Effect of carbohydrate ingestion on metabolism during running and cycling.

The effects of carbohydrate or water ingestion on metabolism were investigated in seven male subjects during two running and two cycling trials lasting 60 min at individual lactate threshold using indirect calorimetry, U-14C-labeled tracer-derived measures of the rates of oxidation of plasma glucose, and direct determination of mixed muscle glycogen content from the vastus 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 before exercise and an additional 2 ml/kg body mass of the same fluid after 20 and 40 min of exercise. Plasma glucose oxidation was greater with CHO than with water during both running (65 +/- 20 vs. 42 +/- 16 g/h; P < 0.01) and cycling (57 +/- 16 vs. 35 +/- 12 g/h; P < 0.01). Accordingly, the contribution from plasma glucose oxidation to total carbohydrate oxidation was greater during both running (33 +/- 4 vs. 23 +/- 3%; P < 0.01) and cycling (36 +/- 5 vs. 22 +/- 3%; P < 0.01) with CHO ingestion. However, muscle glycogen 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 cycling (227 +/- 36 vs. 216 +/- 39 mmol/kg dry mass). We conclude that, compared 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 during 1 h of continuous submaximal exercise at individual lactate threshold.

Pacing strategy in simulated cycle time-trials is based on perceived rather than actual distance.

This study determined the pacing strategies and performance responses of six well-trained cyclists/triathletes (peak O2 uptake 66.4+/-3.7 ml x kg(-1) x min(-1), mean+/-SD) during seven simulated time-trials (TT) conducted on a wind-braked cycle ergometer. All subjects first performed a 40 km familiarisation ride (TT1). They were then informed they would be riding a further four 40 km TT for the purpose of a reliability study. Instead, the actual distances ridden for the next three TT were a random order of 34 (TT2), 40 (TT3) and 46 km (TT4). The only feedback given to subjects during TT1-4 was the percentage distance of that ride remaining. During a further 40 km TT (TT5) subjects were allowed to view their heart rate (HR) responses throughout the ride. Despite the significantly different performance times across the three distances (47:23+/-4:23 vs 55:57+/-3:24 vs 65:41+/-3:56 min for the 34, 40 and 46 km respectively, P<0.001), average power output (296+/-48 vs 294+/-48 vs 286+/-40 W) and HR (173+/-11 vs 174+/-12 vs 173+/-12 beats x min(-1)) were similar. The true nature of the first part of the study was then revealed to subjects who subsequently completed an additional 34 km and 46 km TT TT6-7) in which the actual and perceived distance ridden was the same. Power output and HR responses were similar for both unknown (TT2 and TT6) and known (TT4 and TT7) rides for both distances: 296+/-48 vs 300+/-55 W and 173+/-11 vs 177+/-11 beats x min(-1) (34 km) and 286+/-40 vs 273+/-42 W and 173+/-12 vs 174+/-12 beats x min(-1) (46 km). In conclusion, well-trained cyclists rode at similar power outputs and HR during time trials they perceived to be the same distance, but which varied in actual distance from 34 to 46 km.

Effect of caffeine co-ingested with carbohydrate or fat on metabolism and performance in endurance-trained men.

