## 4 Clinical and Preclinical Relevance and Therapeutic Implications  
An expanding body of clinical and preclinical research highlights the therapeutic potential of nicotinamide (vitamin B3) and pyridoxine (vitamin B6) in mitigating sarcopenia. Through complementary effects on NAD\({}^{+}\) metabolism, mitochondrial function, redox balance, and immune regulation, both vitamins offer promising tools for preserving muscle health in aging populations [22, 24, 146]. In animal models, nicotinamide riboside (NR) supplementation has been shown to replenish declining NAD\({}^{+}\) pools, enhance mitochondrial respiration, and activate muscle stem cells (MuSCs). In aged mice, these effects translated into improved muscle endurance and reversal of age-related gene expression signatures [138]. Similarly, nicotinamide mononucleotide (NMN) supported mitochondrial biogenesis and myogenic differentiation and improved survival in ischemic cardiac tissue through poly(ADP-ribose) polymerase \(1\) (PARP1) inhibition and restoration of mitochondrial enzyme function [144, 145, 146, 147, 148]. Human studies offer encouraging, though occasionally \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mechanism/Pathway** & **Nicotinamide (B3)** & **Pyridoxine (B6)** & **Functional Outcome** & **Reference** \\ \hline NAD\({}^{+}\) metabolism & Precursor for NAD\({}^{+}\), & Indirect support via & Maintains energy & \\  & essential for & amino acid catabolism & homeostasis and redox balance & [18, 54, 74, 123, 124, 125, 126, 127, 128, 129, 130] \\  & mitochondrial function & & & Enhances \\  & Activates SIRT1 and & & & \\  & promotes autophagy & & & mitochondrial health \\  & and mitochondrial & & & and stem cell longevity \\  & biogenesis & & & \\  & Upregulates & & & \\ Redox regulation/Nrf2 & antioxidant enzymes & & & \\ pathway & via the SIRT1–Nrf2 axis & & & \\  & Inhibits NF-kB & & & \\  & downregulates IL-6, & & & \\  & TNF-\(\alpha\) & & & \\ Muscle stem cell & Enhances activation via & & & \\  & AKT/mTOR and & & & \\  & \(\beta\)-catenin & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Molecular mechanisms of action of nicotinamide and pyridoxine in aging skeletal muscle. inconsistent, findings, with some reporting improved muscle strength, metabolic flexibility, and mitochondrial function, while others show minimal or no effect. These discrepancies may stem from differences in study populations (e.g., age, sex, health status), variations in the form and dose of supplementation, as well as inconsistent duration and outcome measures. Additionally, small sample sizes and lack of biomarker-based stratification may contribute to limited reproducibility and conflicting results across trials [149; 150; 151; 152; 153; 154; 155; 156]. A 12-week randomized trial by [PERSON] et al. reported that NR supplementation increased systemic NAD\({}^{+}\) levels and reduced arterial stiffness in older adults [149]. Recent clinical evidence demonstrates that oral nicotinamide riboside (NR) supplementation is bioavailable in aged human skeletal muscle, leading to measurable increases in NAD\({}^{+}\) metabolites and nicotinamide clearance products [150]. Although NR did not significantly alter mitochondrial bioenergetics over the short intervention period, transcriptomic profiling revealed a downregulation of energy metabolism and mitochondrial gene pathways. Importantly, NR supplementation was associated with reduced levels of circulating inflammatory cytokines, suggesting potential anti-inflammatory effects. These findings support the translational relevance of NR for modulating the muscle NAD\({}^{+}\) metabolome and systemic inflammation in older adults, even in the absence of immediate changes in mitochondrial function [150]. In a 5-month randomized trial involving BMI-discordant monozygotic twins, escalating doses of nicotinamide riboside (250-1000 mg/day) enhanced muscle mitochondrial biogenesis, satellite cell differentiation, and gut microbiota diversity and modulated DNA methylation in muscle and adipose tissue.