| THE ORGAN-FUEL INTERACTIONS
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At rest, the brain uses approximately 20% of all oxygen (O2) consumed by the body. As mentioned above, glucose is normally the brain's only fuel, but during starvation the brain can use ketones as an alternate energy source.
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| Gluconeogenesis occurs primarily in the liver
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|
Table 20-1.
Principal anabolic and catabolic pathways, and their main substrates and products. |
| Body_ID: None |
| Utilization and storage of metabotic fuets |
| Body_ID: T020001.50 |
| Pathways | Main substrates | End products |
| Body_ID: T020001.100 |
| Anabolic | | |
| Body_ID: T020001.150 |
| gluconeogenesis | lactate, alanine, glycerol | glucose |
| Body_ID: T020001.200 |
| glycogen synthesis | G-1-P | glycogen |
| Body_ID: T020001.250 |
| protein synthesis | amino acids | proteins |
| Body_ID: T020001.300 |
| lipogenesis | acetyl-CoA, glycerol | fatty acids, triglycerides |
| Body_ID: T020001.350 |
| Catabolic | | |
| Body_ID: T020001.400 |
| glycolysis | glucose | pyruvate, ATP |
| Body_ID: T020001.450 |
| tricarboxylic acid cycle | pyruvate acetyl-CoA, pyruvate | NADH + H+, FADH2, CO2, H2O, ATP |
| Body_ID: T020001.500 |
| glycogenolysis | glycogen | G-1-P, glucose |
| Body_ID: T020001.550 |
| pentose phosphate pathway | G-6-P | NADPH + H+, pentoses, CO2 |
| Body_ID: T020001.600 |
| lipolysis | triglycerides | glycerol, fatty acids |
| Body_ID: T020001.650 |
| proteolysis | proteins | amino acids → glucose, amino acids → ketones |
| Body_ID: T020001.700 |
|
| Body_ID: T020001.750 |
Metabolites, such as pyruvate and acetyl-CoA, link different pathways.
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| page 273 |  | | page 274 |
| When the glucose content of the extracellular fluid decreases, glycogen is mobilized within seconds, providing a short-term supply of endogenous glucose. Subsequently, this supply is complemented by gluconeogenesis, the other source of endogenous glucose. Gluconeogenesis takes place primarily in the liver, with the kidneys contributing during a prolonged fast. The substrates for gluconeogenesis originate from anaerobic glycolysis (lactate) and the breakdown of
either muscle protein (alanine) or adipose tissue triglycerides (glycerol).
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| Muscle handles carbohydrates quite differently to the liver. In contrast to the liver, it does not have glucose-6-phosphatase (Glc-6-Pase) and therefore cannot release glucose into the circulation. Instead, muscle uses glycogen for its own energy needs. It does, however, contribute to endogenous glucose production by releasing lactate, a product of anaerobic glycolysis. Lactate is transported to the liver, where it enters gluconeogenesis. Muscle can use both glucose and fatty acids as energy sources. During intensive exercise, glucose is the preferred fuel. Fatty acids are the main energy source at rest and during prolonged exercise (see Chapter 19).
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