Metabolic and circulatory responses to the ingestion of glucose polymer and glucose/electrolyte solutions during exercise in man

Six men exercised on a cycle ergometer for 60 min on two occasions one week apart, at 68±3% of \(\dot V_{{\text{O}}_{{\text{ 2 max}}} } \) . On one occasion, a dilute glucose/electrolyte solution (E: osmolality 310 mosmol · kg⁻¹, glucose content 200 mmol·l⁻¹) was given orally at a rate of 100 ml every 10 min, beginning immediately prior to exercise. On the other occasion, a glucose polymer solution (P: osmolality 630 mosmol · kg⁻¹, glucose content equivalent to 916 mmol · l⁻¹) was given at the same rate. Blood samples were obtained from a superficial forearm vein immediately prior to exercise and at 15-min intervals during exercise; further samples were obtained at 15-min intervals for 60 min at rest following exercise. Heart rate and rectal temperature were measured at 5-min intervals during exercise. Blood glucose concentration was not different between the two tests during exercise, but rose to a peak of 8.7±1.2 mmol · l⁻¹ (mean±SD) at 30 min post-exercise when P was drunk. Blood glucose remained unchanged during and after exercise when E was drunk. Plasma insulin levels were unchanged during exercise and were the same on both trials, but again a sharp rise in plasma insulin concentration was seen after exercise when P was drunk. The rate of carbohydrate oxidation during exercise, as calculated from \(\dot V_{{\text{O}}_{{\text{ 2}}} } \) and the respiratory exchange ratio, was not different between the two tests. A fall in plasma volume, calculated from changes in haematocrit and haemoglobin concentration, occurred after 15 mins of exercise: the fall was of the same magnitude (9%) at this point on both tests, but thereafter plasma volume was significantly lower with P than with E for the remainder of the exercise period and throughout recovery. Serum osmolality increased during exercise (p<0.05) on the P trial, but was unchanged on the E trial. Heart rate was higher (p<0.05) during the last 20 min of exercise on the P trial. These results suggest that the carbohydrate consumed during the P trial was not available to the working muscles during exercise, and was probably not emptied from the stomach and absorbed to any significant extent until exercise stopped. The differences in plasma volume and osmolality between the two trials are consistent with the net movement of water into the gut which is known to occur at rest when solutions of high osmolality are taken. In more prolonged exercise, this effective dehydration may impair performance.