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When the stored glucose runs low, your liver builds more from other material. This is why a long fast does not send your blood sugar into freefall, and why you can think clearly on day two.
It is the process that keeps your blood glucose in range for as long as the fast continues.
THE PROBLEM
Some tissues require glucose, and the stored supply is limited.
THE SOLUTION
The liver assembles new glucose from non-carbohydrate sources.
THE RESULT
Blood glucose stays in range for as long as the fast continues.
What it draws on
Gluconeogenesis means making new glucose. The inputs are things your body already has circulating or stored.
FROM FAT
When fat is broken down it releases fatty acids and glycerol. The fatty acids become fuel or ketones; the glycerol can be converted into glucose.
FROM PROTEIN
Certain amino acids can be routed into glucose production. This is the input people worry about, and it is covered in detail below.
FROM METABOLISM
Lactate produced by working muscle can be returned to the liver and rebuilt into glucose, then sent back out. A genuine recycling loop.
The muscle question
This is the concern the amino acid route raises, and it deserves a direct answer rather than reassurance. Protein can be used for glucose production, so the question is a fair one.
What the physiology shows is a shift in priority. As ketone production increases, the brain uses more ketones and needs less glucose, which reduces the demand on protein. Fasting is not a state your body enters unprepared.
Burn names this at the Fat Burning stage, where increased growth hormone helps preserve lean muscle. Continuing to train and eating sufficient protein in your window are the levers you control.
A 16 hour fast and a four day fast are different propositions. Extended fasting is where this question stops being theoretical, which is why long fasts belong with a doctor.
What it explains
The intuition that no food means no blood sugar is wrong. Production continues, which is why extended fasting is possible at all.
If you measure your own blood glucose during a fast and it sits in range, that is the system working rather than evidence the fast is not doing anything.
Ketones cover much of the brain's energy needs on a long fast, but not all of them. Some glucose is still required, and this is where it comes from.
Gluconeogenesis, its substrates, and the shift toward ketones as a fast lengthens are standard physiology and well described in the literature. How much protein is used across different fast lengths and training states is an active research area. Claims here are drawn from peer-reviewed research and traced in our evidence policy.
Speak to your doctor before changing your eating pattern if you manage diabetes or take medication timed to meals. Burn stores readings you record yourself and does not estimate them, and it does not measure ketosis, autophagy, or metabolic state.
Burn names all nine stages as you reach them, with the reasoning written out in the app.
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