It's fairly common, at least for some people, to get brain fog during 
the first few weeks of a low/zero carb diet, esp. if they are coming 
from a mixed diet, and esp. if that mixed diet was grain-heavy.  Similar 
to the brain fog many smokers get in the first few weeks of quitting 
smoking (increased oxygen flow,for one - in my case, I was a raving 
idiot for awhile!!)

Day, Wally wrote:
> According to http://proteins-carb-fats.suite101.com/article.cfm/how_lowcarb_diets_can_affect_the_brain
>
> "The results of a new study to be published in the February 2009 issue of Appetite conducted by researchers from the psychology department of Tufts University show those who follow a low-carb diet perform worse on memory-based tasks and have a slower reaction time than those following a calorie-restricted diet. The findings of the study also show that once carbs are reintroduced into the diet of participants following a low-carb diet, cognitive functioning returns to normal, pre-study levels".
>
> http://proteins-carb-fats.suite101.com/article.cfm/how_lowcarb_diets_can_affect_the_brain#ixzz0JhCCU8Ox&D
>
> "While the study was only conducted over a three-week period of time in total, results suggest that after only one week of severe carbohydrate restriction, a diet low in carbohydrates has a negative impact on brain functions such as memory and learning. More research is needed to determine whether this effect on brain functioning continues as a low-carb diet is continued or whether cognitive functioning returns to normal once the body adjusts to being on a new diet."
>
> This coincides with my own experience. As a programmer I need to be sharp the entire day. Mult-million and billion-dollar companies do not like software errors to cost them money. In the past, when I have experimented with zero-carb I would often find myself "thinking through a fog". Once a small amount of carbs were reintroduced, the fog magically lifted. Now, I realize the body compensates somewhat, but I have my doubts about its efficiency at doing so.
>