# Current Uncertainties and Future Challenges Regarding NAD+ Boosting Strategies
## 9 Potential (In)Direct Harmful Effects of Increased NAD+  
It is difficult to detect and investigate indirect adverse health effects in animal studies and human clinical trials. Nevertheless, there are some reports of possible adverse effects. NA and NR decreased physical performance in young rats [149] and the capacity of high-intensity exercise in humans [150]. On the other hand, elderly people seem to benefit from dietary NR supplementation. Specifically, increased NAD(P)H levels, decreased oxidative stress, and improved physical performance were observed only in the elderly participants of the study [121]. Cardiopulmonary exercise performance was improved after 6 weeks of NMN supplementation due to enhanced O\({}_{2}\) utilization of the skeletal muscle in 48 recreationally trained runners [151]. The Japanese authors claim in a pre-print publication that chronic oral supplementation of NMN for 12 weeks significantly improved muscle strength and performance in 10 volunteers [152]. [PERSON] et al. [153] expressed concern that supplementation with compounds involved in redox homeostasis can lead to adverse effects in healthy young people, as their endogenous antioxidant protection is still functioning adequately. An increase in NAD+ levels could also have a negative effect on longevity. Overexpression of SIRT1 did not extend the lifespan of mice on a standard diet, although it improved overall health and decreased carcinomas [154]. Supplementation with NAM improved health span and did not alter lifespan in the mouse model [155]. As mentioned above, the increase in NAD+ levels by NAD+ precursors could also affect the process of cancerogenesis, because increased NAD+ levels have potentially both cancer-promoting and cancer-inhibiting effects. Indeed, NAD+ is a crucial protective factor in early cancer development and could become a detrimental factor later in the phase of cancer development and promotion [24; 25]. None of the animal studies in which NMN or NR were supplemented reported increased cancer incidence. There is also controversy about the involvement of NAD+ in the activation of PARPs and sitruins in carcinogenesis and inflammation/sepsis. Elevated NAD+ levels may have different roles in different stages of sepsis. Activation of SIRT1 has beneficial effect in the initial (proinflammatory) phase, characterized by a cytokine storm, overproduction of reactive oxygen species (ROS), and metabolic shift [156], while SIRT1 expression should be inhibited in the later stages of sepsis [157]. Therefore, the role of increased amounts of SIRT1 substrate, NAD+, cannot be simply defined as beneficial or detrimental in sepsis. Increase in NAD+ could also have negative effects in rheumatoid arthritis and other inflammatory diseases because of stimulation of inflammatory cytokine secretion by leukocytes [158]. Accumulation of toxic degradation products and metabolites of NAD+ precursors, like N-methyl-2-pyridone-5-carboxamide (2-PY), N-methyl nicotinamide (MeNAM), and nicotinic acid adenine dinucleotide (NAAD) is also potentially harmful [118; 159]. Their increase were reported upon the NR supplementation [118; 159]. NAM, NR, and NMN supplementation can increase NAM levels, which could inhibit PARPs and CD38 activities [160]. A potential inhibitory effect of NAM on sitruins was hypothesized [161; 162; 163], although SIRT1 feedback inhibition by NAM may not play a significant role in vivo [164; 165]. Possible adverse effects of a high-dose nicotinamide may alter the methyl pool that is used to methylate DNA and proteins [165] as discussed above. Additionally, it is also necessary to highlight the effect of nicotinamideN-methyltransferase transferase (NNMT), the master regulator of intracellular NAM, on NAD+ levels and malignant transformation. NR and NMN may increase nicotinamide levels, thus enhancing the NNMT activity due to the higher substrate availability [166].