This study was designed to investigate the effect of intense training on muscle glycogen stores under conditions of controlled carbohydrate (CHO) intake. On two separate occasions, 10 highly trained distance runners increased their training load for 5 days (20 km/day, approximately 80% maximal O2 consumption) while eating a diet whose carbohydrate composition either equaled (EQ-CHO) or contained approximately 50% of the runner's estimated daily expenditure (LO-CHO). Total muscle glycogen levels were lower after the LO-CHO regimen. Photometric analysis of the glycogen content in individual fibers revealed that 27% type I and 17% type II fibers had optical densities less than 0.2 U after the LO-CHO regimen, whereas 7% type I and 0% type II were similarly depleted after the EQ-CHO diet. A linear relationship was observed between the histochemical and direct chemical analysis of muscle glycogen content. Treadmill O2 uptake measured at 185 and 238 m/min was higher during the LO-CHO than the EQ-CHO regimen. Ratings of perceived exertion were higher during the 238-m/min run for the LO-CHO regimen. After 3 days of rest, running economy and perception of effort returned to pretraining levels and muscle glycogen stores were approximately 85% of the pretraining values. Thus when CHO intake was only approximately 50% of the energy requirements there was a marked depletion of muscle glycogen stores, particularly in type I fibers, and a concomitant decrease in running economy and increased perception of fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)