I do believe they don't have all of the information. But it seems as though a lot of how Dr. Fung shrugs off that concern is more anecdotal than scientific. I have not seen any actual studies referenced on his blog that show that Fasting doesn't reduce our metabolism. He usually responds to that question, it seems, with more anecdotal evidence, particularly the "caveman" example of, if a caveman doesn't have food to eat for a day or two, would his metabolism just shutdown, and when he puts it that way, it sounds like a ludicrous supposition, but I don't see research on his blog that actually supports that. Granted, I haven't watched all of the videos because of the length (and like others, would prefer to read rather than watch videos), but I think that's a good homework exercise for me.