Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance

We investigated the effect of carbohydrate (CHO) ingestion before and durin g exercise and in combination on glucose kinetics, metabolism and performan ce in seven trained men, who cycled for 120 min (SS) at similar to 63% of p eak, power output, followed by a 7 kJ/kg body wt time trial (TT). On four s eparate occasions, subjects received either a placebo beverage before and d uring 88 (PP); placebo 30 min before and 2 g/kg body wt of CHO in a 6.4% CH O solution throughout 88 (PC); 2 g/kg body wt of CHO in a 25.7% CHO beverag e 30 min before and placebo throughout SS (CP); or 2 g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and 2 g/kg of CHO in a 6.4% CHO solutio n throughout SS (CC). Ingestion of CC and CP markedly (>8 mM) increased pla sma glucose concentration ([glucose]) compared with PP and PC (5 mM). Howev er, plasma [glucose] fell rapidly at the onset of SS so that after 80 min i t was similar (6 mM) between all treatments. After this time, plasma [gluco se] declined in both PP and CP (P < 0.05) but was well maintained in both C C and PC. Ingestion of CC and CP increased rates of glucose appearance (R-a ) and disappearance (Rd) compared with PP and PC at the onset of, and early during, 88 (P ( 0.05). However, late in SS, both glucose R-a and R-d were higher in CC and PC compared with other trials (P < 0.05). Although calcula ted rates of glucose oxidation were different when comparing the four trial s (P < 0.05), total CHO oxidation and total fat oxidation were similar. Des pite this, TT was improved in CC and PC compared with PP (P < 0.05). We con clude that 1) preexercise ingestion of CHO improves performance only when C HO ingestion is maintained throughout exercise, and 2) ingestion of CHO dur ing 120 min of cycling improves subsequent TT performance.