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UTTER, A. C., J. KANG, D. C. NIEMAN, C. L. DUMKE, S. R. MCANULTY, D. M. VINCI, and L. S. MCANULTY. Carbohydrate Supplementation and Perceived Exertion during Prolonged Running. Med. Sci. Sports Exerc., Vol. 36, No. 6, pp. 1036–1041, 2004.The purpose of this study was to investigate the relationship between carbohydrate energy substrate and hormonal regulation on the perception of exertion during prolonged running.Sixteen experienced marathoners ran on treadmills for 3 h at ~70% V̇O2max on two occasions while receiving 1 L·h−1 carbohydrate (C) or placebo (P) beverages. Blood and vastus lateralis muscle biopsy samples were collected before and after exercise.The pattern of change in ratings of perceived exertion (RPE) over time was significantly different between C and P ingestion (P < 0.01) with attenuated RPE responses found in the latter part of the 3 h run. The pattern of change in the respiratory exchange ratio and carbohydrate oxidation rates were significantly greater (P < 0.01) in the C than P condition. Change in muscle glycogen content did not differ between C and P (P = 0.246). C relative to P ingestion was associated with higher plasma levels of glucose, insulin, and lactate and lower levels plasma cortisol.These data indicate that a lower RPE was associated with a higher carbohydrate oxidation, plasma glucose, and insulin levels, and lower plasma cortisol during prolonged running after C supplementation as compared with P feeding despite no differences in muscle glycogen content. These findings support a physiological link between RPE and carbohydrate substrate availability as well as selected hormonal regulation during prolonged running.