Repair
NAD+ in DNA repair
The awkward finding is that repair needs NAD+ and can help exhaust it. I became much less comfortable with the word “more” after following that argument.
PARP1 and the mitochondrial repair enzyme
Mitochondria have DNA of their own. In the purified-enzyme repair experiments, PARP1 inhibited Pol γ, the enzyme involved in repairing mitochondrial DNA, when NAD+ was absent. Raising NAD+ to physiological levels allowed maximal repair activity to resume.
The experiment was reconstructed from biochemical components outside a body. Calling these “human findings” merely because the proteins were human or mammalian would give the result the wrong scale. This was an enzyme experiment, with no treatment outcome in a person.
The sharply bounded question is attractive: how does the availability of NAD+ affect this repair machinery? The answer gives a reason to investigate metabolism alongside DNA damage. It leaves the supplement question for later.
The energy cost of repair
PARP1 responds to damaged DNA and uses NAD+ as a substrate, material consumed during its work. The review of PARP1 activation and cell death describes a consequential limit: excessive activation can deplete NAD+ and cellular energy. A process involved in repair can then contribute to a cell's death.
That does not tell us that repair is undesirable, or that more NAD+ always prevents the problem. It puts metabolism inside the explanation. The state of the cell and the extent of the damage matter.
I had been treating “supports DNA repair” as an uncomplicated compliment. The mechanism deserves a longer description. Supporting a process, controlling it and improving the fate of the whole cell are different experimental questions.
Mouse immune cells under inflammatory stress
In mouse bone-marrow-derived macrophages studied in the laboratory, an inflammatory stimulus led to reactive oxygen species, DNA damage and PARP-mediated NAD+ consumption. The cells became dependent on NAMPT-driven salvage, the recycling route that rebuilds NAD+, to sustain their altered metabolism.
The recycling demand is the revealing part for me. An immune cell dealing with inflammation also has to pay for the consequences of DNA damage. NAD+ consumption and replacement help connect those jobs.
This was work on mouse cells outside a body. It cannot establish that a precursor supplement treats inflammation in humans. The review of NAD+ metabolism in immune responses describes the broader research interest, including autoimmunity and inflammation associated with ageing.
CD38 in ageing mice
PARP1 is one consumer among several. In the mouse study of CD38 and age-related NAD+ decline, CD38 activity rose with age. Mice lacking CD38 preserved NAD+ and had better mitochondrial function and metabolic health as they aged.
Deleting a gene in a mouse changes the question substantially. It helps identify a cause of NAD+ loss in that model; an oral precursor in a human leaves that gene in place. An oral human trial still has to succeed on its own terms.
Together, these experiments make a more demanding story than replenishment alone. NAD+ availability depends on production, recycling and consumption. The clinical findings show how much of that story has reached people so far.