# Current Uncertainties and Future Challenges Regarding NAD+ Boosting Strategies
## 16 Conclusions and Perspectives  
Results from preclinical studies and preliminary results from small human clinical trials imply that promoting intracellular NAD+ anabolism through NAD+ boosters is a promising therapeutic strategy for age-associated degenerative diseases [96]. Little is known about the pharmacokinetics, efficacy, safety, and potential side effects of long-term supplementation with NAD+ precursors. There are many unanswered questions, due to the limited number of human trials and lack of clinical data. Future clinical trials should focus on the safety and risk assessment of NAD+boosting therapies, as there is some evidence of direct and indirect adverse effects. Human studies with larger participant numbers are therefore desired to evaluate long-term toxicological outcomes. It is difficult to extrapolate or transfer results from animal models to humans because of variations in metabolism, longevity, and genetic variability. Current knowledge of the beneficial effects of NAD+ on aging and healthspan is primarily based on cell-culture experiments and model organisms. There are few human studies that have examined key outcomes such as increased vitality, reduction in all causes of death, prolonged health, and lifespan. Further in-depth studies and clinical trials are needed to understand the fundamental aspects of NAD+ biology and physiology and to clarify the use of NAD+ precursors for prevention or treatment and to promote health and longevity.  
The effects of various NAD+ boosters in different disorders should be additionally investigated. Special attention should be given to study synergistic/antagonistic effects between NAD+ precursors, CD38 and/or PARP1 inhibitors, and the use of other drugs and supplements. Data from human studies may be biased by lifestyle factors such as diet, exercise, age, and genetic factors. Special attention should also be paid to interindividual variations/differences in NAD+ amounts of participants at baseline in clinical trials. Need for NAD+ supplementation should be precisely determined. Potential serious risks such as tumor development and progression, adverse effects on inflammation, senescence, circadian rhythm disruption, and feedback inhibition should be additionally investigated, as well as the potential hormetic effect of increased NAD+ where a low-to-moderate dose might stimulate beneficial effects, and a high (over)dose might induce an inhibitory or even toxic effect. For example, supraphysiological dose of NAM was shown to decrease lifespan in _C. elegans_ and yeast [144; 19; 145]. In this regard, special attention is needed in supplementation with NAD+ precursors NR and NMN, which induce high levels of NAM to prevent undesirable effects [146]. Since the effects of NAD+ occur via the regulation of DNA repair, stress responses, and energy metabolism, it seems worthwhile to test NAD+ precursors in a manner of drug development to better understand the therapeutic role of NAD+ precursors in the prevention and treatment of human disease.  
**Author Contributions:** All authors (B.P., V.K. and I.M.) equally contributed to the writing of the manuscript. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Slovenian Research Agency (Research Core Funding No. P3-0388 and P3-0019). We would like to thank [PERSON], for her help in preparing Figure 1.  
**Conflicts of Interest:** The authors declare no conflict of interest.