Exactly. Depending on your goals in the weight room, glycogen is the main energy substrate during exercise intensity above 70% of maximal oxygen uptake (
Vo2max) and fatigue develops when the glycogen stores are depleted in the active muscles. Most all weight training is done above 70% unless you do a lot of texting.
In humans the majority of glycogen is stored in skeletal muscles (∼500 g) and the liver (∼100 g). Those stores are easily depleted with intense training that last 45min-1 hour. After exercise, the rate of glycogen synthesis is increased to replete glycogen stores. So for those of us who actually lift weights it is important to have these muscle glycogen stores filled before exercise and then refilled after exercise.
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Muscle glycogen stores are also very important to
sarcoplasmic hypertrophy, which causes the sarcoplasm of the muscle cell to super saturate, kicking off myofibrillar hypertrophy.
1. Higher glycogen levels let you train harder.
2. Higher glycogen levels improve recovery.
Low muscle glycogen levels lead to overtraining, fasting and low carb diets deplete muscle glycogen, increase cortisol and reduce testosterone levels in athletes. Low-carb or fasting also reduces insulin levels. Insulin is needed to shuttle nutrients to the muscle cell and insulin also has powerful anti-catabolic properties. Exactly why pro bodybuilders use insulin before and after training to increase hypertrophic and anti-catabolic effects.
Again, it all depends on your preferences and goals. Here is a good paper...
Body composition changes associated with fasted versus non-fasted aerobic exercise
It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering...
jissn.biomedcentral.com
Conclusion
In conclusion, our findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training. Hence, those seeking to lose body fat conceivably can choose to train either before or after eating based on preference. It should be noted that given the small sample size and short study duration, we cannot rule out the possibility that either condition might confer a small benefit over the other with respect to fat loss. Further study is warranted in a longer term trial with a greater number of participants.
Yet another very good paper....
Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training
Exercise and nutrition are often used in combination to lose body fat and reduce weight. In this respect, exercise programs are as important as correct nutrition. Several issues are still controversial in this field, and among them there are contrasting reports on whether training in a fasting...
pubmed.ncbi.nlm.nih.gov
The authors conclude that when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization; rather, physical activity after a light meal is advisable.