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Amino acid metabolism
Nitrogen and urea cycle
Citric acid cycle
Electron transport chain and oxidative phosphorylation
Pentose phosphate pathway
Physiological changes during exercise
Fatty acid oxidation
Fatty acid synthesis
Ketone body metabolism
Maple syrup urine disease
Ornithine transcarbamylase deficiency
Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Hereditary fructose intolerance
Pyruvate dehydrogenase deficiency
Glycogen storage disease type I
Glycogen storage disease type II (NORD)
Glycogen storage disease type III
Glycogen storage disease type IV
Glycogen storage disease type V
Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)
Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)
Fabry disease (NORD)
Gaucher disease (NORD)
Metachromatic leukodystrophy (NORD)
Niemann-Pick disease type C
Niemann-Pick disease types A and B (NORD)
Tay-Sachs disease (NORD)
Disorders of amino acid metabolism: Pathology review
Disorders of carbohydrate metabolism: Pathology review
Disorders of fatty acid metabolism: Pathology review
Dyslipidemias: Pathology review
Glycogen storage disorders: Pathology review
Lysosomal storage disorders: Pathology review
Ketogenesis and Ketogenolysis
Lipid Metabolism and Ketogenesis
In life, it’s helpful to have a plan B in case plan A doesn’t work out.
In terms of energy, the body’s plan A is to generate energy from carbohydrates, fats, and proteins - basically in that order.
But if these main fuels aren’t readily available, then plan B is to use an alternative fuel source - ketone bodies.
Ketone bodies are a group of carbon-containing molecules produced by liver mitochondria using a 2-carbon molecule called acetyl-CoA.
The liver makes ketone bodies in physiologic states like prolonged fasting or exercise, as well as in pathological states like type 1 diabetes mellitus or alcoholism.
Ketone bodies can be released into the circulation and get picked up by the majority of cells.
Inside the cells, they’re reconverted back into acetyl-CoA, at which point they can then enter the mitochondria and produce ATP.
The 3 primary ketone bodies are acetoacetate, beta-hydroxybutyrate, and acetone.
Alright, so let’s say you decide to go on a 5-day fast.
About 12 hours into your fast, your blood glucose levels start to dip.
In response, glucagon is secreted from the pancreas and stimulates hepatic glycogenolysis - meaning that the liver begins to break down glycogen into glucose and release that glucose into the blood.
About 24 hours into your fast, your liver begins running out of glycogen, so it starts the process of gluconeogenesis which is where it makes new glucose molecules from substrates like amino acids.
Then, around 1 to 3 days into your fast, your body begins to run out of the necessary substrates to make new glucose.
So, it switches to breaking down fatty acids for energy.
Fatty acids are mobilized from fat stores and are broken down to acetyl CoA through beta oxidation in the mitochondria of most cells - except for brain cells.
Ketone bodies like beta-hydroxybutyrate and acetoacetate are an alternative source of energy in states of prolonged starvation. Ketone body synthesis occurs in the liver in physiologic states like prolonged fasting or exercise, as well as in pathological states like type 1 diabetes mellitus or alcoholism. Once synthesized, ketone bodies can leave the liver, and enter into peripheral cells such as the brain, skeletal muscle and kidney to serve as energy sources.
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