Neurons generally lack pathways to synthesize NAD from tryptophan and nicotinic acid, making them vulnerable to dysfunction if NAD synthesis pathways are impaired.

“But um the more concerning thing is in cells like neurons right where if they don't have a pathway to make NAD from tryptophan and they generally don't have a pathway to make NAD from nicotinic acid then what happens in a neuron that has high bioenergetic requirements if it starts losing expression of one or the other pathways and you get a lot of, you know, cell and tissue dysfunction in those conditions.”