Ketone Body Production in the Liver

In the liver, fatty acids and amino acids are metabolized to acetoacetate and β-hydroxybutyrate (to be used in muscle and brain).

In prolonged starvation and diabetic ketoacidosis, oxaloacetate is depleted for gluconeogenesis. With chronic alcohol overuse, high NADH state leads to accumulation of oxaloacetate (downregulated TCA cycle), shunting it to malate.

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Ketone Body Characteristics and Utilization

Ketone bodies include acetone, acetoacetate, and β-hydroxybutyrate. Acetone is responsible for the fruity odor often detected on the breath.

A urine test for ketones can detect acetoacetate, but not β-hydroxybutyrate. It’s important to note that red blood cells, or RBCs, cannot utilize ketone bodies; they strictly rely on glucose for energy.

HMG-CoA lyase is crucial for ketone body production, while HMG-CoA reductase is involved in cholesterol synthesis.

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Fasted vs Fed State: Adipocyte Metabolism in Fasted State

In the fasted state, adipocytes play a crucial role in providing energy. Hormones like epinephrine and glucagon stimulate the breakdown of triglycerides.

This process begins with epinephrine and glucagon activating adenylate cyclase, which converts ATP to cAMP. cAMP then activates Protein Kinase A, which phosphorylates and activates Hormone-Sensitive Lipase, or HSL.

Activated HSL, or HSL-P, hydrolyzes triglycerides (TG) into free fatty acids (FFA) and glycerol. These FFAs and glycerol are then released into the blood to be used by other tissues for energy.

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Fasted vs Fed State: Adipocyte Metabolism in Fed State

In contrast, during the fed state, the body is in an anabolic mode, storing excess energy. Insulin plays a central role in this process.

When glucose levels are high after a meal, insulin is released. Insulin promotes the uptake of glucose into adipocytes. Inside the adipocyte, glucose is converted to DHAP, or dihydroxyacetone phosphate, and then to Glycerol 3-P.

Simultaneously, lipoprotein lipase, or LPL, which is activated by Apo C-II, breaks down triglycerides from circulating lipoproteins. The resulting free fatty acids are taken up by the adipocyte.

These free fatty acids, along with Glycerol 3-P, are then re-esterified to form new triglycerides (TG) for storage within the adipocyte.

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Topics Covered:

• Hepatic Ketone Body Synthesis
• Ketone Body Properties and Usage
• Adipocyte Metabolism: Fasted State
• Adipocyte Metabolism: Fed State