Aging is associated with a reduction in tissue NAD levels. In turn, this is believed to be associated with physical decline. It is not entirely clear what makes the NAD level decline, but it seems that it might be caused by the accumulation of senescent cells. As we age, we tend to accumulate these non-functional and slightly harmful “dead” cells called “senescent cells”. We now have drugs called senolytics that can remove some of these senescent cells, with at least one ongoing clinical trial.