# ATP and NAD+
## 4 Discussion
Our data demonstrating significantly reduced NAD in PD subjects is consistent with a recent pilot study that observed altered in vivo NAD+ in the brains of early onset PD subjects [47]. Unfortunately, we were not able to quantify NADH values as reported for the brain for several reasons: (1) The NAD and NADH peaks are obscured by the much higher [ATP] in muscle (8.2 mM) relative to brain (2.8 mM) [48]; (2) ATP and nicotinamide in muscle appear to have longer correlation times/shorter T\({}_{2}\)'s than brain. For example, [PERSON] et al. reported just under 8 Hz line width average at 4 T for alpha-ATP [49]. Report values ranging from \(\sim\)3 to \(\sim\)9, whereas [PERSON] et al. 4-8 [49; 50], while values for the NAD/NADH in skeletal muscle are much higher, with estimates as high as 540 for cytosol or six for mitochondria [51]. Despite the inability to quantify the NAD redox ratio, data showing the decrease in NAD in the skeletal muscle of PD subjects is an important new insight because of the central role in both metabolism and stress response pathways. Interventions to manipulate NAD levels based on supplementing steps in NAD synthesis or salvage in the cell are now a major industry (e.g., NR, NMN, NAM). Elevating NAD in animal models and cells from idiopathic PD patients supports investigating NAD as a therapeutic target. In cultured neurons, NAD shows effects of axonal protection [52]. Nicotinamide riboside (NR), a precursor of NAD\({}^{+}\), prevents noise-induced hearing loss and degeneration of neurites in the spinal ganglion [53]. In patient-derived induced pluripotent stem cells, increasing NAD\({}^{+}\) with NR significantly ameliorated mitochondrial dysfunction in neurons [54]. In fly models of _GBA_-related PD, NR prevented loss of dopaminergic neurons and motor decline. The authors conclude that NR is a viable clinical avenue for PD neuroprotection research [29]. The biology of NAD in PD and results from animal and cell models has led to multiple ongoing clinical trials for NAD targeted interventions in PD [28]. To fully develop NAD targeted strategies, it will be important to be able to measure changes in nicotinamide levels in vivo as well as to assess the effects of these changes on PD pathology. Results reported here suggest that \({}^{31}\)P MRS in skeletal muscle may be an important tool in the development and testing of NAD based interventions. Muscle fatigue and weakness are two of the most common and debilitating symptoms of PD. The underlying factors driving muscle fatigue and weakness in PD are not well understood. However, most focus has been on the role of central factors as a primary cause, with some suggestions of a role for circulating factors. In the leg muscles in this study, we have identified significant muscle weakness and reduced fatigue resistance in the skeletal muscles of PD patients associated with low ATPmax and disrupted metabolic homeostasis. Although this study was not designed to test mechanisms of muscle dysfunction, the metabolic defects in the muscles suggest that there is a potential role for skeletal muscle intrinsic pathology in PD-related muscle dysfunction. However, the absence of a decline in ATPmax in the hand despite similar weakness and fatigue to that observed in the leg supports an important role for central factors as drivers of muscle fatigue in PD. The higher the leg NAD, the less RBD and apathy reported by participants. Both hand and leg ATPmax levels were associated with the motor subset (part 3) of the UPDRS, and hand ATPmax was associated with mild, but statistically significant improvement in total UPDRS scores. Greater leg ATPmax was associated with less patient-reported symptom severity, specifically slowness, constipation, gait, falls, tremor, balance, visual disturbances, and nausea. This was an exploratory analysis and therefore no adjustments were made for multiple comparisons.