We have glucose in our blood at all times. Where does it come from?
The first and most obvious source is from the carbohydrates in our food. Most of the complex carbs we eat, from sugar to pasta, can be broken down into glucose monosaccharides and will be absorbed into the circulation in that form. The illustration above is taken from page 329 of Lippincott's Illustrated Reviews-Biochemistry. It shows the effect of a meal of 100 grams of pure glucose. The large spike of glucose is metabolized or stored within about three hours.
If we do not eat any more food after that 100 grams of glucose, our blood glucose levels nevertheless must be maintained. If we are well-fed, our livers store about 80-100 grams of glucose as glycogen. The liver is able to convert glycogen back into glucose by means of a process called glycogenolysis. The blue line on the graph shows that, over the next few hours, the liver releases glucose back into the blood on an as-needed basis.
What if we continue to fast? Eventually, we run out of liver glycogen. If you look at the green line in the graph, you'll see that it gradually rises. This line represents gluconeogenesis, which we have discussed before. In the absence of ingested carbohydrates, and even after our liver glycogen is depleted, our liver is able to make glucose out of glycogenic amino acids and the glycerol backbones of fatty acids. It takes several hours or even days for this process to ramp up.
What if we are not fasting but are simply low-carbing? The ramping-up of gluconeogenesis is one of the reasons that people who are new to low-carbing experience something called the "Atkins flu" when they start eating 20 grams of carbs or less per day for the first time. They are not starving, but their bodies have to get used to manufacturing new glucose out of amino acids and glycerol rather than absorbing ready-made glucose from the gut. Once gluconeogenesis is established, our bodies can readily make glucose from the protein we eat.
However, even when it is well established, gluconeogenesis is not a quick-response system. Because it takes us several hours to digest a protein meal into its constituent amino acids, followed by more time to convert the amino acids into glucose, the process of gluconeogenesis is relatively slow. However, the glucose we produce can be used to replenish the glycogen in the liver, and liver glycogen is a quick-response system.
Once we are adapted to a low intake of carbohydrates, we have two means of stabilizing our blood sugar. Gluconeogeneis provides a baseline of glucose production from the protein we eat. Because the fuel needed for gluconeogenesis ebbs and flows, the new glucose made by this process ebbs and flows. Fortunately our stored liver glycogen is available to make up the difference, assuring a steady stream of glucose for baseline functions for our brain, red blood cells and other tissues that require glucose as fuel. If we need an extra burst of energy, or if a big protein meal causes our insulin to spike and our glucose to plummet, our stored liver glycogen is also there to step in the gap and provide the glucose necessary to keep our blood glucose within normal limits.
When we first start low-carbing, it's hard to understand that (if we choose to) we never need to eat another gram of carbohydrate again. But once our good friend the liver is adapted to the process, it can provide all the carbs we need through gluconeogenesis and glycogenolysis.