Well, because I looked up previous ketogenic Soylent recipes, and they used standard multivitamin powders containing, e.g., way too much manganese and the wrong form of selenium. (You get all the manganese you need from ordinary drinking water, if it hasn’t been distilled or bottled. Excess amounts may be neurotoxic. One of the leading hypotheses for why multivitamins aren’t found to produce net health improvement, despite having many individual components found to be helpful, is that multivitamins contain 100% of the US RDA of manganese. Similarly, if a multivitamin includes sodium selenite instead of, e.g., se-methyl-selenocysteine, it’s the equivalent of handing you a lump of charcoal and saying, “You’re a carbon-based lifeform; this has carbon in it, right?”)
I started taking multivitamins somewhat recently to get additional iron, vitamin B12, and vitamin D (I'm vegan). Specifically I am taking this multivitamin. I also take creatine. Taking these supplements was partially informed by this post, though I have been taking creatine on and off for quite a while.
I picked multivitamins instead of individual supplements because it was cheaper and more convenient. I also thought that covering additional vitamin/minerals would almost certainly be either neutral or slightly positive. However I now place somewhat more probability in the these additional vitamins/minerals being harmful in net (via excess vitamin A, manganese, or similar). How concerned should I be?
At the moment, I think the additional manganese I am getting from this multivitamin (2.3 mg or 100% DV) is probably not harmful. Precisely, I estimate that the probability of taking the supplement causing modest negative health effects (for instance, reducing expected IQ by >0.5 points or reducing expected life span by >1 month) is 20%. This is primarily based on noting that many common foods have high manganese content and not seeing any arguments against this dosage of manganese after briefly googling. I have seen arguments against higher dosages of manganese in supplements; this may be what Eliezer is discussing.
More generally beyond just manganese, how should the expected value of the 'unknown unknowns' of taking a multivitamin be estimated? It isn't immediately clear to me what the sign would be. I am currently leaning toward slightly negative expected value due to general principles about side effects.
Additionally, are there any other supplement I should be taking? I think my diet is generally healthy; however, I may get too little protein occasionally.
(edit: used to be 'less than 20%', but this wasn't what I meant)