My experiences of longer fasts have all been positive, except for the first one--I was very hungry the whole time then, but I started fairly lean.
I think Nancy's right that Dr. Fung plays loose with the science here. The equivalence of fasting to bariatric surgery is uncertain when it comes to metabolic rate. I'm not even certain that the maintenance of a higher metabolism is that much of a plus--what do you want, more muscle, or a faster metabolic rate?
Fung also oversimplifies with lean mass loss, significant amounts are lost. Not necessarily a big deal, as long as you don't push things too far and you have appropriate medical support for longer fasts. What ever you lose, you'll have to gain back.
Getting in electrolytes is important. Vitamins make sense as well. The thiamin thing--a person can feel just fine while fasting, and then get in trouble when they refeed, especially carbohydrate because the requirement for thiamin and some other nutrients is higher in the fed than in the fasted state.
I don't really worry about a metabolic slowdown that's particular to fasting, but not necessarily for the same reasons that Dr. Fung gives. Other than David Ludwig's study where he dieted people down on a standard low calorie diet and then put them on a low fat, mediterranean, or a low carb diet, where the low carb diet resulted in less of a decrease in metabolism than the other two diets, I haven't really seen anything to convince me that how people lose weight has that much of a difference to metabolic rate.
Over four days of continuous fasting, basal metabolism does not drop.
Yes--but over four days of fasting, you don't lose that
much weight. The fair test is having one group lose fifty pounds with a standard diet, and another through some sort of fasting/refeeding program. There's a lot of apples and oranges comparisons going on.
I do think Dr. Fung has some hypotheses worth testing. And I think there's enough good data about fasting to make it useful. Oversimplification probably makes it easier to coach people along while they're fasting, but it's going to turn some people off, which is unfortunate, because I do think there's enough data to suggest that it deserves a place in the toolbox.