The silent information regulator (SIR) genes (sirtuins) comprise a highly conserved family of proteins, with one or more sirtuins present in virtually all species from bacteria to mammals. In mammals seven sirtuin genes – SIRT1 to SIRT7 – have been identified. Emerging from research on the sirtuins is a growing appreciation that they are a very complicated biological response system that influences many other regulator molecules and pathways in complex manners. Part 1 of this article provided an overview of the mammalian sirtuin system, discussed the dietary, lifestyle, and environmental factors that influence sirtuin activity, and summarized research on the importance of vitamin B3 in supporting sirtuin enzyme activity, as well as the role specifically of the amide form of this vitamin – nicotinamide – to inhibit sirtuin enzyme activity. In Part 2 of this review, clinical situations where sirtuins might play a significant role, including longevity, obesity, fatty liver disease, cardiovascular health, neurological disease, and cancer are discussed. Research on the ability of nutritional substances, especially resveratrol, to influence sirtuin expression and function, and hence alter the courses of some clinical situations, is also reviewed.