Whey or Casein Hydrolysate with Carbohydrate for Metabolism and Performance in Cycling

The protein type most suitable for ingestion during endurance exercise is undefined. This study compared co-ingestion of either 15g/h whey or casein hydrolysate with 63g/h fructose: maltodextrin (0.8:1) on exogenous carbohydrate oxidation, exercise metabolism and performance. 2h postprandial, 8 male cyclists ingested either: carbohydrate-only, carbohydrate-whey hydrolysate, carbohydrate-casein hydrolysate or placebo-water in a crossover, double-blind design during 2h of exercise at 60%W-max followed by a 16-km time trial. Data were evaluated by magnitude-based inferential statistics. Exogenous carbohydrate oxidation, measured from (CO2)-C-13 breath enrichment, was not substantially influenced by co-ingestion of either protein hydrolysate. However, only co-ingestion of carbohydrate-casein hydrolysate substantially decreased (98% very likely decrease) total carbohydrate oxidation (mean +/- SD, 242 +/- 44; 258 +/- 47; 277 +/- 33g for carbohydrate-casein, carbohydrate-whey and carbohydrate-only, respectively) and substantially increased (93% likely increase) total fat oxidation (92 +/- 14; 83 +/- 27; 73 +/- 19g) compared with carbohydrate-only. Furthermore, only carbohydrate-casein hydrolysate ingestion resulted in a faster time trial (-3.6%; 90% CI: +/- 3.2%) compared with placebo-water (95% likely benefit). However, neither protein hydrolysate enhanced time trial performance when compared with carbohydrate-only. Under the conditions of this study, ingesting carbohydrate-casein, but not carbohydrate-whey hydrolysate, favourably alters metabolism during prolonged moderate-strenuous cycling without substantially altering cycling performance compared with carbohydrate-only.