Effect of carbohydrate ingestion on metabolism during running and cycling

The effects of carbohydrate or water ingestion on metabolism were investiga ted in seven male subjects during two running and two cycling trials lastin g 60 min at individual lactate threshold using indirect calorimetry, U-C-14 -labeled tracer-derived measures of the rates of oxidation of plasma glucos e, and direct determination of mixed muscle glycogen content from the vastu s lateralis before and after exercise. Subjects ingested 8 ml/kg body mass of either a 6.4% carbohydrate-electrolyte solution (CHO) or water 10 min be fore exercise and an additional 2 ml/kg body mass of the same fluid after 2 0 and 40 min of exercise. Plasma glucose oxidation was greater with CHO tha n with water during both running (65 +/- 20 vs. 42 +/- 16 g/ h; P < 0.01) a nd cycling (57 +/- 16 vs. 35 +/- 12 g/h; P < 0.01). Accordingly, the contri bution from plasma glucose oxidation to total carbohydrate oxidation was gr eater during both running (33 +/- 4 vs. 23 +/- 3%; P < 0.01) and cycling (3 6 +/- 5 vs. 22 +/- 3%; P < 0.01) with CHO ingestion. However, muscle glycog en utilization was not reduced by the ingestion of CHO compared with water during either running (112 +/- 32 vs. 141 +/- 34 mmol/ kg dry mass) or cycl ing (227 +/- 36 vs. 216 +/- 39 mmol/ kg dry mass). We conclude that, compar ed with water, 1) the ingestion of carbohydrate during running and cycling enhanced the contribution of plasma glucose oxidation to total carbohydrate oxidation but 2) did not attenuate mixed muscle glycogen utilization durin g 1 h of continuous submaximal exercise at individual lactate threshold.