Here's What Happens To Your Body Each Hour Of Your Fast

Fasting is a popular biohack for optimizing well-being. Whether it's intermittent fasting or reverse intermittent fasting, the premise is very similar — abstain or reduce the amount of food being consumed to certain hours of the day. Not only can this aid in weight loss, but Boulder Medical Center suggests other health benefits include enhancing cognitive performance, reducing inflammation, and decreasing the risk of various chronic diseases. Beyond the health benefits, what's actually happening to your body each hour during your fast?

According to MindBodyGreen, the moment you stop eating (and begin your fast), your pancreas starts secreting a hormone called insulin. During the "anabolic growth phase" (0 to 4 hours) it focuses on creating and storing energy, as well as growing tissues and cells (via MindBodyGreen). However, at around the 3- to 4-hour mark, insulin and blood sugar levels begin declining, and the body starts entering into an early fasting state (per Healthline).

The next phase is a breakdown phase or "catabolic phase," which lasts up to 16 hours (via MindBodyGreen). This is when your body gets energy from converting glycogen into glucose, but once it runs out, your body starts pulling fuel from stored fat cells. The process of autophagy then begins and forces your body to deep clean its cells, removing any damaged or dead material.

This is what happens after 16 hours of fasting

By the time you've reached 16 hours, glycogen and glucose levels are nearly depleted, and stored fat cells become your only source of energy (via MindBodyGreen). When fat cells are converted into energy, the body starts to produce ketone bodies, according to a 2017 study published in Cell Metabolism. This production of ketone bodies also marks the start of ketosis (via Healthline). During ketosis, your body only relies on stored fat for energy. Once this happens, it's a tell-tell sign your body is in full fasting mode.

While elevated amounts of ketone bodies is a sign that your body has reached ketosis, there are other common signs. One 2015 study published in Frontiers in Psychology notes that other markers include fruity or smelly breath, reduced appetite, tiredness, and fat loss. Once you hit the 24-hour mark, you may experience a brain boost from the burning of ketones and the production of brain-derived nootropic factor (via MindBodyGreen).

The starvation state is reached once you've fasted for over 48 hours (via Healthline). In this stage, you'll likely experience the breakdown of branch-chain amino acids, more mental clarity, and weight loss (via Healthline). If you're attempting long-term fasting, it's highly recommended to seek medical supervision first.

It's important to note that people may enter fasting stages quicker or slower than others. Healthline points out that fasting states are also influenced by your last meal (i.e. size, foods eaten, etc.) and genetics.