Dr Jason Fung doing an interview with some good information on both fasting and ketogenic diet.

My wife and daughter in the last couple months have done some specific fasting. Think my daughter went 41 hours once and wife did 36 hours. They might do 24 hours once a week. Women tend to have stalls on weight loss and intermittent fasting can get them over the "hump" to start loosing again. I have done 24 hours a couple times just to see what it is like. Once a person if "fat adapted" (not on the high carb tread mill) it really is no big deal. You get a little hungry around normal meal time, then it passes in a half hour to an hour. I get a little weak/tired for about the same time, then the liver kicks in and does its gluconeogenesis thing and it passes.

I had an interesting observation one time quite by accident. I had went to the YMCA gym and did some weight machines then happened to run into a guy that played racquetball. I like to exercise fasted so this was early morning and I had not had anything to eat since the night before. I over did it. Played three games. Felt totally wasted energy wise when finished (he kicked my butt and was at least 5 years older than me). Did a quick shower and back home in probably 30 minutes. Thought, hmmmm, be a good time to test blood sugar. Should be WAY down, or so I thought. Tested it and it was 175. WHAAAAT????? How could that be. Keep in mind I had not seen a reading above 150 for a long time. So I did some research. I had heard of gluconeogenesis, but can't say I understood it very well. Well that was exactly what explained my high blood sugar reading. My body under very heavy energy requirement burned through all my glucose in the blood, all my ketones (I would have been in ketosis because of exercising fasted) and all the glycogen in my liver. The body says "hey, we gotta have energy!" so gluconeogenesis kicks in and the liver produces glucose out of my body fat. Since the exercise and requirement were very high, it gave me a big bolus. But I had suddenly stopped the exercise and energy need. So the glucose bolus showed up in my blood. Checked it twenty minutes later and it had dropped back down to a more normal level (can't remember for sure but I think around 120). So it was a temporary thing. My body responded to my needs and provided the glucose the body required without eating any carbs that normally supply the glucose for most people. Mine just came from body fat via the liver and gluconeogenesis.

The liver can make all the glucose we need, even without ingestion of carbohydrates. Was an interesting lesson for me.

John