Fasting has become a trendy dietary choice because many people believe it can help burn stored fat and improve insulin levels, heart health, and brain function. It can also trigger autophagy, a natural cellular recycling system that helps to clear out old and damaged cells to rebuild fresh new ones.
Much of the information you’ll find tells you exactly when autophagy begins during fasting but fails to mention that the timing depends largely on several factors, such as when you last ate, your activity levels, and whether your metabolic health is optimal. Because you’re unique, the timing of what you experience during fasting may vary, so this timeline is approximate.
Hours 0 to 4
In the first few hours after eating, known as the anabolic phase, your body is still breaking down the food from your last meal. During this time, it’s as if you haven’t really started fasting because your body is still digesting your food.
During digestion, your body breaks down everything you’ve eaten into smaller nutrients, or the basic building blocks such as amino acids and simple sugars, so it can use them for energy and other bodily processes.
Once simple sugars are converted into glucose and enter the bloodstream, insulin is released to help cells use that glucose for energy. Any leftover glucose is sent to the liver and muscles to be stored as glycogen, the stored form of glucose that acts as an energy reserve.
- Your body is still breaking down the food from your last meal, known as the anabolic phase.
- Food is broken into basic building blocks such as amino acids and simple sugars.
- Insulin helps cells use glucose for energy; leftover glucose is stored as glycogen.
Hours 4 to 12
After glucose is gone from the blood and insulin levels drop because they no longer have a task to do, glycogenolysis begins. This process breaks down stored glycogen in the liver and muscles, keeping blood sugar from dropping dangerously low.
As your body empties its glycogen stores to nothing, a process known as glycogen drawdown, you experience a metabolic switch. Since there is no more glucose in your body to be used for energy, it has to start shifting toward using something else as energy so that it can still perform basic functions.
The hunger hormone ghrelin may also spike during this time, especially if it lines up with your regular mealtimes, as the body gets used to eating at certain times and notices a lack of food. This is known as your circadian mealtime.
These hunger pangs generally last about 20 to 45 minutes if you don’t eat, and they subside once the eating window passes.
- Glycogenolysis breaks down stored glycogen, keeping blood sugar from dropping dangerously low.
- When glycogen runs out, your body makes a metabolic switch toward another fuel.
- Ghrelin may spike at your regular mealtimes; hunger pangs generally last about 20 to 45 minutes.
Hours 12 to 18
Hours 12 to 18 is a crucial time during a fast because the body has no glucose energy left. It’s been a long time since you’ve eaten, so your body starts pulling energy from fat stores. As fat cells get broken down to be used as energy, acidic byproducts known as ketones are produced by the liver. The longer fat is broken down for fuel, the more ketones the body produces, so their levels rise.
But that’s not the only change. Because your body has shifted toward burning fat for fuel, you will likely also experience a surge in ketones, a state known as ketosis, where your body burns fat instead of glucose.
This happens because human growth hormone (HGH) surges, acting as a muscle preserver during fasting while encouraging continued fat burning by breaking down triglycerides (fats) in tissues and converting them into free fatty acids, which can then be converted into more ketones.
The hormone ghrelin rises and falls throughout the fast, making you hungry at some times and not hungry at others, even as you move further from your last meal. As you get closer to the 18-hour mark, your ghrelin levels may rise again, reaching their highest levels since you last ate before going back down again. Because ghrelin operates in waves, you won’t feel a steady state of hunger the longer you forgo food.
- Your body starts pulling energy from fat stores, and the liver produces ketones.
- Ketosis begins: your body burns fat instead of glucose.
- Human growth hormone surges, acting as a muscle preserver while encouraging fat burning.
- Ghrelin comes in waves and may peak again near the 18-hour mark.
Hours 18 to 24
After 18 to 24 hours of fasting, your body starts the process of autophagy. That said, there is no specific hour during this window where the process is guaranteed to start. Research shows that while it typically happens during hours 18 to 24, it can start as early as hour 16 in some cases.
This specific type of autophagy is known as macroautophagy. During this process, cells collect waste, such as leftover proteins or damaged parts, in a double-walled bubble called an autophagosome. It acts like a garbage bag, holding everything the body no longer needs in one place so it can be sent to the lysosome, the body’s recycling center.
Once at the lysosome, the bubble is destroyed and converted into clean, undamaged building blocks the body can use as needed. The organs that become the focus during this process are the brain, liver, and muscles because they often generate the most cellular waste and require the most upkeep.
At the 24-hour mark, HGH also tends to surge dramatically, lending its hand in fat burning even more significantly.
- Autophagy typically starts during hours 18 to 24, as early as hour 16 in some cases.
- Cells collect waste in an autophagosome and send it to the lysosome to be recycled.
- The brain, liver and muscles get the most attention.
- HGH tends to surge dramatically at the 24-hour mark.
Beyond 24 Hours
After you’ve fasted for 24 hours, autophagy continues to increase, while HGH continues to support fat burning. This is a state of deep ketosis. Research shows that while this may help in some ways, it can also hurt in others.
For example, a narrative review examining prolonged fasting looked at its connection to weight loss and found that fasting for 5 to 20 days could lead to mild to moderate weight loss of 2 to 10%. However, only about one-third of the weight loss was fat mass, with most of it being lean muscle mass.
The narrative review also showed that fasting improved blood pressure and glycemic control in adults with normal blood sugar, with reductions in fasting glucose, insulin resistance, and long-term averages of blood sugar levels. However, participants with either type of diabetes did not see these blood glucose improvements.
All metabolic benefits reversed within three to four months after participants returned to eating, so any weight loss, blood pressure, and blood sugar management benefits were only temporary.
There were also some adverse events reported in the review, such as headaches, insomnia, hunger, and metabolic acidosis, which is a condition that occurs when there is too much acid in the blood, leading to a dangerously low pH level.
- Autophagy keeps increasing while HGH keeps supporting fat burning.
- Blood pressure and blood sugar improved in adults with normal blood sugar, but not in people with diabetes.
- All metabolic benefits reversed within three to four months of returning to eating.
- Reported side effects included headaches, insomnia, hunger and metabolic acidosis.
Who Should Not Fast
While short-term fasting has been shown to provide health benefits, some people should avoid it because it can do more harm than good.
Fasting for short windows can be beneficial if done correctly, but there are caveats to it that people should know before they start refraining from food. Depending on your level of health, fasting may also be dangerous.
Before starting any new fasting program, talk to your healthcare provider about why you think it will help and whether it’s safe for you. They can guide you toward a time window that works for you, if one exists, so you can achieve your goals with fasting without harming your overall health or risking serious complications.
Angelica Bottaro is a medical writer and researcher with a background in psychology and journalism.
Is your neighborhood healthy?
Get a free report on your tap water, air quality, walkability and more
