The effects of alterations in dietary carbohydrate intake on the performance of high-intensity exercise in trained individuals

This study was designed to examine the effects of alterations in dietary ca rbohydrate (CHO) intake on the performance of high-intensity exercise lasti ng approximately 10 min (EXP 1) and 30 min (EXP 2). Trained subjects exerci sed to exhaustion on four occasions on a cycle ergometer at 90% of maximal oxygen consumption (VO2max; EXP 1, n = 5) and 80% of VO2max (EXP 2, n = 7). The first two tests were familiarisation trials and were carried out follo wing the subjects' normal diet. Normal training was continued but standardi sed during the periods of dietary control. The subsequent two tests were pe rformed 7 weeks apart after 7 days of dietary manipulation. The two diets w ere a 70% and a 40% CHO diet, isoenergetic with each subject's normal diet and administered in a randomised order. At both exercise intensities, time to exhaustion following the high CHO and low CHO diets was not different [m ean (SD) EXP 1. 11.56 (3.78) min and 8.95 (2.35) min, P = 0.22; EXP 2. 26.9 (7.4) min and 26.5 (6.5) min, P = 0.90]. No differences in resting blood m etabolite concentrations were found apart from a lower P-hydroxybutyrate (b eta-HB) level following the high CHO diet in EXP 2. Blood lactate was highe r after exercise at 90% of VO2max following the high CHO diet. Blood lactat e was higher, and beta-HB lower during exercise at 80% of VO2max following the high CHO diet. No differences were found in the other blood metabolites tested. The respiratory exchange ratio after 15 min of exercise at 80% of VO2max was higher on the high CHO diet. No differences in oxygen uptake, he art rate (EXP 2) or ratings of perceived exertion (both experiments) were f ound between conditions. These results indicate that moderate changes in di et composition during: training do not affect the performance of high-inten sity exercise in trained individuals when the total energy intake is modera tely high.