My 36-Hour Fast: What Happens Inside The Body
I recently completed a 36-hour fast with only water and black coffee. I went into it curious and open-minded, and I came out of it feeling calm, steady, and clear. My mind felt sharp. My energy stayed stable. Hunger was pretty quiet, almost peaceful. The experience made me want to understand what was happening inside my body, so I spent time reading the research on extended fasting and the changes that unfold once your body shifts from burning glucose to burning fat.
During a fast of this length, your body moves through its stored glucose and begins to rely on fat for fuel. This shift usually happens somewhere between twelve and sixteen hours. Once glycogen is used up, fat becomes the primary energy source. Your liver breaks down stored fat into fatty acids and converts them into ketones. These ketones give your body, and especially your brain, a steady and efficient stream of energy. Many people feel more focused during a fast because the brain runs smoothly on ketones. I felt that same clarity. It was gentle and grounding.
The body also increases the rate of fat burning during this time. As stored fat breaks down, insulin levels fall and insulin sensitivity improves. Growth hormone rises as well. Research shows that human growth hormone increases during a fast, and this helps protect muscle and support metabolism. This shift helps your body use fat more effectively while keeping your muscle tissue stable.
Another change that happens during a longer fast is autophagy. This is the body’s natural clean up process, and the process I was most interested to understand. Autophagy gives your cells a chance to clear out what is worn down or no longer useful. Old proteins are broken apart and recycled. Damaged cell parts are removed, so healthier ones can take their place. Your body does this on a small scale every day, but fasting increases the intensity of this work. When you are not digesting food, your cells shift their focus to repair. This internal reset supports better function in every tissue, including your brain, liver, muscles, and immune system. Researchers study autophagy because it is linked to healthier aging, steadier energy, better metabolic health, and stronger cellular resilience. It is a powerful reminder of how much your body does to maintain balance when given the time and space to restore itself.
Your digestive system rests during a fast. The gut lining gets a break from constant processing, which allows it to repair. Inflammation quiets down. The balance of gut bacteria improves. Some studies show that fasting helps activate intestinal stem cells, which support the rebuilding of the gut lining and strengthen long term digestive health. Giving the gut a moment of rest can bring a noticeable sense of calm to the entire body.
The liver benefits too. Without the steady flow of incoming food, the liver can focus on clearing toxins and waste products more efficiently. This is one reason people often feel lighter and more clear after an extended fast. The body has space to do deeper internal work.
As I moved through the fast, I felt grateful for how capable this human body is when we give it the right environment. The benefits reach far beyond a single day. Better fat burning, steadier energy, improved mental clarity, cellular repair, gut renewal, and a more efficient liver all work together in a way that feels both natural and supportive.
My 36-hour fast left me with a deep respect for how much healing happens behind the scenes. The human body is an amazing super computer of sorts. When you give it the right fuel and remove the overload of constant eating, digestion, and processing, it shifts into repair mode. Systems that usually stay busy with incoming food can redirect their energy toward healing, restoring balance, and strengthening long term health.
If you are thinking about trying a longer fast, listen to your body and speak with your healthcare provider, especially if you have medical conditions. Fasting is a simple practice, but the changes it brings are meaningful. You might feel a level of clarity and strength that surprises you.
Disclaimer: The content shared here is for informational and educational purposes only and should never be taken as medical advice.
In writing this blog post, my goal is to distill research findings into a clear, approachable format that encourages critical thinking and empowers you to make informed decisions about your health.
