Here Dr Ali presents where he made a mistake on extended fasting. The data Phinney presented was measruing as I recall the remnants of protein breakdown to measure lean body mass loss. What was not realized was that what was being measuring in the urine that denoted protein breakdown was also measuring autophagy, the body doing "house cleaning" of recycling old tissue (which is a good thing).

Now there are tools such as the dexa scan that can measure actual lean body tissue rather than have to rely on urine sample to estimate the loss of protein. Science advances.

So in this video he changes his message on extended fasting and gives the science behind it.

That is my understanding. Watch it yourself to make sure I interpreted it correctly.

https://www.youtube.com/watch?v=SiYuTP5ac8Y&t=886s

John