Niacin Toxicity
Symptoms of toxicity include: Flushing of the skin, primarily on the face, arms, and chest *This side effect may occur at doses as low as 30 mg/day. Itching. Nausea. Vomiting.
3 oz (85 grams) of "Chicken breast, meat only, grilled" contains 10.3 grams of the stuff[1], and average adults get about 30mg/day. Best I can tell this is about nicotinic acid specifically, and might be IV rather than ingestion, which is very much an apples and oranges situation.
1: https://ods.od.nih.gov/factsheets/Niacin-HealthProfessional/Fact is though, that here in Europe there is no need or value in adding this supplement, and generally the EU is wary of any adulterated foods without reason.
That's off by a factor of 1000x -- it says 10.3 milligrams.
In some cases these arguments are made in good faith.
In even rarer cases these arguments are true.
But initially it's wise to take them with a grain of salt.
America has a longstanding tradition of food fortification. Niacin is also used to supplement US diets featuring a corn staple -- corn's niacin is not bioavailable without additional processing traditional to Mesoamerican cultures. Probably more recognized today is iodized salt, which is responsible for perhaps a decade's worth of the Flynn effect in the US.
In the case of flour, Europe is lagging behind: https://www.ffinetwork.org/europe. Apparently Denmark went the extra step of banning enriched / fortified foods. You could probably argue that regional diets will naturally vary and that something like iodine supplements aren't required for Italians when they have ready access to seafood that Oklahomans don't. But since I'm pretty sure these enrichments aren't harmful to those who do have access, I imagine some of the divide is simply protectionism -- protecting your local markets and local manufacturers from cheaper American brands.