What is the purpose of these shakes? I've lost enough weight pre-op to have a BMI of 37 so my liver is not big. Why do some drs want this and others just say nothing after midnight night before surgery?

Actually having a low BMI isn't an indicator of liver size from my understanding. Everyone in my program does 2 week regardless of BMI.

I can’t speak for all drs but mine told me it was to shrink the size of my liver for easier access to my stomach during surgery.

overall BMI does not correspond to fatty liver necessarily. basically it is a precaution to ensure they have as much room to operate as they can