Lightheadedness

I'm four days into the 800-calorie liquid diet that the Cleveland Clinic requires pre-surgery.  I've experienced a couple of bouts of lightheadedness - nothing major, just surprising.  Has any one else had that?  Comments?  I have pre-ops on Tuesday, so I'll follow up then. 

i am pre op too.  I was having the same issues along the shakes. My nutritionist said my suger was low and to drink some fruit juice.  I drank a gatoraid G2 instead 12oz for a few days and never had trouble with it again. I am now 18 days into a pre op liquid diet

I get lightheaded all the time, I am close to 6 months post-op...  i don't have the problem pin-pointed yet, but it is not dehydration (which can cause it), i think it is associated with the workouts i am putting in.