According to ChatGPT:
"Nicotinic acid (NA), also known as niacin or vitamin B3, is considered a more direct precursor of nicotinamide adenine dinucleotide (NAD+) compared to nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN). NA can be converted into NAD+ through a relatively simple pathway involving fewer enzymatic steps [7].
NR and NMN, on the other hand, require additional enzymatic reactions to be converted into NAD+ [6][8][10]. NR is first converted into NMN by the enzyme NR kinase, and then NMN is further converted into NAD+ by NMN adenylyltransferase [6][10]. The conversion of NR and NMN to NAD+ involves more complex metabolic processes compared to the direct conversion of NA to NAD+ [7].
It's worth noting that while NA may be a more direct precursor of NAD+, the effectiveness of NAD+ elevation through NA supplementation alone might be limited due to potential side effects such as flushing, which can be attributed to the release of prostaglandins [1][2]. In contrast, NR and NMN supplementation have been shown to effectively increase NAD+ levels in various preclinical and clinical studies [4][9]."