Journal of Applied Physiology, Vol 67, Issue 5 2066-2071, Copyright © 1989 by American Physiological Society
Effects of 4- and 8-h preexercise feedings on substrate use and performance
M. G. Flynn, T. J. Michaud, J. Rodriguez-Zayas, C. P. Lambert, J. B. Boone and R. W. Moleski
Department of Health Promotion and Human Performance, University of Toledo, Ohio 43606.
Seven well-trained male cyclists were studied during 105 min of cycling
(65% of maximal oxygen uptake) and a 15-min "performance ride" to compare
the effects of 4- and 8-h preexercise carbohydrate (CHO) feedings on
substrate use and performance. A high CHO meal was given 1) 4-h preexercise
(M-4), 2) 8-h preexercise (M-8), 3) 4-h preexercise with CHO feedings
during exercise (M-4CHO), and 4) 8-h preexercise with CHO feedings during
exercise (M-8CHO). Blood samples were obtained at 0, 15, 60, 105, and 120
min and analyzed for lactate, glucose, insulin, and glycerol. Total work
output during the performance ride was similar for the M-4 (217,893 +/-
13,348 N/m) and M-8 trials (216,542 +/- 13,905) and was somewhat higher for
the M-4CHO (223,994 +/- 14,387) and M-8CHO (224,702 +/- 15,709) trials (P =
0.059, NS). Glucose was significantly elevated throughout exercise, and
insulin levels were significantly elevated at 15 and 60 min during M-4CHO
and M-8CHO compared with M-4 and M-8 trials. Glycerol levels were
significantly lower during the CHO feeding trials compared with placebo and
were not significantly different during exercise when the subject had
fasted an additional 4 h. The results of this study suggest that when
preexercise meals are ingested 4 or 8 h before submaximal cycling exercise,
substrate use and performance are similar.
