this abstract seems to contradict itself a bit (is endogenous synthesis enough, or not?), but you can see that at least functional conversion of tryptophan to niacin has a role to play in overall niacin nutrition:
Nutritional Aspect of Tryptophan Metabolism (2013)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3729278/
"Mammals, including humans, can synthesize the vitamin nicotinamide from tryptophan in the liver. The resultant nicotinamide is distributed to non-hepatic tissues. We have studied the effects of changes in tryptophan–nicotinamide metabolism on niacin nutritional status. The liver plays a critical role in nicotinamide supply. ... Human studies have shown that 1 mg of nicotinamide is produced from 67 mg of tryptophan intake, and that the conversion ratio of tryptophan to nicotinamide is enhanced from mid to late pregnancy. These findings have contributed to the determination of dietary reference intakes for niacin recommended in the Dietary Reference Intakes for Japanese 2010. Our findings suggest that the conversion of nicotinamide from tryptophan is important in maintaining niacin nutrition. ... Dietary surveys have shown that the amount of nicotinamide biosynthesized from tryptophan is equal to the amount of pre-formed niacin from food intake in Japan, and matches the niacin requirement in humans.1,3,4 Although niacin can be supplied from amino acid tryptophan, nicotinamide biosynthesized from tryptophan is considered to be a byproduct of the kynurenine pathway, and the amount is thought not to be sufficient to meet the requirement. Thus niacin has been recognized as a vitamin."