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One study found that increased homocysteine levels in the central nervous system characterize patients fulfilling the criteria for both CFS and fibromyalgia. A significant positive correlation occurred between both high cerebrospinal fluid (CSF)-homocysteine and low CSF-B12 levels and fatigability. Vitamin B12 deficiency causes a deficient remethylation of homocysteine and therefore probably contributed to the increased homocysteine levels found.
(Source: Concise Encyclopedia of Chronic Fatigue Syndrome, by Roberto Patarca-Montero, M.D., Ph.D.)