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. 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. 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 material3, which can otherwise contribute to aging, cancer, and chronic disease. 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. 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)4. 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. Research suggests longer fasts can help your body become more resilient5 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.
