# Advances in the Synthesis and Physiological Metabolic Regulation of Nicotinamide Mononucleotide
## 5 Conclusions and Outlook  
NMN is considered the best supplementary precursor of NAD\({}^{+}\), and its biological activity is largely the result of its efficient conversion into NAD\({}^{+}\), which acts as a fulcrum of energy metabolism and is involved in many physiological processes [88; 108]. As a consequence, it affects mitochondrial function, the tricarboxylic acid cycle, lipid metabolism, and DNA damage repair [81]. So far, direct supplementation with NMN has been found to alleviate inadequate levels of NAD\({}^{+}\) in organisms under specific conditions such as aging, obesity, and metabolic disorders, which has been confirmed by numerous experimental and clinical studies in cells, animals, and human patients [164]. As a consequence, NMN has been applied in health products, cosmetics, and food additives. With the increasing market demand for NMN and related products, high-yield, low-cost, safe, and environmentally friendly NMN synthesis methods have become an important problem in need of a solution. Although the physiological metabolic regulation function of NMN and research on its mechanism have been widely reported, excessive intake of NMN may cause side effects, such as cellular metabolic disorders and oxidative stress [7]. Animal studies have shown that high-dose NMN supplementation may affect blood sugar metabolism, cause weight gain, or change lipid metabolism [165]. Preliminary clinical trials have evaluated high-dose intake of NMN. Although no serious side effects were found in the short term, long-term safety needs further verification. Hence, most of the existing studies focused on the effects of NMN supplementation at low to medium doses. Research on the potential side effects of high-dose NMN is still in its infancy, and more long-term studies are needed to evaluate its long-term safety. While the chemical synthesis of NMN has been industrialized, with easily controllable process technology, challenges arise from excessive impurities and the presence of chiral by-products, causing difficulties in separation and resulting in a diminished overall yield. Moreover, the substantial use of organic solvents raises concerns of environmental pollution that cannot be overlooked. In response to these challenges, the current focus of research lies in the biological synthesis of NMN. These pathways include a sequence of enzymatic reactions involving nicotinamide, nicotinamide ribose, and ATP. Biocatalysis offers the advantages of environmental safety and high substrate conversion rates with almost no impurities. However, obstacles include the cost and difficulty of obtaining NR and PRPP as substrates. Additionally, another significant hurdle is the substantial consumption of ATP in the enzymatic catalysis process, leading to elevated costs of biosynthesis that still prohibit its large-scale industrial application. Utilizing synthetic biology strategies, significant progress has been made in metabolic engineering for the enhancement of NMN synthesis. This includes screening and modifying exogenous, highly active NMN synthases, increasing the accumulation of substrates, improving NMN transport efficiency, and knocking out genes related to NMN degradation. Despite significant progress, there is still room for improvement in the productivity of NMN synthesis. For example, studies on NMN biosynthesis mostly used _E. coli_ as the engineering chassis and only analyzed the titer of the final product, with few studies focusing on improving the ATP supply system. Therefore, strategies that should be tested in the future include other excellent chassis cells and selecting different promoters for expression, detecting the concentration of the intermediate products during the reaction process to determine the limiting step of the multienzyme pathway, introducing a recycling system to optimize the supply of ATP, and designing different synthetic routes from relatively cheap and simple raw materials.