In summary: Manganese is an essential nutrient and too much will kill you.
On one hand we have people exposed to manganese is the air from industrial processes. These people definitely get Parkinson’s like symptoms, and in the aggregate score lower on various intellectual tasks. Monkey infants on soy formula (which has more manganese than milk based formulas) had mild neurological effects, so you can take in enough from food alone to be dangerous.
On the other hand, manganese superoxide dismutase is the only thing keeping your mitochondria from melting, and it does several other things besides. So we can’t just throw up our hands and say “no more manganese”, we have to actually figure out a safe level.
The World Health Organization has set the No Observed Adverse Effects Level at 11mg/day, based on the observation that some humans naturally get that much through their diet. As mentioned previously, we know you can get manganese poisoning through food alone, so this is not very reassuring to me.
The most comprehensive source I found on this was this thread on the soylent message boards (they cite several of the same things I did above, all correctly, so I trust their work ). According to their sources, most adults already eat enough Manganese to meet the RDA. I don’t want to put too much stock in the RDA, because neither I nor they could find out how the FDA (or the Linus Pauling Institute) set it. However even if you accept the number, if people are already getting it through diet, and too much is toxic, it doesn’t belong in a multivitamin.
This is worrisome enough to me that I’m tossing my multivitamin and focusing on getting individual missing nutrients through specific supplements (if anyone knows of a balanced B supplement, please let me know). It’s not so bad I’m tossing my protein/vitamin powder, because it has a lower volume of manganese and I don’t eat much at a time- however when it’s gone, I’ll probably switch to a straight protein powder.