Physiological changes of muscle glycogen reserves and fat oxidation rate during marathon training cycle

This study examined the physiological adaptations in muscle glycogen storage and fat oxidation rates over a 16-week marathon training cycle. Forty recreational marathon runners (25 males, 15 females; age 32.6 ± 5.4 years) undertook periodized training with detailed physiological tests on four occasions. Muscle glycogen concentrations and fat oxidation rates were determined during incremental exercise tests using standardized methods. The results showed significant alterations in both variables throughout the training period. Muscle glycogen levels followed a typical pattern, decreasing by 42% at the most intense training periods and showing supercompensation of 15% above baseline values during subsequent recovery periods. The ability to oxidize fat increased substantially from 0.42 ± 0.08 g/min to 0.67 ± 0.11 g/min (p < 0.001) after 12 weeks, with peak fat oxidation occurring at higher exercise intensities (52% ± 6% VO2max). Three unique categories of responders were identified, with 35% exhibiting substantial adaptation responses (greater than 60% improvement in fat oxidation capacity). The research concludes that intentional modulation of muscle glycogen levels via periodized training can significantly improve fat oxidation capacity while maintaining performance levels. These results yield practical insights for refining marathon training regimens through tailored strategies that consider individual metabolic response patterns.