EFFECT OF TIMING OF CARBOHYDRATE INGESTION ON ENDURANCE EXERCISE PERFORMANCE

This study compared the effects of carbohydrate ingestion throughout e xercise with ingestion of an equal amount of carbohydrate late in exer cise. Eight well-trained men cycled 2 h at 70 +/- 1% VO2 peak, followe d immediately by a 15-min performance ride, while ingesting either a 7 % carbohydrate-electrolyte solution (CHO-7), an artificially sweetened placebo (CON), or the placebo fur the first 90 min then a 21% glucose solution (CHO-0/21). At the start of the performance ride, plasma glu cose averaged 4.2 +/- 0.2, 5.2 +/- 0.1, and 5.7 +/- 0.2 mmol.l(-1) in CON, CHO-7, and CHO-0/21 respectively (all different, P < 0.05). Plasm a insulin levels were similar just prior to the performance ride in CH O-7 and CHO-0/21, with both higher than CON. A similar pattern was obs erved with respiratory exchange ratio (RER). Work performed during the performance ride was significantly greater in CHO-7 (268 +/- 8 kJ) co mpared with CON (242 +/- 9 kJ). Performance in CHO-0/21 (253 +/- 10 kJ ), however, was not improved compared with CON, despite higher plasma glucose levels and plasma insulin levels similar to CHO-7. Seven of th e eight subjects performed best in CHO-7. In conclusion, performance w as improved, relative to the control trial, only when carbohydrate was ingested throughout exercise. Carbohydrate ingestion late in exercise did nor improve performance despite increases in plasma glucose and i nsulin.