI meal was 72% greater than the area resulting from the consumption of the LGI meal (HGI: 297.1±42.7mmol per minute per L⁻¹ vs. LGI: 172.4±27.0mmol per minute per L⁻¹, P<0.01).
• All subjects completed the 90-minute R1 in both main trials. During R2, the run time in the HGI trial was 15% longer than that in the LGI trial (HGI vs. LGI: 86.6±10.7 minutes vs. 75.2±8.1 minutes, P<0.05, ES=0.53). The range of run times in the HGI trial was 48.8 minutes to 142.0 minutes compared with 46.0 minutes to 108.8 minutes in the LGI trial. No difference was found when time to exhaustion for the treatments was analyzed by order (Trial One vs. Trial Two: 78.0±8.4 minutes vs. 83.8±10.9 minutes, NS). The prescribed exercise intensities were attained in both trials in T1 (HGI vs. LGI: 70.6±0.6 vs. 70.1±1.0% VO_2max; NS) and T2 (HGI vs. LGI: 69.8±0.4% and 70.1±0.7% VO_2max, NS). In R1 and R2, the blood glucose concentrations were maintained at near resting levels in both main trials.
• The blood glucose concentrations had increased significantly compared with their pre-R1 values (60 minutes vs. pre-R1: HGI: 6.1±0.4mmol vs. 4.2±0.1mmol per L⁻¹; LGI: 5.9±0.3mmol vs. 4.2±0.1mmol per L⁻¹; P<0.01). During REC, the incremental area under the two-hour blood glucose response curve after ingestion of the HGI meal was 40% greater than that after the LGI meal (HGI: 157.0±32.5mmol per minute per L⁻¹ vs. LGI: 112.5±27.3mmol per minute per L⁻¹, P<0.01).
• Serum insulin concentrations in the course of the HGI and LGI trials were almost identical during R1. Following the ingestion of the HGI and LGI meals, serum insulin concentrations rose above pre-R1 values (P<0.01). Serum insulin concentrations in the HGI trial were higher than those in the LGI trial, the difference did not reach statistical significance.
• Resting blood lactate concentrations were similar prior to R1 in the HGI and LGI trials. Exercise resulted in an elevation in the blood lactate concentration above resting values, but there were no differences in the response between trials. In REC, the blood lactate concentrations returned to their pre-R1 values, and during R2, the concentrations were again elevated, similar to those in R1.
• The R1 induced a rise in plasma FFA and glycerol concentrations, as did R2. At exhaustion, both parameters were higher in HGI trials than in the LGI trials (P<0.01).
• The rate of CHO and fat oxidation was not different when the HGI and LGI trials were compared. The total amount of CHO and fat oxidized during R2 was similar in both trials.
• Serum Na + increased for the duration of R1 (P<0.05), but it returned to the initial levels after the REC in both trials.There were no treatment effects at any point in time between the trials. Similarly, serum K+ increased during exercise in R1 in both treatments. No significant differences were observed at any point in time between the two conditions. Serum osmolality was not different between trials during R1, REC and R2.
• The R1 and R2 caused the subjects to lose approximately 3% of their pre-exercise body mass. There was no difference in body mass loss between the two trials. The total volume of water prescribed was 2,529±257ml and 2,636±204ml for the HGI and LGI trials, respectively. The subjects restored their body mass to pre-R1 values after the REC and were in positive fluid balance of 186±118ml in the HGI trial and 243±107ml in the LGI trial.
• The percent rehydration was 112.7±8.7% in the HGI trial and 115.5±7.1% in the LGI trial.  The changes in plasma volume were similar in both trials after R1 (HGI: −3.0±3.8% vs. LGI: −5.0±3.8%; NS). After the first hour of the REC, plasma volume was restored above the initial level in both the HGI and LGI trials. Cumulative urine output during REC was similar between the HGI and LGI trials (HGI: 768.1±177.2ml vs. LGI: 809.4±116ml; NS).
• The pattern of responses in terms of HR, RPE, PT and AD were similar in R1 REC, and R2 in the HGI and LGI trials.

• The consumption of a HGI meal during a four-hour recovery improved endurance capacity by 15% in a subsequent run; however, the precise mechanism(s) by which this takes place is yet to be clarified
• Subject responses in terms of blood glucose and lactate, serum insulin, plasma FFA, glycerol, sodium and potassium, and heart rate and RPE were not different
• Restoration of fluid balance could be achieved using a large volume of plain water (150% of fluid losses) combined with either meal.

• Very small size; these findings need to be tested in a large population
• No control group. Well-controlled double-blinded clinical trials are required in males and females. Other demographics like height, weights are also required.