Ok, good point. The evidence indicates that humans can deal with it in larger amounts than in non-organic forms (20mg being the largest amount that we know humans have consumed regularly without observed side effects).
Unfortunately, I did not realize it was dead until I had already dug it up. My subscription email suggested it as a good topic. I apologize. I guess I cannot trust the emails to give me up-to-date information on what is going on in the forums.
You are totally right that this is an important topic. And it is true that different compounds of a given element tend to have some shared traits. I am going from a point of view here that we do have evidence supporting the conclusion that organic manganese compounds are less toxic than inorganic ones. (And there are known cases of other metals where this has been fairly well proven. Magnesium is one that is highly toxic in inorganic forms but no toxicity of organic forms has been ever observed, even when consumed in very large amounts.)
You did also ask what the difference is between consuming it in water and consuming it in food. The difference is that the compounds are dramatically different. One interesting thing about compounds is that more complex compounds tend to exhibit fewer traits of the elements they are composed of. Organic compounds are more complex than inorganic ones on a very extreme level (many common organic molecules are composes of millions or billions of atoms). I said that I don’t know the specific details for organic manganese compounds, but it is possible (even probable) that the other constituent elements totally overpower the manganese traits.
What it comes down to is that thus far, the evidence indicates that most people consuming large amounts of food with high amounts of manganese do not have adverse reactions. We have between 10 and 15 years during which these diets have been common enough to expect at least a few recorded instances of side effects. What this means is that diets containing up to 20mg of manganese from organic (in the chemical sense, not in the “organically grown” sense; I think this ambiguity is why “food source” is commonly used here) sources won’t cause any observable side effects, for at least 15 years. It is possible that 20mg over 20 years will start to cause problems, though the odds are very low.
Really though, there are too many unknowns. There are not many known compounds that cause problems that are only observed after 20+ years (ethanol is one, and high glycemic index foods are another). This means that the odds are low that manganese from organic sources is toxic at the 20mg/day level. Still, I agree that this does deserve attention. I would argue that amounts in soylent that are a little above 11mg/day is safe enough that we will probably know enough more about this before it is too late to avoid any bad consequences.
Anyhow, when it comes to organic compounds, we cannot assume that they will have any of the properties of the elements they are composed of. You are right though: This does not mean that we should automatically treat them as safe.