From a New York Times article by Jane E. Brody (Feb 17, 2020):
“It takes 10 to 12 hours to use up the calories in the liver before a metabolic shift occurs to using stored fat,” Dr. Mattson told me. After meals, glucose is used for energy and fat is stored in fat tissue, but during fasts, once glucose is depleted, fat is broken down and used for energy.
I was skeptical, but it turns out there is something to be said for practicing a rather prolonged diurnal fast, preferably one lasting at least 16 hours. Mark P. Mattson, neuroscientist at the National Institute on Aging and Johns Hopkins University School of Medicine, explained that the liver stores glucose, which the body uses preferentially for energy before it turns to burning body fat.
For example, human studies of intermittent fasting found that it improved such disease indicators as insulin resistance, blood fat abnormalities, high blood pressure and inflammation, even independently of weight loss. In patients with multiple sclerosis, intermittent fasting reduced symptoms in just two months, a research team in Baltimore reported in 2018.