Glycemic Index
Recovery from prolonged exercise following low GI meal
Carbohydrate has long been accepted as an essential aid to recovery from endurance exercise but does the type of carbohydrate you eat influence future performance?
This study aimed to find out by looking at the effects of the glycemic index (GI) of post-exercise carbohydrate intake on endurance capacity the following day (Stevenson et al. (2005) Improved Recovery from Prolonged Exercise Following the Consumption of Low Glycemic Index Carbohydrate Meals. International Journal of Sports Nutrition and Exercise Metabolism, 2005, 15(4), 333-349.).
9 subjects took part in this study which required them to complete 2 trials on consecutive days. On the first day, they ran for 90minutes at 70% VO2 max and on the second day they ran to exhaustion at the same intensity. In between the two trials, the subjects were given either a high GI or low GI diet both of which provided 8g/kg body mass of carbohydrate.
Results showed that time to exhaustion in the second trial was longer following consumption of the low GI diet (~108.9min) as compared to the high GI diet (~96.9min). In addition to this, fat oxidation and free fatty acid concentration were higher in the low GI trial. The researchers suggested that the results indicate that the increased endurance capacity following the low GI recovery diet was largely due to increased fat oxidation.
No effect of GI on recovery & performance in high intensity running
An intermittent, prolonged, high intensity shuttle run test was used during this study to examine the effect of GI on performance and recovery (Erith et al, (2006) The Effect of High Carbohydrate Meals with Different Glycemic Indixes on Recovery of Performance During Prolonged Intermittent High-Intensity Shuttle Running. International Journal of Sports Nutrition and Exercise Metabolism, 2006, 16(4), 393-404.).
7 semi-professional footballers acted as subjects during this study which required them to participate in 2 trials. On day 1 of the trials, the subjects performed 90 minutes of intermittent, high intensity running in accordance with the Loughborough Intermittent Shuttle Test (LIST) protocol. Following this, the subjects were given a high carbohydrate diet consisting of 8g/kg of either high GI (GI = 70) or low GI (GI = 35) carbohydrate foods. 22 hours later, on day 2 of the trial, the subjects were required to complete 75 minutes of the LIST followed by alternate sprinting and jogging to exhaustion.
The results of this study showed that the GI of the recovery diet had no significant effect on time to exhaustion, sprint performance or distance covered during the second part of the second trial. The researchers concluded that GI of diet during a 22 hour recovery period did not affect sprint and endurance performance.
Low GI pre-exercise meal improves endurance capacity
8 recreational runners acted as subjects in this trial which investigated the effect of pre-exercise low and high GI carbohydrate meals on endurance running capacity (Wu & Williams (2006) A Low Glycemic Index Meal Before Exercise Improves Endurance Running Capacity in Men. International Journal of Sports Nutrition and Exercise Metabolism, 16(5), 510-527.).
The runners completed 2 trials in which they consumed either a low GI (GI = 37) or high GI (GI = 77) meal prior to running at 70% VO2 max until exhaustion - the trials were separated by 7 days. Following an overnight fast, the subjects consumed their meals which contained 2g/kg carbohydrate, 3 hours prior to beginning their run.
Time to exhaustion was significantly longer following the low GI meal (~108.8 minutes) than the high GI meal (~101.4 minutes) and fat oxidation rates were high during the low GI trial. The enhanced running performance following the consumption of a low GI meal could be attributed to increase fat oxidation during exercise.
Low GI foods increase plasma glucose levels in prolonged exercise
This study looked into whether consumption of low GI foods would increase blood glucose concentration and therefore fuel availability toward the end of exercise (Thomas et al, (1994) Plasma Glucose Levels After Prolonged Strenuous Exercise Correlate Inversely with Glycemic Response to Food Consumed Before Exercise. International Journal of Sports Nutrition, 1994, 4(4), 361-373.).
The 6 trained cyclists who took part in the study were required to cycle at 65-75% VO2 max, 60 minutes after ingesting either a low GI powdered food, a high GI powdered food, a low GI breakfast cereal or a high GI breakfast cereal all of which provided 1g/kg carbohydrate.
After more than 90 minutes of pedalling on cycle ergometers, plasma glucose levels were found to correlate inversely with the GI of the foods which had been consumed. In addition, free fatty acid concentrations were also found to inversely correlate with the GI of the foods. These results suggest that the slower digestion of low GI foods in pre-event meals leads to increased availability of fuels towards the end of endurance exercise.