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Energy metabolism of carbohydrates, fats and proteins
Last reviewed: 04.07.2025
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The accumulation of energy-containing nutrients - carbohydrates (glucose), proteins (amino acids) and fats (fatty acids) - is a single process. Excess of these substances accumulates as fats. Glucose can be used to synthesize amino acids, and some amino acids - to synthesize glucose. However, these processes lead to energy costs, for example, 5% of energy is lost when glucose is stored in muscles as glycogen instead of being used directly to produce ATP. This figure increases to 28% when glucose is converted to fatty acids for storage.
The energy systems that use these nutrients do not operate one after the other (first the ATP-CrP system, then the anaerobic glycolysis system, and finally aerobic metabolism), but are activated simultaneously, and their contributions change depending on the level of accumulation, the availability of oxygen, and the level of motor activity.
For example, the availability of oxygen influences which substrate is used for energy production. For every carbon atom of a fatty acid, 8.2 molecules of ATP are produced, whereas for every carbon atom of a glucose molecule, only 6.2 molecules of ATP are produced. When oxygen is limited, 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 via the aerobic system. However, the use of fatty acids depends on the simultaneous flow of carbohydrates into energy pathways to regenerate intermediates in the Krebs cycle.
Without adequate dietary carbohydrates, fatty acids switch to a different metabolic pathway. So instead of leading to ATP production, fatty acids produce ketones. Only certain tissues, such as the brain, can use ketones for energy. If carbohydrate stores are low, ketone levels can increase and cause fatigue and metabolic imbalance.
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