Fasting, hour by hour
What happens after 12 hours of fasting?
Around 12 hours, your body reaches the Fuel Switch: its sugar stores run low and fat steps in
- 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
- 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
- 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
What's happening in your body
For the first hours after a meal your body runs on glucose from your food. As insulin falls, your liver keeps your blood sugar steady, first from its stored glycogen and increasingly by making new glucose 2. Around 12 hours the glycogen is running low and the brake on your fat cells comes off. Fat becomes a growing share of your fuel; ketones come later 1.
| Stage | Fuel Switch (12 to 16 h) |
|---|---|
| Main fuel | Glycogen, then more and more fat |
| Insulin | Low |
How you might feel
Hunger may peak around your usual mealtimes, and some people feel a little low on energy while the switch happens. Hunger comes in waves and usually passes on its own 3.
What helps
- A light walk helps your body tap into its fat stores.
- Fasting through the night makes this stage much easier.
- If you feel lightheaded, a pinch of salt in water can help.
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
- Natalucci G, Riedl S, Gleiss A, et al. Spontaneous 24-h ghrelin secretion pattern in fasting subjects: maintenance of a meal-related pattern. Eur J Endocrinol. 2005;152(6):845-50. doi.org/10.1530/eje.1.01919
Keep reading
- What happens after 16 hours of fasting?Around 16 hours, your body reaches Fat Burning: stored fat becomes a major fuel
- 13:11 intermittent fastingA 13-hour overnight fast and an 11-hour eating window: the gentlest way to start fasting
The metabolic switchHow your body moves from burning sugar to burning fat, when it happens and what speeds it up
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.