During fasting our energy storage process reverses, insulin falls, and that decline is a signal that the body should start using some of the stored food energy to power the body.
Dr. George Cahill described the 5 stages between eating and prolonged starvation/fasting as follows;
The first few hours after you have eaten;
Insulin is high and you still mostly drawn from the glucose you have eaten.
All the tissue of the body can use glucose
You still store food energy as glycogen, excess is stored as body fat.
4 to 16 hours after eating;
Exogenous glucose is no longer available as energy
You start to rely on body stores
The most readily available source of energy is glycogen in the liver
Glycogen is broken down into component glucose molecules and send out into the body for energy
Glycogen stores last for approximately for 24 hours
16 to 30 hours after eating;
Stores of glycogen start to run out
Body fat is not yet available
So you bridge the gap by producing glucose from protein (gluconeogenesis)
Gluconeogenesis = Creation of new glucose
In this stage your body transitions from using glucose (which is becoming scarce) to using fat and protein (from available body stores)
If you exercise after 24 hours this will help to speed up the process of burning fat and because growth hormone is high will help to build muscle.
30 hours to 24 day’s after eating
The body mobilises stores of body fat for energy.
Most tissue of the body has switched to fat burning triglycerides for energy. The brain, red blood cells and kidney must still use glucose.
Some of the backbone of triglyceride molecules is turned into glucose and some still derive from the breakdown of protein.
Protein breakdown is further reduced and the body mainly burns fat
Clinical evidence shows that alternating 24 hour repeated fasting/feeding does not cause muscle loss.
The proteins that turn over the most rapidly are the first proteins to be catabolized for glucose including the skin and intestinal lining. That means you will, with longer fasts, also loose your excess skin.
Dr. Fung in his intensive dietary program (www.DMprogram.com) which uses therapeutic fasting for weight loss has not yet sent any of his clients for skin removal surgery even those who lost over more than 100 pounds.
There is a significant difference in metabolizing protein between lean and obese people.
Obese people will burn 2 to 3 times less protein compared to lean people.
This means when people have more fat to burn, their body will use more of it.
If there is less fat available the body will be forced to rely on protein.
So when will autophagy kick in?
When you fast, you drop insulin and increase glucagon, which stimulates autophagy. However, it's not quite that simple: to induce autophagy, you need low liver glycogen, which is usually only achieved after about 14–16 hours of fasting, but is even more likely to happen after 24 hours.
So that means when you cut out of your diet as much sugar as possible this will help to reduce the glycogen in your liver which will activate autophagy already with intermittent fasting.
Exercise to induce autophagy
Autophagy occurs in response to stress. And, in fact, exercise is one of the ways by which you boost autophagy. Exercising creates mild damage to your muscles and tissues that your body then repairs, and by doing so makes your body stronger.
Exercise also helps flush out toxins by sweating and is helpful for just about any detox program. In fact, many consider exercise a foundational aspect of effective detoxification.
The amount of exercise required to induce autophagy is still unknown however intense (HIIT) exercise is believed to stimulate autophagy more than mild exercise.
Apparently, when you spend around 30% of your exercise on high intensity exercise rather than on mild exercise it is shown to increase longevity by 13% with an amount of 150 min to 450 min exercise a week.