A while back, I noticed that my Blood Sugar peaks around the second day of extended fasting. George Cahill did the seminal work measuring blood markers during starvation (Cahill, George. Fuel Metabolism in Starvation.). Here’s an interesting chart from that study that explains the sources of glucose during starvation.
This demonstrates the increase in blood sugar around day 2-3. Diabetics are particularly adept at GNG. Eventually though, even that reduces as the body becomes physiologically Insulin Resistant.
The chart can provide some idea of what happens in a ketogenic diet. Although someone on a ketogenic diet is eating enough food, their exogenous glucose is greatly reduced due to the low carbohydrate content of the diet. Glycogen stores lower next. When the glycogen stores get low the body then upregulates Glyconeogenesis (GNG).
This could also explain why when I see an increase in blood sugars on one morning I often see a drop in weight the following morning. The body is signalling that it is switching fuel to up-regulated GNG due to dropped Glycogen stores. Although these two sources are of the same magnitude in Cahill’s chart above they could well be less equally matched in a diabetic. It is possible that GNG in a diabetic outpaces the ability to pull from Glycogen stores.