The importance of having carbohydrates during exercise

Muscle glycogen is the body's main source of carbohydrates (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 widely among individuals depending on factors such as food intake and training conditions. The muscle glycogen store in a non-athlete is approximately 80-90 mmol kg of raw muscle tissue. Carbohydrate loading increases muscle glycogen store to 210-230 mmol kg of raw muscle tissue.

Exercise energetics has shown that carbohydrate is the preferred fuel source for exercise at 65% V02max (maximal oxygen consumption - a measure of the body's maximum capacity to transport and utilize oxygen during exercise) and above, the levels at which most athletes train and compete. Fat oxidation cannot supply ATP rapidly enough to support strenuous exercise. While exercise can be performed at low to moderate levels (< 60% V02max) and with low muscle glycogen and blood glucose levels, it is not possible to meet the ATP demands of greater exercise with depleted energy sources. Muscle glycogen is used most rapidly in the early stages of exercise and is exponentially dependent on exercise intensity.

There is a strong relationship between pre-exercise muscle glycogen content and the time of exercise at 70% V02max: the higher the pre-exercise glycogen content, the higher the endurance potential. Bergstrom et al. compared the time of exhaustive exercise performed at 75% V02max over 3 days with diets of different carbohydrate contents. The mixed diet (50% of calories from carbohydrate) produced 106 mmol kg muscle glycogen and allowed subjects to work for 115 min, the low-carbohydrate diet (<5% of calories from carbohydrate) -38 mmol kg glycogen and provided exercise for only 1 h, and the high-carbohydrate diet (>82% of calories from carbohydrate) - 204 mmol kg muscle glycogen provided a 170-min exercise.

Liver glycogen stores maintain blood glucose levels both at rest and during exercise. At rest, the brain and central nervous system (CNS) use most of the blood glucose, and muscles utilize less than 20%. However, during exercise, muscle glucose uptake increases 30-fold, depending on the intensity and duration of exercise. Initially, most liver glucose is obtained from glycogenolysis, but as exercise duration increases and liver glycogen decreases, the contribution of glucose from gluconeogenesis increases.

At the onset of exercise, liver glucose output meets the increased muscle glucose uptake and blood glucose levels remain close to resting levels. Although muscle glycogen is the primary energy source at exercise intensities of 65% VO2max, blood glucose becomes the most important source of oxidation as muscle glycogen stores become depleted. When liver glucose output can no longer support muscle glucose uptake during prolonged exercise, blood glucose levels fall. While some athletes experienced CNS symptoms typical of hypoglycemia, most athletes experienced local muscle fatigue and had to reduce exercise intensity.

Liver glycogen stores can be depleted by a 15-day fast and decrease from a typical level of 490 mmol on a mixed diet to 60 mmol on a low-carbohydrate diet. A high-carbohydrate diet can increase liver glycogen to approximately 900 mmol.

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