Sirt1 (Sirtuin type 1), a member of the conserved sirtuin family, is a NAD+ -dependent histone deacetylase. It is intimately related to cell proliferation, differentiation, apoptosis, and metabolism. Its role in senescence and metabolism has recently come to light. Sirt1 promotes fat mobilization in white adipocytes by repressing PPARgamma and inhibits myoblast differentiation by down-regulating muscle gene expression. Therefore, Sirt1 is not only an important longevity factor, but also may play a key modulatory role in animal fat deposition and muscle development.