Since the thread is titled, "What causes Sjogren's", I think this next comparison is still on topic. I think the same thing that causes Sjogren's, also causes CFS and Fibro. Another thing they share in common is restless legs syndrome. A study published in the Journal of Clinical Sleep Medicine found that 33% of patients with fibro had restless legs snydrome. RLS is a common finding in CFS also. Here is a study that shows it is also common in Sjogren's.
http://www.ncbi.nlm.nih.gov/pubmed/8252317
So, what causes restless legs syndrome? Here is a study that shows it is linked to low dopamine and iron.
http://www.ncbi.nlm.nih.gov/pubmed/15222997
The study I posted above showed that 51% of the Sjogren's patients tested had low iron. Dopamine is necessary for adrenaline (autonomic nervous system neurotransmitter) Dopamine comes from tyrosine. (Both of the thyroid hormones come from tyrosine.) 33% of Sjogrens patients had Hashimoto's.
33% of fibro patients have RLS. Dr. Woods, the Scientific Director of the Fibromyalgia Association of America has discovered that fibro patients lack dopamine and have low iron.
Studies confirm that CFS patients also lack iron and have low dopamine. In the study entitled, "Serum concentrations of some metals and steroids in patients with chronic fatigue syndrome with reference to neurological and cognitive abnormalities" it states, "The results indicate that patients had SIGNIFICANTLY increased aluminum and DECREASED IRON compared to controls.
So, what causes the lack of dopamine and iron in these diseases?
Dopamine is derived from tyrosine and tyrosine is derived from phenylalanine. Phenylalanine is found in HIGH PROTEIN FOODS. If you lacked the same enzymes that are responsible for regulating tumor necrosis factor (proteases) you would also not be able to break down high protein foods and release phenylalanine. Iron is also found primarily in these same foods.
There is another reason though why there is a lack of iron in all of these diseases. The same proteases that regulate tumor necrosis factor, and break down high protein foods, also regulate iron absorption in the body. Here is a study that shows this.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2430097/