A  

Humans can endogenously convert the amino acid tryptophan to nicotinic acid/nicotinamide (niacin), whereas pantothenic acid, vitamin B12, and folic acid must be obtained from diet or microbial sources. Mammalian studies confirm that “Mammals, including humans, can synthesize the vitamin nicotinamide from tryptophan in the liver” [Fukuwatari & Shibata, 2013, PMID 23922498].