The accumulation of nutrients that contain energy - carbohydrates (glucose), proteins (amino acids) and fats (fatty acids) - represents a single process. Surpluses of these substances accumulate in the form of fats. Glucose can be used to synthesize amino acids, and some amino acids can be used for the synthesis of glucose. However, these processes lead to energy costs, for example, 5% of energy is lost when glucose is accumulated in muscles in the form of glycogen instead of directly used for the production of ATP. This figure is increased to 28% when glucose is converted to fatty acids for deposition.
Energy systems using these nutrients work not one after another (first the ATP-KrF system, then the anaerobic glycolysis system and finally the aerobic metabolism), but are switched on simultaneously, and their contributions vary depending on the accumulation level, the presence of oxygen and the level motor activity.
For example, the presence of oxygen affects which substrate is used to generate energy. For one fatty acid carbon atom, 8.2 molecules of ATP are produced, and only 6.2 molecules of ATP are produced per carbon atom of the glucose molecule. With a limited amount of oxygen, glucose is the preferred source for aerobic metabolism and the only one for anaerobic oxidation. Hormonal changes, as a consequence of diet and exercise, significantly affect energy flows. Fatty acids produce energy with the help of an aerobic system. However, the use of fatty acids depends on the simultaneous flow of carbohydrates in the energy pathways for the regeneration of intermediate compounds in the Krebs cycle.
Without an adequate amount of carbohydrates, fatty acids are transferred to a different pathway of metabolism. Therefore, instead of leading to the production of ATP, fatty acids produce ketones. Only certain tissues, such as the brain, can use ketones to produce energy. If the carbohydrate stores are small, the ketone content may increase and cause fatigue and imbalance in metabolism.
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