Figure 2.

Diet supplies nicotinamide, nicotinic acid, and nicotinamide riboside as well as the essential amino acid tryptophan. Tryptophan can be degraded to synthesise nicotinamide when there is dietary stress by the kynurenine ‘de novo’ ‘immune tolerance’ pathway. Symbionts, whether in gut or TB, are a backup source as is ‘autocarnivory’. Salvage pathways are extensive and efficient to conserve NAD/nicotinamide as NAD consumers mean that there is a continuous need for replenishment. Many are involved in repair and disease processes and ageing. Excess nicotinamide can be excreted after a methylation reaction by NNMT. NNMT, indicates nicotinamide N-methyltransferase; TB, tuberculosis.