Maybe you have an ammonia issue:

From: http://www.cancertutor.com/Autism/ComprehensiveGuide.doc

"Hyperammonemia means that ammonia, instead of being discharged by the liver, is recirculated into the blood stream. It is apparently caused by a deficiency of four Amino Acids: Citrulline, Aspartic Acid, Threonine, and Arginine. Vegetarians are especially susceptible to Hyperammonemia because of the lack of essential, Medium-Chained Amino Acids (L-Leucine, L-Isoleucine, and L-Valine) that in turn cause a deficiency of those Amino Acids named above. [color:"red"] Thus, a hyperammonemic state yields the spacy “brain fog” reaction, or in more severe instances may lead to seizures. [/color] "