Rodger, I. M., & Bosch, A. N. (2000). Delaying carbohydrate feeding for 90 min during 3.5 h cycling reduces performance. Medicine and Science in Sports and Exercise, 32(5), Supplement abstract 1442.

This study investigated the effects on metabolism and performance of delaying CHO feeding for 90 minutes compared with feeding from the start of exercise. Ss (N = 8) pedaled on a cycle ergometer for three hours at 70% VO2max followed by a 20-km time trial under two conditions. The first condition involved drinking 150 ml of 10% glucose solution every 15 minutes to determine CHO oxidation. In the delayed condition, Ss drank a placebo for 90 minutes and thereafter, followed the same CHO protocol as the other condition.

Time-trial performances were significantly worse in the delayed-CHO condition. Plasma glucose concentrations were lower in the delayed-CHO condition for the first 88 minutes of the exercise bout. At the start of the time trial, plasma glucose and insulin concentrations were similar between the conditions. In the delayed-CHO condition, CHO oxidation was higher in the first 90 minutes.

Delaying CHO feeding for 90 minutes in a 3.5-hour task resulted in a performance decline despite CHO levels being adequate. It was speculated that the decline could be due to lower muscle glycogen concentration at the start of the time-trial that resulted from increased muscle glycogen oxidation when feeding was delayed.

Implication. Extended endurance performance is compromised when CHO feeding is delayed.

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