NAD+ in blood and muscle
A blood sample is accessible. The tissues doing the work are harder to examine, and the difference runs through the human research.
A result inside muscle
The human result that draws me towards muscle comes from a 10-week NMN trial in prediabetic women after menopause. Muscle insulin sensitivity improved: the muscle responded better to insulin. Body composition and HbA1c, the longer-term blood-sugar measure, did not change.
The muscle result feels usefully specific to me. There is a tissue, a defined metabolic problem and an outcome within that tissue. Its scope stays with the women studied and the oral NMN they received. Broad claims about weight loss, extra energy or a longer life would require other results.
NAD+ itself is a coenzyme, a helper molecule for enzymes. It carries electrons during energy metabolism and is also consumed by proteins involved in repair and signalling. The review of NAD+ metabolism describes these overlapping cellular jobs. NMN and NR are precursors from which the body can make NAD+; they are distinct substances.
The blood results
In the eight-week oral NR trial in healthy overweight adults, whole-blood NAD+ rose by 22%, 51% and 142% across the studied groups. The investigators found no significant adverse-event differences from placebo. Those are substantial blood changes, recorded over a limited period.
The NMN trial in healthy middle-aged adults found higher blood NAD+ at 30 and 60 days, together with improvements in walking distance and quality-of-life scores compared with placebo. No safety issues were reported in the tested groups. The functional outcomes deserve their own place alongside the blood measurements.
General replenishment claims outrun my enthusiasm. The 2025 review of human precursor supplementation concludes that clinical efficacy remains limited and tissue-specific data are sparse. The uncertainty includes where NAD+ changes within the body, as well as what those changes accomplish. A blood result provides one view of that larger problem.
Consumption inside cells
Supply is only one side of cellular NAD+ biology. In the CD38 mouse experiment, removing an NAD+-consuming enzyme protected against the age-related fall in NAD+, preserved SIRT3 activity and improved mitochondrial and metabolic measures. This was a genetic intervention in mice.
The empty-container picture leaves out the activity that interests me: the cell is continually using and rebuilding its NAD+. Changing consumption and adding a precursor are different interventions, even when both affect the amount available.
HMGCS2 in mouse hearts
The heart-failure work using human myocardium and a mouse model examined HMGCS2, an enzyme involved in ketone production. NAD+ restoration improved fat oxidation and cardiac function in the mice. When HMGCS2 was reduced in heart muscle cells, the intervention failed to rescue the mouse heart-failure phenotype.
The dependency impresses me: a particular piece of cellular machinery had to be working. The human heart tissue informed the investigation; the reported rescue of heart function occurred in mice. A human treatment benefit remains a separate clinical question.
Route and safety
Oral NAD+, oral NMN, oral NR and infused NAD+ have different delivery questions. The human precursor review cannot turn a finding about swallowed NMN or NR into evidence for an NAD+ injection. Long-term safety and broad health benefits remain unsettled despite reassuring findings in some short oral trials.
Compounded injectable NAD+ preparations are not FDA-approved. Their quality and contamination risks add another problem beyond the molecule's biology. The cellular importance of NAD+ makes the subject compelling; the route-specific human evidence determines how far a treatment claim can travel.
GHK-Cu encounters an outer skin barrier. Here the practical question continues within the body: what happened in the tissue whose function was supposed to improve?