Journal of Applied Physiology, Vol 55, Issue 3 830-833, Copyright © 1983 by American Physiological Society
Influence of fasting on glycogen depletion in rats during exercise
G. L. Dohm, E. B. Tapscott, H. A. Barakat and G. J. Kasperek
We recently observed that a 24-h fasted group of rats could run longer than
an ad libitum fed control group before becoming exhausted. Because of the
demonstrated importance of glycogen levels and free fatty acid availability
during endurance exercise, we have investigated several parameters of
carbohydrate and lipid metabolism in exercised and nonexercised rats that
were either fed ad libitum or fasted for 24 h. A 24-h fast depleted liver
glycogen, lowered plasma glucose concentration, decreased muscle glycogen
levels, and increased free fatty acid and beta-hydroxybutyrate
concentrations in plasma. During exercise the fasted group had lower plasma
glucose concentration, higher plasma concentration of free fatty acids and
beta-hydroxybutyrate, and a lower muscle glycogen depletion rate than did
the ad libitum fed group. Since fasted rats were able to continue running
even when plasma glucose had dropped to levels lower than those of
fed-exhausted rats, it seems unlikely that blood glucose level, per se, is
a factor in causing exhaustion. These results suggest that fasting
increases fatty acid utilization during exercise and the resulting
"glycogen sparing" effect may result in increased endurance.
