I just found the same information from the NIH. According to them:
Brazil nuts, for example, contain very high amounts of selenium (68–91 mcg per nut) and can cause you to go over the upper limit if you eat too many.
It seems grossly negligent to say this, but not mention that eating one serving per day is “too many” for anyone, and over double the limit for children.
It also seems grossly negligent that the FDA literally mandates that companies use a serving size on the label which is above the upper tolerable limit for adults.
And then there is this article from WebMD, which touts the health benefits of Brazil nuts. It mentions near the bottom that “Eating too many Brazil nuts may also lead to selenium toxicity.”, but doesn’t say that “too many” is literally one serving. They DO say earlier in the article that “A 1-ounce serving of Brazil nuts has nearly 1,000% of your recommended daily allowance of selenium.”, but they don’t mention that is actually a problematic amount. There are lots of nutrients (e.g. Vitamin C) where it is fine to get 1000% of your RDA.
The WebMD article also mentions that Brazil nuts are becoming more popular because of keto & vegans.
If you look at the US government’s My Plate Protein Foods recommendations, they recommend that you can eat nuts, seeds, beans, lentils, and peas as vegan protein foods, and people should be eating a minimum amount of those every day. They don’t say anything about Brazil nuts specifically, but their info does say that 1 oz of nuts would meet 1/3 of your protein food requirement for the day. So that seems like a reasonable amount someone might eat, if trying to replace meat with legumes and nuts.
I guess this hasn’t been much of an issue because most people don’t eat very many Brazil nuts anyway, so it doesn’t come up a lot. But as people add more nuts to their diet – which is something that the government and many other people are literally recommending right now (they keep recommending people eat more non-meat alternatives) – this could end up being a major issue with a bunch of people accidentally giving themselves selenium poisoning.
I don’t see why anyone would assume that it would be problematic to eat just 1-2 servings of a food every day. That seems totally within reasonable & moderate amounts.
There are issues with some foods, like fish & mercury, but the government puts out a lot of warnings about not eating, e.g., too much tuna. They don’t seem to be putting out the same kind of information about Brazil nuts.