Contrary to common wisdom pre-loading carbohydrates does not improve performance. From this study (Medicine and Science in Sports [01 Jan 1979, 11(1):1-5]. Effects of preexercise feedings on endurance performance. Foster C , Costill DL , Fink WJ.):
Eight male and female students were studied during exercise to exhaustion on a bicycle ergometer at 80 and 100% of Vo2max following the ingestion of water (W), 75 g of glucose (G) or a liquid meal (M) (10 g protein, 12.5 g fat, 15 g CHO).
When compared to the endurance ride (80% Vo2max) in the W treatment, endurance performance time was reduced by 19%, (p less than .05) (53.2 to 43.2 min) as a result of the preexercise glucose feeding (Trial G).
No difference in performance at 80% Vo2max was found between the W and M trials. The preexercise feedings had no effect on exercise time to exhaustion at 100% Vo2max.
During the G and M trials at 80% Vo2max, most of the subjects demonstrated a transient decline in serum glucose (less than 3.5 mM). After 30-40 min. of exercise, however, serum glucose returned to normal and was seldom low at the time of exhaustion.
Serum free fatty acids (FFA) were depressed throughout the G trial.
The results of these experiments indicate impaired lipid mobilization following CHO ingestion. The present data support our earlier findings (11) which demonstrate that glucose feedings 30-45 minutes before endurance exercise increase the rate of CHO oxidation and impede the mobilization of FFA, thereby reducing exercise time to exhaustion.