Plasma glucose homeostasis is critical for the functioning of mammalian organisms; thus, glucose levels should be strictly regulated according to nutrient conditions and energy demands. To maintain glucose homeostasis at the cellular level and to adapt to various challenges such as high-nutrient condition, disuse, and sarcopenia, it is necessary to improve or stabilize mitochondrial function, number, and size, which are important for maintaining the cellular NAD
+ pool [
16,
19,
20,
21,
22,
23].
PGC-1α is a master regulator that interacts with various transcription factors involved in cellular metabolic functions [
24]; thus, PGC-1α mediates the transcriptional activity and biological response related to them [
24]. SIRT1 (
Sirtuin 1) is involved in the regulation of systemic metabolism via the control of glucose and lipid homeostasis by deacetylating various targets, especially PGC-1α [
25]. Therefore, the NAD
+–SIRTs–PGC-1α pathway plays a vital role in cellular metabolic function. NAD
+ depletion is a characteristic of diabetes [
26], and
sirtuins, including SIRT1-3 and SIRT6, influence cellular functions such as glucose metabolism, mitochondrial function, and oxidative stress [
25,
27,
28,
29]. It is well documented that PGC-1α expression is reduced in T2DM muscle [
30,
31,
32].
The NAD
+ pool is important for cell physiological and metabolic functions for cell integrity; however, metabolic diseases, such as insulin resistance in tissues and diabetes, increase NAD
+ consumption; thus, lower levels of cellular NAD
+ are clearly linked to metabolic diseases [
33,
34,
35,
36,
37,
38,
39]. In this context, SIRT1, which consumes NAD
+ for cellular metabolic function, is downregulated in several cells and tissues, including myotubes, HEK293, peripheral blood mononuclear cell, human skeletal muscle, and adipose tissue, in insulin-resistant states [
39,
40,
41]. A previous study has shown that SIRT1 regulates glucose homeostasis by regulating the secretion of insulin and protecting beta (β)-cells in the pancreas [
42], enhancing mitochondrial biogenesis and glucose uptake in skeletal muscle [
43], and promoting glucose production and fatty acid oxidation in the liver [
44]. β-cell-specific SIRT1 overexpression in mice improves insulin secretion and glucose tolerance in response to glucose [
42]. Age-related downregulation of SIRT1 activity due to a lack of systemic NAD
+ biosynthesis results in a decrease in insulin secretion from β-cells in response to glucose; however, treatment with nicotinamide mononucleotide (NMN), which is a derivative of niacin and an intermediate in NAD
+ biosynthesis in the salvage pathway, restores insulin secretion and improves glucose tolerance in aged mice with β-cell-specific SIRT1 overexpression. Therefore, SIRTs regulate glucose–lipid metabolism and mitochondrial biogenesis via PGC-1α [
42,
43,
44,
45]. Overall, NAD
+ boosting can be one of the strategies to improve metabolic dysfunction via SIRTs–PGC-1α; therefore, we will discuss the role of the SIRTs–PGC-1α pathway in increasing NAD
+ biosynthesis and decreasing NAD
+ consumption.