Here is a good summary of the
Nature Medicine article from
Ars Technica:
https://arstechnica.com/science/2024...ck-and-stroke/
In it is the relevant salvage pathway:
The concern is that everything that contains wheat (bread, cereal, etc) is mandated to contain what is now suspected to be an over-dose of niacin. If you take pretty much any over-the-counter multi-vitamin, you are getting > 100% RDA of niacin, just in one vitamin pill. If you take specific B-supplements, you are probably getting a much larger over-dose.
Vitamin supplements, in general, are associated with an increased mortality rate. The reasons for this are not well understood. Although speculative, it is not impossible that part of this is due to excess niacin.
Lack of niacin in the diet was a major public health problem, as the
Ars Technica article points out.