Effects of ^<13>C-labeled carbohydrate ingestion on the cardiovascular responses to prolonged exercise

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Carbohydrate ingestion during prolonged exercise is known to improve endurance performance. It is, however, unclear whether carbohydrate ingestion also affects the cardiovascular adjustments to exercise. To verify this problem, we investigated time courses of oxidation of ingested glucose and fructose and the cardiovascular responses during prolonged exercise. Ten female volunteers exercised on a cycle ergometer at 60 % of maximal workload for first 40 miri (Ex 1) and then 60 min (Ex 2) on three occasions while subjects ingested water only (W) or 5 % of glucose (G) or fructose (F) solution. The subject drank a single bolus of W or G solution (3.4 ml/kg body wt) during 3.5-min resting period spaced between Ex 1 and Ex 2. In G or F solution, ^<13>C-labeled stable isotope glucose or fructose (8.5 mg/kg body wt) was dissolved as a tracer. Oxidation of the ingested glucose or fructose was evaluated by V^<13>CO_2 per V^<12>CO_2 ratio (^<13>C/^<12>-C ratio) in expired gases. Oxygen uptake, me … More an arterial blood pressure (MAP), heart rate (HR), and muscle blood flow (MBF) in thigh muscles measured by near-infrared spectroscopy were continuously recorded. The baseline value was defined as the average during the last 10 min of Ex 1, and the changes from the baseline was calculated for each variable during Ex 2. Rating of perceived exertion (RPE) was also monitored every 5 min during Ex 2. The oxygen uptake during Ex 2 was identical among G, F, and W. The ^<13>C/^<12>C ratio began to increase significantly from 15 mm in G and from!6 min in F and peaked at 37 min inG and at 53 min in F during Ex2. The MAP inG showed a significant decrease before the increase of ^<13>C/^<12>C ratio and sustained at the decreased level till the end of exercise, whereas the MAP in F showed similar levels as seen in W. The attenuated MAP responses in G were accompanied by lower RPE, HR, and MBF. These results indicated that the glucose ingestion had attenuating effects on the MAP adjustment to prolonged exercise whereas the fructose ingestion did not alter the MAP responses. The preceding decrease of MAP observed before the increase of ^<13>C/^<12>C ratio in G was independent of utilization of ingested glucose and probably mediated by insulin-induced vasodilatation in the gastrointestinal tract for digestion. The later sustained decreases in MAP in G were probably mediated by less activation in central command and/or the reduced activation of muscle fibers required to perform the identical exercise. Less