What Actually Happens To Your Body During A Fast, Hour By Hour 

Contributing writer By Alexis Shields, N.D.
Contributing writer
Alexis Shields, ND, is a licensed Naturopathic Doctor and an expert in Functional Medicine. She earned her doctorate at the National University of Natural Medicine and currently does virtual health consulting with busy professionals and families worldwide.
Medical review by Bindiya Gandhi, M.D.
Physician
Dr. Bindiya Gandhi is an American Board Family Medicine–certified physician who completed her family medicine training at Georgia Regents University/Medical College of Georgia.
Young woman in the desert

Fasting can have many powerful effects on your body. From weight management and cellular cleanup to digestion support and mental clarity, fasting can offer a number of benefits. But what's actually happening in your body, hour by hour, while you're fasting? 

Let's take a look at the various stages your body progresses through so you can get an idea of all the effects you're experiencing at different points throughout your fast. 

(Just note, if you do decide to do a prolonged fast, always do so with guidance from a medical professional.)

0 to 4 hours

The first four hours after you eat are known as the anabolic growth phase. Your body is using up the energy you just ate to power your current activity and for cellular and tissue growth.

Your pancreas produces the hormone insulin. This allows your body to use up the glucose that was released into your bloodstream after your meal and to store any excess energy in your cells for later use. 

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4 to 16 hours

The second phase kicks off after four hours and lasts up until about 16 hours from your last meal. This is the "catabolic," or breakdown, phase, when all those extra nutrients start being released from storage to be used for energy. 

Once the energy stored in your cells runs out, your body starts to rely instead on stored fat. The process of releasing fat and burning it up for energy releases chemicals known as ketone bodies for energy, which usually happens around the 16-hour mark. 

The rate at which you reach this stage really depends on what you ate for the last couple of meals before your fast. If you ate a lot of carbs and starch, it will take a bit longer than if you ate mostly fats and protein.

One of the most powerful features of fasting, called autophagy, also kicks off during this phase. 

Autophagy is triggered by a reduction in a growth regulator called MTOR, and this process is basically a spring cleaning for your cells. It gets rid of any dead or damaged cellular material, which can otherwise contribute to aging, cancer, and chronic disease.

16 to 24 hours

Once you pass the 16-hour mark, glucose in the cells and glycogen in the liver and muscles begin to dwindle rapidly, which causes you to burn stored fat in order to keep up with your body's need for energy. 

Your energy demands at this stage likely haven't changed much—you're still waking up, working, walking, interacting with people, maybe exercising—so the amount of energy that's needed can be quite significant.

The production of another chemical, AMP-activated protein kinase (AMPK), is also triggered at this point. This really increases the amount of autophagy happening all around your body.

24 to 72 hours

Again, it's important to work with a physician if you plan to do a prolonged fast.

Once you pass 24 hours of fasting, your body will enter into a state called ketosis, where you're reliant on burning your fat stores for energy. As fat cells are broken down for energy, ketone bodies are created and released into the bloodstream.

Ketone bodies act as fuel for the brain when glucose is scarce. Glucose is our brain's primary source of fuel, but ketone bodies provide the brain with additional benefits. 

Burning ketones can lead to a major uptick in your cognitive performance, mental clarity, and general sense of energy and well-being as you enter into a multiday fast. 

Your brain also gets a boost around the 24-hour mark from an increase in production of brain-derived nootropic factor (BDNF).

BDNF supports the growth of brain neurons. Not only is it correlated with improvements in long-term memory, coordination, and learning, but it's also thought to be key in reducing the risk of developing Alzheimer's disease in later life. 

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72+ hours

Once you've been fasting for three days or more, your body enters a deep state of ketosis. All the previous benefits: Autophagy, the uptick in the production of beneficial chemicals and hormones, fat loss, and mental clarity continue to increase. That said, your thyroid hormones also start to get affected, as your body thinks it's starving.

Research suggests longer fasts can help your body become more resilient to stress and exposure to toxins and that certain hormones produced at this stage can have beneficial anti-cancer and anti-aging effects. That said, people with a history of eating disorders should avoid prolonged fasts, and it's always a good idea to seek medical supervision when fasting.

Bottom line.

Fasting is a powerful tool for improving your body's longevity, and it can have a really transformative impact on your health. Once again, if you plan to do any kind of extended fast, it's very important to work with your medical practitioner. By understanding these stages, you can choose the right type and duration of fasts for your body and your goals. 

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