Journal of Applied Physiology, Vol 69, Issue 5 1849-1855, Copyright © 1990 by American Physiological Society
Effects of a 36-hour fast on human endurance and substrate utilization
B. A. Zinker, K. Britz and G. A. Brooks
Exercise Physiology Laboratory, University of California, Berkeley 94720.
To determine if prolonged fasting affects substrate utilization and
endurance time, seven trained men exercised to exhaustion on a cycle
ergometer at 50% maximum oxygen consumption (VO2max) in an overnight-fasted
[postabsorptive (PA)] state and after a 36-h fast (F). Fasting produced
significant elevations in the resting concentrations of blood free fatty
acids (FFA; 1.16 +/- 0.05 vs. 0.56 +/- 0.06 mM, F vs. PA, respectively, a
107% increase), beta-hydroxybutyrate (beta-OH, 2.06 +/- 0.66 vs. 0.15 +/-
0.06 mM, a 1,270% increase), and glycerol (0.12 +/- 0.03 vs. 0.04 +/- 0.01
mM, a 200% increase), with a significant decline in glucose (79.79 +/- 2.12
vs. 98.88 +/- 3.11 mg/dl, a 19% decrease). Exercise in the F trial
increased FFA, decreased glucose, and significantly elevated beta-OH and
glycerol over the PA trial. There was no difference in blood glucose
concentration between trials at exhaustion. However, F produced a
significant decrement in exercise endurance time compared with the PA trial
(88.9 +/- 18.3 vs. 144.4 +/- 22.6 min, F vs. PA, a 38% decrease). Based on
the respiratory exchange ratio, fasting led to a greater utilization of
lipids during rest and exercise. It was concluded that 1) a 36-h fast
significantly altered substrate utilization at rest and throughout exercise
to exhaustion, 2) glucose levels do not appear to be the single determinant
of time to exhaustion in submaximal exercise, and 3) despite the apparent
sparing of carbohydrate utilization with the 36-h fast, endurance
performance was significantly decreased.
