The muscle glycogen is the main source of carbohydrates in the body (300-400 g or 1200-1600 kcal), followed by liver glycogen (75-100 g or 300-400 kcal) and finally blood glucose (25 g or 100 kcal). These values vary in a wide range in people, depending on factors such as eating and the conditions of training sessions. The stock of muscle glycogen in non-athletes is about 80-90 mmol-kg of raw muscle tissue. Carbohydrate load increases the muscle glycogen reserve to 210-230 mmol-kg of raw muscle tissue.
The energy of the training process showed that carbohydrates are the preferred source for exercise at 65% V02max (maximum oxygen consumption is an indicator of the maximum capacity of the human body to transport and use oxygen during exercise) and more - the levels at which most athletes train and compete . Oxidation of fat can not supply ATP quickly enough to provide a strenuous workout. If training is possible at low and medium levels (<60% V02max) and at low levels of muscle glycogen and blood glucose, then it is impossible to satisfy the requirement for ATP needed for a greater load with depleted energy sources. Muscle glycogen is most rapidly used in the early stages of exercise and exponentially depends on their intensity.
There is a strict relationship between the content of muscle glycogen before exercise and the time of exercise at 70% V02max: the higher the glycogen content before the load, the higher the endurance potential. Bergstrom et al. Compared the time of a depleting load, performed at 75% V02max after 3 days with rations with different carbohydrate content. The mixed diet (50% of calories from carbohydrates) produced 106 mmol-kg of muscle glycogen and allowed subjects to work 115 min, low-carb diet less than 5% of calories due to carbohydrates) -38 mmol-kg glycogen and provided a load only for 1 hour, and a high-carbohydrate diet (> 82% of calories from carbohydrates) - 204 mmol-kg of muscle glycogen provided 170-minute physical activity.
Stocks of glycogen in the liver maintain the level of glucose in the blood both at rest and at a load. At rest, the brain and the central nervous system (CNS) use most of the blood glucose, and the muscles utilize less than 20%. However, during physical exertion, glucose uptake by muscles increases 30-fold, depending on the intensity and duration of the load. First, most of the hepatic glucose is obtained as a result of glycogenolysis, but with increasing duration of the load and a decrease in the amount of glycogen in the liver, the contribution of glucose due to gluconeogenesis is increasing.
At the beginning of the load, the yield of hepatic glucose satisfies the increased intake of muscle glucose and the blood glucose level remains close to the resting level. Although muscle glycogen is the main source of energy at a 65% VO2max load, blood glucose becomes the most important source of oxidation when depleting muscle glycogen stores. When the output of hepatic glucose can no longer maintain the absorption of muscle glucose during prolonged exercise, the amount of glucose in the blood drops. While some athletes in the central nervous system exhibited symptoms typical of hypoglycemia, most athletes felt local muscle fatigue and had to reduce the intensity of the load.
Stocks of liver glycogen can be depleted by 15-day fasting and drop from a typical level of 490 mmol with a mixed diet to 60 mmol with a low-carb diet. A high-carbohydrate diet can increase the liver glycogen content to about 900 mmol.
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