Quick Process of Ketosis During a Long Fast

1. Glycogen Runs Out (12–24 hours)

• Liver glycogen is used first to maintain blood glucose.
• Once depleted, the body must switch fuels.

2. Insulin Drops, Glucagon Rises

• Low insulin = fat cells open up.
• Glucagon signals the liver to start producing alternative fuels.

3. Fat Mobilization Begins (24–48 hours)

• Adipose tissue releases free fatty acids (FFAs) into the bloodstream.
• Muscles and liver take them up.

4. Liver Converts Fatty Acids → Acetyl‑CoA

• Beta‑oxidation in the liver mitochondria breaks fatty acids into Acetyl‑CoA.
• But without carbs, the Krebs cycle slows (low oxaloacetate).

5. Excess Acetyl‑CoA → Ketone Bodies (48–72 hours)

The liver converts Acetyl‑CoA into:

• Acetoacetate
• β‑Hydroxybutyrate (BHB)
• Acetone

These are released into the bloodstream.

6. Brain and Muscles Switch to Ketones (3–7 days)

• Brain begins using ketones for up to 60% of its energy.
• Muscles use fatty acids directly, sparing glucose.

7. Protein Sparing Phase (after ~1 week)

• As ketone levels rise, the body reduces muscle breakdown.
• This is why long fasts don’t automatically cause massive muscle loss- ketones protect lean tissue.

8. Deep Ketosis (weeks into fasting)

• Very low insulin
• High glucagon
• High growth hormone
• Stable ketone production
• Fat becomes the primary fuel source
• Mental clarity increases due to steady BHB supply

In One Sentence

Ketosis is the metabolic shift from glucose to fat‑derived ketones as the primary fuel once glycogen is depleted and insulin drops.