## 4 Clinical and Preclinical Relevance and Therapeutic Implications
However, NR supplementation did not lead to improvements in adiposity or overall metabolic health [151]. In a randomized crossover trial, six weeks of nicotinamide riboside (1000 mg/day) in overweight and obese individuals increased skeletal muscle NAD\({}^{+}\) metabolites and acetyl-carnitine concentrations and modestly improved fat-free mass and sleeping metabolic rate. However, no significant effects were observed on insulin sensitivity, mitochondrial function, cardiac performance, or inflammatory markers [152]. Recent preclinical work by [PERSON] et al. identified a synergistic effect of nicotinamide and pyridoxine on muscle stem cell (MuSC) activation and regeneration. The combination of these food-derived nutrients enhanced MuSC proliferation and differentiation through CK1-dependent \(\beta\)-catenin activation and AKT signaling. In aged mice, oral administration of NAM and PN significantly improved muscle strength and regeneration following injury. Furthermore, their levels declined with age and were positively associated with muscle mass and walking speed in a cohort of 186 elderly individuals, highlighting their clinical relevance and potential as nutritional rejuvenators of MuSC function [22]. These mixed results underscore the need for refined strategies that consider dosage, timing, formulation, and individual variability in aging physiology. Although direct evidence of their synergy in skeletal muscle is still limited, preclinical models demonstrate that nicotinamide enhances mitochondrial biogenesis, autophagy, and satellite cell activation, while pyridoxine modulates amino acid metabolism and inflammatory signaling [153; 154; 155; 156]. Further investigation into combined supplementation strategies is warranted to determine their potential interactive effects on muscle strength, regeneration, and function in aging populations. Pyridoxine, particularly in its active form pyridoxal-5\({}^{\prime}\)-phosphate (PLP), has been consistently associated with better muscle function in observational studies. Additional evidence suggests that physical exercise may influence vitamin B6 dynamics in both humans and animals, further amplifying its role in muscle physiology [153; 154; 155; 156]. Multiple human studies have reported transient increases in plasma PLP levels following endurance exercise. For example, adolescent runners exhibited a significant rise in plasma PLP and total B6 concentrations after 4500-m races [157]. Similar results were observed in trained and untrained women following cycling at 80% VO\({}_{2}\)max for 20 min, where PLPand 4-pyridoxic acid (4-PA) levels rose post-exercise [158; 159]. Plasma PLP also increased during prolonged running in men [160] and following graded cycling challenges in healthy adults [161]. [PERSON] et al. confirmed these findings in young male athletes, showing significant post-exercise elevations in PLP after a maximal aerobic test [162], while [PERSON] et al. observed increased serum PLP and PMP after a half-marathon [163]. In animal models, similar trends have been documented. Rats exposed to forced swimming for varying durations showed time-dependent increases in plasma PLP, muscle B6 vitamers, and liver total B6 content [164]. Chronic endurance training over several weeks also elevated tissue levels of PLP, PMP, and PL in skeletal muscle and liver [165]. Notably, [PERSON] et al. demonstrated that exercise reversed the negative effects of a B6-deficient diet on mitochondrial enzymes and muscle B6 levels [166]. While one study reported no change in plasma PLP after an 8-week swimming protocol, this appears to be an exception rather than the norm [167]. Collectively, these findings indicate that endurance exercise enhances tissue and circulating levels of vitamin B6, potentially improving its functional availability during muscle remodeling and repair. This adaptive redistribution of B6 vitamers may augment the vitamin's antioxidant and anti-inflammatory capacity, particularly in the context of physical rehabilitation or sarcopenia intervention.