FNDC5 and Alzheimer disease: Studies have shown that NAD+ can activate mitochondrial function to mitigate neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) (Hikosaka et al., 2019), to enhance sirtuin activity in the treatment of heart and kidney diseases (Hershberger et al., 2017) and to stimulate Fndc5/irisin factor to relieve non-alcoholic fatty liver disease (Li et al., 2021).