You're forgetting that SIRT1 provides good effects in a process that converts NAD+ to nicotinamide, so nicotinic acid -> NAD+ -> nicotinamide is beneficial.
You mean, because the nicotinamidase longevity gene PNC1 (or human PBEF) reconverts niacinamide back to niacin under adverse conditions (such as CR or taking resveratrol, or even metformin)?
I haven't given much thought to that. My reasoning was that if you want SIRT1 to do something nice for you, you have to feed it NAD+ and it gives you niacinamide. This niacinamide product thus is something you can live with. Ingested niacinamide would be a negative because it would give you inhibition of the process without having received the beneficial effect of the process.
This paper says that niacin is better than niacinamide at increasing NAD+.
J Biol Chem. 2007 Aug 24;282(34):24574-82.
Elevation of cellular NAD levels by nicotinic acid and involvement of nicotinic acid phosphoribosyltransferase in human cells.
Hara N, Yamada K, Shibata T, Osago H, Hashimoto T, Tsuchiya M.
Department of Biochemistry, Shimane University Faculty of Medicine, 89-1, Izumo, Shimane 693-8501, Japan. nhara@shimane-u.ac.jp
NAD plays critical roles in various biological processes through the function of SIRT1. Although classical studies in mammals showed that nicotinic acid (NA) is a better precursor than nicotinamide (Nam) in elevating tissue NAD levels, molecular details of NAD synthesis from NA remain largely unknown. We here identified NA phosphoribosyltransferase (NAPRT) in humans and provided direct evidence of tight link between NAPRT and the increase in cellular NAD levels. The enzyme was abundantly expressed in the small intestine, liver, and kidney in mice and mediated [(14)C]NAD synthesis from [(14)C]NA in human cells. In cells expressing endogenous NAPRT, the addition of NA but not Nam almost doubled cellular NAD contents and decreased cytotoxicity by H(2)O(2). Both effects were reversed by knockdown of NAPRT expression. These results indicate that NAPRT is essential for NA to increase cellular NAD levels and, thus, to prevent oxidative stress of the cells. Kinetic analyses revealed that NAPRT, but not Nam phosphoribosyltransferase (NamPRT, also known as pre-B-cell colony-enhancing factor or visfatin), is insensitive to the physiological concentration of NAD. Together, we conclude that NA elevates cellular NAD levels through NAPRT function and, thus, protects the cells against stress, partly due to lack of feedback inhibition of NAPRT but not NamPRT by NAD. The ability of NA to increase cellular NAD contents may account for some of the clinically observed effects of the vitamin and further implies a novel application of the vitamin to treat diseases such as those associated with the depletion of cellular NAD pools.
PMID: 17604275