George F Cahill Jr, Department of Medicine, Harvard School, published an article in 2006 in the Annual Review of Nutrition titled ‘Fuel Metabolism in Starvation’.* Here are the findings:
In the first 3 to 4 hours after a meal, all of the blood glucose comes from the food you’ve eaten [exogenous], and all of your tissues are using glucose for fuel, including your brain.
As that source is depleted, your body and brain pretty much continue to use glucose for fuel. The initial source of this additional fuel is the glucose your liver has stored as glycogen, but your liver also begins to create new glucose [gluconeogenesis]. After about 30 hours, the glycogen stores in your liver has been completely depleted, but gluconeogenesis continues (though at a reduced rate,) and your body switches to using ketone bodies [from stored fat] as its major source of fuel.
So, while shorter fasts are still beneficial, longer ones are also indicated, especially if one of your goals is to lose weight.