Maybe It's the Niacin, but I am not completely sold. Iodine has a very convincing mechanism for softening fat. Most soapmakers know that each oil has an Iodine value, which is proportional to the degree of hydrogen unsaturation: or Double bonds.
Iodine will chemically react with UFA's in vitro. Remember reading about the iodinated oil prophylaxis of schoolchildren experiment? That is not Iodine in oil, that is iodinated oil; oil covalently bonded to iodine.
I suppose niacin could reduce blood lipids by some other mechanism, but Iodine removing fat in the bloodstream seems intuitive based on fat/iodine chemistry.