The influence of a whey protein preload prior to carbohydrate consumption on cycling performance

The addition of whey protein co-ingested with a carbohydrate source during aerobic exercise has been theorized to augment insulin secretion, invoke a muscle glycogen sparing effect, improve glycemic control, and ultimately improve endurance performance. However, previous literature has reported a discrepancy between the performance measures outcomes. This study aimed to examine if 0.7 g/kg/LBM of whey protein isolate administered as a preload to a glucose bolus of 0.9 g/kg/LBM influences cycling performance and the metabolic profile during a 60 min cycling performance trial. Ten recreationally trained cyclists and triathletes (age 32.2 ± 8.7 years; weight 81.4 ± 11.5 kg; body fat 23.7 ± 5.1%; lactate threshold 203 ± 37.6 W) completed two experimental trials. Each trial was assigned in a counter-balanced order and separated by at least one week. For each experimental trial, participant reported to the exercise physiology lab in a 10-12 hour fasted state. Each participant was required to perform a cycling performance test, which consisted of cycling for 30 min at 90% LT, followed by a 30 min time trial. Prior to the cycling performance test, participants consumed either a whey protein isolate preload (0.7 g/kg/LBM) or a placebo 20 min prior to the consumption of a glucose beverage (0.9 g/kg/LBM). The glucose beverage was consumed 10 min prior to the cycling performance test. Following the completion of both experimental trials, results indicated there were no significant differences in overall time trial performance (WP 16.8 ± 0.34 km; PL 17 ± 0.4 km; p = .346). The whey protein stimulated a significant increase in plasma insulin concentrations at time point 0 (WP = 222.88 ± 45.1 pg/ml; PL = 85.95 ± 45.1 pg/ml; p = .047) compared to the placebo trial. The increase in insulin during WP stimulated a significant interaction effect for plasma glucose concentrations (p = 0.009) between the two trials, however, there were no significant differences. The whey protein stimulated a significant increase in plasma glucagon concentrations for timepoint -10, 0, 15, 30, 45, and 60 when compared to the PL trial (all values p < .05). There were no reported differences in RER, NEFA, or any other variables between the two experimental trials. Although there were significant metabolic alterations due to the consumption of the whey protein preload, this did not influence overall cycling performance or substrate utilization.