Hi Steve,
When a person transitions from lying to standing, more blood pressure is required to maintain proper blood flow to the brain. The autonomic nervous system controls the regulation of how much pressure / pumping is needed. If the autonomic nervous system is malfunctioning, it won't adjust properly for the change to standing fast enough, and the person feels light headed due to a lack of oxygen reaching the brain (Called orthostatic hypotension). The oppposite can also happen if the autonomic system over responds and increases the heart rate too much when standing(Called POTS).
Before I was diagnosed with AS, I had a severe problem with orthostatic hypotension. Almost everytime I stood up quickly, I would feel dizzy and my vision would be off, feeling like I might pass out. Interesting, dysfunction of the autonomic nervous system is closely associated with thiamine deficiency. I found that my thiamine level was low due to malabsorption caused by AS (at one point, the level was undetectable - causing beriberi), and upon supplementing vitamin B1, my orthostatic hypotension completely resolved, along with a host of dysfunctions of the autonomic system.
I haven't seen much other information on thiamine deficiency in AS, but I think it may be the culprit behind autonomic dysfunction in others. Thiamine is also essential for the cnoversion of fuel to energy, so a deficiency causes severe fatigue, the fuel is there, it just can't be utilized properly. The body only stores about two to three weeks of thiamine, so if you are malabsorbing, it is one of the first substances to go deficient.
Check out the work of Dr. Derrick Lonsdale regarding Thiamine and autonomic dysfunction.
-Ryan
