Thank you for the citations. Interesting stuff that propelled me to do more research.
This all brought up three more immediate questions for me:
1. What substance are we talking about? B6 or P5P? They are not the same thing and do not affect at lease some people the same way. This is something to keep in mind when looking at the results of studies.
"Based on my initial review of the medical literature, it appears that P5P supplements may not be needed by most individuals since regular vitamin B6 supplementation raises levels of P5P. However, there may be rare individuals who have a deficiency in enzymes that convert pyridoxine into pyridoxal 5 phosphate, and hence may benefit from P5P supplementation."
http://www.raysahelian.com/pyridoxal5phophatep5p.html
(But perhaps it's not so rare; I'm reminded of the evolution of research around active versus artificial B12s.)
2. What exactly is too much? Toxicity occurs at what dose?
1,000mg/day, and that at long-term use. For a smaller set of people, 500/mg day.
"Although vitamin B6 is a water-soluble vitamin and is excreted in the urine, long-term supplementation with very high doses of pyridoxine may result in painful neurological symptoms known as sensory neuropathy. Symptoms include pain and numbness of the extremities and in severe cases, difficulty walking. Sensory neuropathy typically develops at doses of pyridoxine in excess of 1,000 mg per day. However, there have been a few case reports of individuals who developed sensory neuropathies at doses of less than 500 mg daily over a period of months. Yet, none of the studies in which an objective neurological examination was performed reported evidence of sensory nerve damage at intakes below 200 mg pyridoxine daily"
http://lpi.oregonstate.edu/mic/vitamins/vitamin-B6
NOTE: that this does not address the question of whether the reported neuropathy at this high dose might have been caused in part or in whole by some other co-factor, like not enough methylgroups to support a changing biochemical system.
However it does help outline the range of safety, which I find useful.
3. What is the goal? Studies are showing improvement on everything from immune system disfunction to cardiovascular disease to kidney stones to CTS at daily doses anywhere up to 500mg/day, for some period of time (probably not "long-term".) For some people, the risk of (apparently reversible) neuropathy is probably worthwhile.
"Growing evidence from experimental and clinical studies suggests that systemic inflammation underlying most chronic diseases may impair vitamin B6 metabolism"
Interesting stuff.
My summary conclusions are that I'm glad for the information; it tells me that my current short-term 100mg/day experiment is unlikely to be a problem.
My favorite site on the subject:
http://lpi.oregonstate.edu/mic/vitamins/vitamin-B6