The science
The metabolic switch
How your body moves from burning sugar to burning fat, when it happens and what speeds it up
Your body runs on two main fuels: glucose, a sugar, and fat. After a meal it burns mostly glucose; during a fast it gradually shifts to fat and to ketones, a fuel your liver makes from fat. Researchers call this shift the metabolic switch, and it's the main thing that makes a fast different from simply eating less 1.
Two fuel tanks
Think of your body as having two fuel tanks. The first is small and fast: glucose in your blood and glycogen, the form in which your liver and muscles store glucose. The second is large and slow: the fat in your fat cells, which holds far more energy. The liver's glycogen tank is small: within the first day and a half or so of a fast, it supplies only a small share of your glucose 2.
The switch, step by step
Most daily fasting plans, such as 16:8, sit right at the start of the switch. That's one reason researchers are interested in them: they repeat this transition every day, without the strain of long fasts 1,4.
Metabolic flexibility
A healthy body switches smoothly between glucose and fat as conditions change. Scientists call this metabolic flexibility. People with obesity or type 2 diabetes tend to be less flexible, and researchers are studying how exercise and changes in eating can restore it 5.
How to reach the switch sooner
The switch comes when your liver's glycogen runs low, so anything that empties it faster brings it forward, and anything that fills it up pushes it back 1.
Sources
- Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
- Rothman DL, Magnusson I, Katz LD, et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573-6. doi.org/10.1126/science.1948033
- Owen OE, Morgan AP, Kemp HG, et al. Brain metabolism during fasting. J Clin Invest. 1967;46(10):1589-95. doi.org/10.1172/JCI105650
- Mattson MP, Moehl K, Ghena N, et al. Intermittent metabolic switching, neuroplasticity and brain health. Nat Rev Neurosci. 2018;19(2):63-80. doi.org/10.1038/nrn.2017.156
- Goodpaster BH, Sparks LM. Metabolic Flexibility in Health and Disease. Cell Metab. 2017;25(5):1027-1036. doi.org/10.1016/j.cmet.2017.04.015
Quick answers
The science
- Fasting stages, hour by hourWhat your body may be doing from your last meal to 72 hours of fasting, and roughly when each stage begins
Autophagy: your cells' recycling systemWhat autophagy is, what fasting does to it, and why nobody knows the exact hour it starts
Insulin and fastingThe hormone that decides whether you store or burn, how insulin resistance develops and what fasting can do about it
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.