Two parallel fuel pipelines show glucose dimming and ketones flowing as the fasting threshold valve flips around hour 14-16.
A free, animated glucose to ketone fuel switch you can read here or embed on any website, from Scrollchart.
Glucose to Ketone Fuel Switch
The body maintains a continuous glucose supply from diet, liver glycogen, and gluconeogenesis. During fasting, as liver glycogen depletes and insulin falls, hepatic fatty acid beta-oxidation accelerates and produces acetyl-CoA in excess of TCA capacity. The overflow is diverted into ketogenesis: acetoacetate, beta-hydroxybutyrate (BHB), and acetone.
The brain, which cannot oxidize fatty acids directly, up-regulates monocarboxylate transporters and begins replacing glucose with BHB as its primary fuel. Full metabolic flexibility includes a smooth, reversible transition between these fuel states.
The key regulatory switch is malonyl-CoA, a product of insulin-stimulated ACC activity. In the fed state, high malonyl-CoA inhibits CPT1 and blocks mitochondrial fatty acid import. As insulin falls (12-16h into a fast), ACC activity drops, malonyl-CoA falls, CPT1 opens, and beta-oxidation begins in earnest.
The liver converts acetyl-CoA to ketones via HMG-CoA lyase. Blood BHB rises from a fasting baseline of ~0.1 mM to 1-3 mM in nutritional ketosis and 5-7 mM during extended fasting. The brain can derive up to 60-70% of its energy from BHB during a prolonged fast.
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Source & accuracy
This glucose to ketone fuel switch is an editorial illustration built to represent the concept accurately. Where it shows figures, they are typical or representative values chosen to make the relationship clear, not a single underlying dataset. The diagram and its explainer are reviewed and maintained centrally, and updated over time as understanding improves.
Two fuel lines that trade places
In the fed state, glucose is the body's readily available fuel and circulating insulin keeps fat largely in storage. As fasting continues and liver glycogen is drawn down, glucose availability falls and the body shifts toward fat-derived fuel. Fatty acids are oxidized directly by many tissues, and the liver converts surplus acetyl-CoA into ketone bodies that the brain and other tissues can use.
Reading the timing of the switch
The transition is gradual rather than a single moment, and the often-cited window of roughly 12 to 16 hours is an approximation that depends heavily on the prior meal, glycogen stores, activity, and individual metabolism. Some people shift earlier and some later, and the body uses a blend of fuels throughout rather than flipping cleanly from one to the other. General educational information about metabolism, not medical advice; fasting is not suitable for everyone, including people with diabetes or who are pregnant.
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Reference
What this is
A free, embeddable, animated glucose to ketone fuel switch for any website.
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Nutrition blogs, Biohacker blogs, Health blogs, Weight-loss sites.
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Frequently asked questions
Where can I get a free animated "Glucose to Ketone Fuel Switch" for my website?
Scrollchart provides "Glucose to Ketone Fuel Switch" as a free, embeddable animated diagram you can add to any website with one line of HTML. No signup is required and there is no watermark. The diagram and its explainer text are served from scrollchart.com, so the embed stays current without any maintenance on your end.
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