Adding to what mrsD cited:

" YES, NIAGEN works at NAD+ levels. We just don’t know how much the increased NAD+ levels benefit in restoring youthful function to aging cell."

This is a reference to what is called a "surrogate outcome" study. It is not a surprise that NIAGEN increases blood NAD+ because it is a biochemical precursor of NAD+. Surrogate outcome studies are often very unreliable as predictors of therapeutic benefits.

Drug companies are fond of surrogate outcome studies because they are quick and cheap to do.

Using intentionally silly words to make my point: A drug company might report "Our WonderDrug77 blocks the action of the Zogbeta4 receptor. Over-active Zogbeta4 receptor is a risk factor for Fred's Encephalopathy.". The inference that is intended to be drawn from this is that the drug is therapeutically beneficial. This does not follow.

A real equivalent of this is that many surrogate outcome studies have shown that statins decrease blood cholesterol levels. This is no surprise because statins inhibit HMGCoA reductase, which catalyses a step in cholesterol biosynthesis. It does not follow from this that statins are, with acceptably low adverse side-effects, useful in treating patients who are at increased risk of cardio-vascular disease because of elevated cholesterol levels.

The contrast with a surrogate outcome study is what is called a "real-life outcome" study. Often drug companies do not like them because they are expensive and take a long time.

A real-life outcome study poses the question "Does drug X, compared to placebo, lead to improved health outcomes with acceptably low adverse side-effects?".

Bottom line; surrogate outcome studies mean very little. Only the results of a well-designed real-life outcome study will help people to make an informed choice about whether a drug may help them.