Preexercise feedings of glucose, fructose or sucrose: effects on substrate depletion in rats

Male Wistar rats (n = 72) trained to run on a treadmill were fasted overnight and fed by gavage 3 ml unsupplemented water or a solution containing 2 g of glucose, fructose or sucrose 30 min prior to exercise. Six rats from each dietary group were killed after 0, 1 or 2 h of exercise. Blood glucose levels decreased in all groups over the 2-h exercise period; however, the largest decline in blood glucose was exhibited by the rats fed unsupplemented water (-32.1%), followed by those fed fructose (-26.9%), sucrose (-13.9%) and glucose (-8.3%). The water-fed control rats also had the largest increase in circulating free fatty acids (+215.4%), followed by those fed fructose (+120.8%), sucrose (+69.2%) and glucose (+57.5%). The fructose-fed animals exhibited the greatest depletion of liver glycogen and the smallest decline in soleus and red vastus lateralis muscle glycogen of the rats fed the different carbohydrates. The data indicate that exercise-induced changes in substrate levels can be modified by the type of carbohydrate given prior to exercise.