We examined the effect of caffeine co-ingested with either carbohydrate or fat on metabolism and performance in eight endurance-trained subjects who performed a random order of four experimental trials consisting of 120 min of steady-state ergometer cycling at 70 % of maximal O(2) uptake (SS) followed by a time trial in which subjects completed a set amount of work (7 kJ kg-1) as quickly as possible. One hour before SS subjects ingested either 2.6 g kg-1 carbohydrate (CHO); 2.6 g kg-1 CHO + 6 mg kg-1 caffeine (CHO + CAF); 1.2 g kg-1 fat with 2000 U I.V. heparin (FAT); or 1.2 g kg-1 fat with 2000 U I.V. heparin + 6 mg kg-1 caffeine (FAT + CAF). The rate of carbohydrate oxidation was higher (micromol kg-1 min-1: CHO, 243 +/- 39 and CHO + CAF, 239 +/- 30 vs. FAT, 196 +/- 48 and FAT + CAF, 191 +/- 55; P < 0.05, values are means +/- S.D.) and the rate of fat oxidation lower (micromol kg-1 min-1: CHO, 19 +/- 8 and CHO + CAF, 22 +/- 7 vs. FAT, 35 +/- 19 and FAT + CAF, 37 +/- 17; P < 0.05) with carbohydrate than fat ingestion. Yet despite lower carbohydrate use with fat feeding, the time taken to complete the time trial was less after carbohydrate than after fat ingestion (min: CHO, 30.37 +/- 7.42 and CHO + CAF, 29.12 +/- 5.62 vs. FAT, 33.02 +/- 8.50 and FAT + CAF, 32.78 +/- 7.70; P < 0.05). We conclude that (1) caffeine co-ingested with either carbohydrate or fat meals has no additive effect on substrate utilization or exercise performance and (2) carbohydrate ingestion before exercise improves subsequent time trial performance compared with fat ingestion. Experimental Physiology (2001) 86.1, 137-144.

Carbohydrate ingestion attenuates the increase in plasma interleukin-6, but not skeletal muscle interleukin-6 mRNA, during exercise in humans.

1. The present study was undertaken to examine the effects of exercise and carbohydrate (CHO) ingestion on interleukin-6 (IL-6) gene expression in skeletal muscle and plasma IL-6 concentration. 2. Seven moderately trained men completed 60 min of exercise at a workload corresponding to each individual's lactate threshold on four randomised occasions. Two trials were conducted on a bicycle ergometer (Cyc) and two on a running treadmill (Run) either with (CHO) or without (Con) the ingestion of a CHO beverage throughout the exercise. Muscle biopsies were obtained from the vastus lateralis before and immediately after exercise and IL-6 gene expression in these samples was determined using real-time PCR. In addition, venous blood samples were collected at rest, and after 30 min during and at the cessation of exercise. These samples were analysed for plasma IL-6. 3. Irrespective of exercise mode or CHO ingestion, exercise resulted in a 21 +/- 4-fold increase (P < 0.01; main exercise effect) in IL-6 mRNA expression. In contrast, while the mode of exercise did not affect the exercise-induced increase in plasma IL-6, CHO ingestion blunted (P < 0.01) this response. 4. These data demonstrate that CHO ingestion attenuates the plasma IL-6 concentration during both cycling and running exercise. However, because IL-6 mRNA expression was unaffected by CHO ingestion, it is likely that the ingestion of CHO during exercise attenuates IL-6 production by tissues other than skeletal muscle.

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

We investigated the effect of carbohydrate (CHO) ingestion before and during exercise and in combination on glucose kinetics, metabolism and performance in seven trained men, who cycled for 120 min (SS) at approximately 63% of peak power output, followed by a 7 kJ/kg body wt time trial (TT). On four separate occasions, subjects received either a placebo beverage before and during SS (PP); placebo 30 min before and 2 g/kg body wt of CHO in a 6.4% CHO solution throughout SS (PC); 2 g/kg body wt of CHO in a 25.7% CHO beverage 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 solution throughout SS (CC). Ingestion of CC and CP markedly (>8 mM) increased plasma glucose concentration ([glucose]) compared with PP and PC (5 mM). However, plasma [glucose] fell rapidly at the onset of SS so that after 80 min it was similar (6 mM) between all treatments. After this time, plasma [glucose] declined in both PP and CP (P < 0.05) but was well maintained in both CC and PC. Ingestion of CC and CP increased rates of glucose appearance (R(a)) and disappearance (R(d)) compared with PP and PC at the onset of, and early during, SS (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 calculated rates of glucose oxidation were different when comparing the four trials (P < 0.05), total CHO oxidation and total fat oxidation were similar. Despite this, TT was improved in CC and PC compared with PP (P < 0.05). We conclude that 1) preexercise ingestion of CHO improves performance only when CHO ingestion is maintained throughout exercise, and 2) ingestion of CHO during 120 min of cycling improves subsequent TT performance.