The books are being rewritten on Lactate, for a long time it has been considered a necessary evil in high intensity exercise, but recently I read some stuff that indicated lactate was a key part of our day to day metabolism, even at rest.
Some sources indicated it was actually the prefered fuel for the heart, brain and some other tissues, regarding the brain stuff search "George Brooks brain damage recovery", or similar, he has been using a treatment protocol of flooding the brain of accident victims with lactate and getting better recovery results.
Apparantly the skeletal muscle produces a relatively constant supply of lactate under all conditions.
The other key factor to consider is the abscence of Mitochondria in mature Red Blood cells (RBC), this means they have no capability of aerobic respiration and the only way they can get their energy is via the Pyruvate/Lactate pathway and hence are net Lactate producers, all day every day.
So when we talk about essential glucose requirements, it is not only the 90g/day or so required to fuel the brain, the RBC's also have an essential requirement for glucose and there may be other tissues as well that we are not aware of, so as always, the plot thickens.