The fasting state occurs around 18 hours after the beginning of your fast and can last for up to 2 days. Because your glycogen stores have most likely been completely depleted by this point, your body has probably already started breaking down stored fats (aka triglycerides) and proteins and converting them into energy.
To dive right into the nitty gritty details, this is all done using a process called lipolysis5, which relies on a specific enzyme known as hormone-sensitive lipase. It also results in the production of ketones8, which are chemicals that help supply extra energy1 to tissues throughout the body, including the brain.
Over time, this causes your body to enter ketosis, a metabolic state in which your body relies on fat instead of sugar9 as its main source of energy. This can also be achieved without fasting, using very low-carb diets like keto.
This basically means that your body is being powered by ketones instead of carbs because you’re not eating.
How long it takes you to enter ketosis can vary depending on many factors10, including your age, activity level, and metabolism. It can also be influenced by your usual diet. For instance, if sugary snacks, sweets, and soda are a staple in your daily diet, it may take you a little longer to completely deplete your glycogen stores.
Interestingly, some research also suggests that this stage of fasting may trigger autophagy, a catabolic process used to clean out damaged cells and replace them with healthy new ones. In fact, one study showed that markers of autophagy were detected11 in white blood cells after just 24 hours of fasting. Plus, levels of mTOR, a protein that blocks autophagy12, are also decreased during this stage.
“Research suggests that autophagy13 may have a protective effect against aging, cancer, and other diseases,” explains Humaira Jamshed, Ph.D., a nutrition and TRE researcher at the Dhanani School of Science and Engineering.
Impressively enough, some researchers even believe that enhancing autophagy could extend the lifespan14 and slow signs of aging.
