The Metabolic Timeline
In order to make sense of how fasting works, we need to peer into the bodies of two kinds of people: the evolutionary “normal” fasting individual and the modern “snacker”.
The Fasting Metabolic Timeline
Lets create a timeline of what happens in the body when someone stops eating food.
- Food Absorption (0-6 hours): The first few hours after eating food, your body begins to bring in nutrients from your intestines. While we are busy shuttling the food into our cells and storing fat, our bodies are able to survive on all the new macromolecules in the bloodstream. Insulin has peaked at this time.
- Glycogen Burning (6-24 hours): After all of the food has been stashed away in storage, our bodies once again need to begin using internal energy to sustain itself. As the insulin levels begin to fall, our liver begins to break down its glycogen stands into sugar for the body. This process can provide energy for up to one day.
- Gluconeogenesis (24-36 hours): After one day of not eating, the liver has run out of sugar for the body. In order to provide energy, the body resorts to breaking down protein for energy. This does not mean muscle breakdown! Our bodies are not stupid enough to waste away muscle in order to produce energy. Instead, our bodies seek out old, dysfunctional cells in our organs and eat them up. This process is known as autophagy and it is one of the best tools our bodies have against fighting diseases such as cancer.
In autophagy, our bodies undergo massive cell recycling. In nature, this would happen regularly since food was scarce all the time. Because of food abundance, people do not reach this stage of fasting often, resulting in junk building up in the body. Cells such as those of loose skin and precancerous cells are primary targets of this system.
- Fat Burning (36-72 hours): After this cleaning process has completed, insulin bottom lines and our bodies begin to tap into the huge energy stores present in fat. Almost all of our cells begin to burn fat for energy as the remaining sugar is saved for the brain. The ketones produced as fat is broken down begins to fuel the brain until up to 80% of the brain is running glucose-free.
- Protein Conservation (3 days-low body fat): As fasting continues for longer and longer, the body jacks up growth hormone production in order to make sure muscle is not being lost. This preservation mechanism will last until your body hits low body fat levels, at which point the muscle will be consumed.
As you can see, periods of lack of food are not only harmless but also incredibly beneficial for preserving healthy cells in your body. When this person does eat again, the body will be more efficient at adding protein back to important tissue and distributing energy more efficiently.
The Snacker: A Metabolic Timeline
The process that occurs in the average human in developed nations is quite different than the above example.
- Food Absorption (0-6 hours): In a chronic snacker, this is the stage that the body stays in for the entire waking day. Since a significant amount of food is always entering the body, insulin remains high and the body is trapped in a high sugar, high insulin state.
- Sugar Deloading (Sleeping): All of the sugar that was forced into the body cells as a result of constant eating is being burned during sleep. As a result, liver glycogen is barely touched.