First, this one is a helper
NAD+ is a coenzyme, a helper used in cell reactions. It has jobs in energy handling and repair. It isn't a chain of amino acids, so it isn't a peptide, despite often appearing in the same conversations.
The biology review explains the interest. An essential job doesn't prove that increasing the supply improves health. To see what was tested, let's look at the starting materials.
The starting material changes the question
NR and NMN are precursors: substances cells can use to make NAD+. Their full names are nicotinamide riboside and nicotinamide mononucleotide. No need to memorize them; just keep them distinct from NAD+.
In an eight-week trial of oral NR, whole-blood NAD+ increased in healthy adults with overweight. That shows a change in a blood measurement. It doesn't show longer life or establish the benefits of an NAD+ infusion. Another precursor trial looked beyond the blood reading.
A useful result can be uneven
Muscle responded better to insulin in an NMN trial involving postmenopausal women with prediabetes. That's what researchers mean by improved insulin sensitivity. The study didn't find clear changes in body composition or HbA1c, a measure of longer-term blood sugar.
That's more useful than calling the whole experiment a success or failure. One response improved; the study didn't find improvements in the others. It doesn't establish the same result in younger adults, men or people receiving a different substance.
Back with the precursor studies, the question is what improved for the people taking part. A changed marker can be a clue without being the health benefit we hoped for.
The aging claims are bigger than the findings
The review of human precursor research describes limited, inconsistent clinical benefits and gaps in tissue measurements. A blood sample can't answer every question about muscle, liver or brain. The location matters as well as the number.
Short trials leave long-term safety questions open. Several weeks can't tell us what years of use would bring. That uncertainty belongs beside the encouraging result.
Compounded injectable medicines are not FDA-approved. Findings from oral precursors don't establish an injection's benefits or safety. The FDA's compounding explanation covers that separate approval question.
So NAD+ earns a place in the conversation, with its own name and limits. The comparison keeps the substance, route and result together. That's a better starting point than treating every mention of cellular energy as the same health claim.