The feed/fast cycle refers to the chain of events that occur after you have eaten a meal. It is a look at the big picture with regards to metabolism. Let's look at the feed fast cycle looking at an average timeline after you have eaten a meal.
0 - 4 hrs
After you have just eaten a meal, your body absorbs the sugar into the blood, which causes your blood sugar to spike up. This stimulates your pancreas to release insulin, which causes most cells in your body to take in glucose from the blood.
4 - 24 hrs
Insulin levels have dropped because the glucose from the meal is gone. Most cells are using fat as their main energy source; glucose is being reserved primarily for the brain. Now blood glucose is maintained by the liver, through the breakdown of glycogen stores. These stores are normaly depleted within 24 hrs.
1 - 2 day
After the glycogen stores are gone, the liver maintains blood glucose levels through gluconeogenesis. Thus, the liver makes glucose from either fat or proteins. Making glucose from protein means the body is breaking down muscle tissue and other essential proteins because the body does not store any protein as an intended energy reserve.
2 days - several weeks
As the fat cells release more fat into the blood, the liver starts to release ketone bodies. The brain begins to use ketone bodies for energy and thus, requires less glucose. This fall in glucose depend results in less gluconeogenesis, which means less protein breakdown.
After the fat reserves are depleted (timeline depends on individual body fat), the sole method of blood glucose maintenance is through the breakdown of proteins. The body basically cannabilises itself. People that die from starvation do not die because they run out of protein; they die because they eat away essential proteins like the heart. Note, many people die from complications of starvation before they get to this point.
That sums up the feed/fast cycle. The next metabolism post will talk about the science behind the Atkins diet and other low carb weight loss gimicks