# NAD+ in blood, muscle and heart tissue > Blood NAD+ and muscle | Really Research Review — Really Research Review examines NAD+ precursor trials, selective muscle findings, mouse experiments and the remaining safety questions. **02** The blood response is fairly dependable. Blood chemistry persuades me less readily than a useful muscle response. ## The supplement trials NMN and NR supply ingredients that cells can use to make NAD+. The distinction matters because much of the human research concerns these precursors, rather than NAD+ given directly. A [multicenter NMN trial in healthy middle-aged adults](https://pubmed.ncbi.nlm.nih.gov/36482258/) found higher blood NAD+, improved walking distance and improved quality-of-life scores compared with placebo. Those last two measures give the chemistry some human company. I’d weigh the walking result alongside the blood increase before celebrating the chemistry. An [NR trial in healthy overweight adults](https://pubmed.ncbi.nlm.nih.gov/31278280/) also found increased whole-blood NAD+, with no significant difference in adverse events from placebo. The repeated blood response supports a specific conclusion: these oral precursors can raise that measure. The broader clinical story remains uneven. The [2025 review of human precursor trials](https://pubmed.ncbi.nlm.nih.gov/41083806/) judged efficacy limited and tissue-specific data sparse. Even the often-repeated age-related decline in human NAD+ had been consistently observed in only a limited number of studies. My assumption of a tidy human ageing story fares poorly in this review. ![Walking shoes and a stopwatch beside a long, empty indoor corridor](images/walking-test-corridor.webp) *An imagined walking-assessment setting, illustrating the functional measure in the NMN trial.* ## NMN after menopause In a [10-week NMN trial in prediabetic, postmenopausal women](https://pubmed.ncbi.nlm.nih.gov/33888596/), muscle became more responsive to insulin, the hormone that helps control blood sugar. Body composition and HbA1c, a longer-term blood-sugar measure, stayed unchanged. I like this selective result precisely because it resists inflation. Muscle insulin sensitivity is a real physiological outcome, and the unchanged measures help locate it. Describing the result as general metabolic rejuvenation would throw away the most informative feature: some things moved and others did not. The participants’ sex, menopausal status and prediabetes are essential to the finding. Extending it to everyone interested in energy would require other treatment comparisons. ## A mouse missing CD38 NAD+ carries electrons during the chemistry that releases energy from food. Other cellular work consumes it. A [review of NAD+ metabolism](https://pubmed.ncbi.nlm.nih.gov/33353981/) describes the enzymes involved in DNA repair, gene regulation and inflammation that use this supply. The [CD38 experiment](https://pubmed.ncbi.nlm.nih.gov/27304511/) approached the supply from its disappearing end. Mice lacking this NAD-consuming enzyme were protected against age-related NAD+ decline and maintained better mitochondrial function. Mitochondria are the cell structures heavily involved in energy production. Removing an enzyme in a mouse is a substantial biological intervention. It helps explain a mechanism; a supplement trial in people has to establish its own outcome. ## The heart tissue experiment Another experiment combined [human heart tissue with a mouse model of heart failure](https://pubmed.ncbi.nlm.nih.gov/40211954/). NAD+ repletion restored activity of HMGCS2, an enzyme involved in making ketone fuel, and improved cardiac function in the mice. Reducing HMGCS2 in heart muscle prevented the same rescue. I like the second half of that experiment particularly. Removing a proposed mechanism gave the explanation a demanding test. The human material was heart tissue; the functional treatment result was in mice. A clinical heart-failure benefit in living people remains a further question. ## NAD+ tolerability The [NMN trial reported no safety issues in its groups](https://pubmed.ncbi.nlm.nih.gov/36482258/), and the [NR trial found no significant adverse-event difference from placebo](https://pubmed.ncbi.nlm.nih.gov/31278280/). These are reassuring findings within the products, participants and observation periods tested. Longer-term human safety remains uncertain. NAD+ itself, oral precursors and compounded infusions have different exposures and quality questions. The oral trials cannot validate the contents or safety of an infusion. Claims about longer life, disease prevention and healthier organs also remain beyond the demonstrated clinical benefits in the [human evidence review](https://pubmed.ncbi.nlm.nih.gov/41083806/). The research raises a theoretical concern that extra NAD+ could also support the metabolism of an existing cancer. This is an unresolved biological concern, not a demonstrated cancer harm in these human supplement trials. The [metabolism review](https://pubmed.ncbi.nlm.nih.gov/33353981/) supplies the wider cellular context. I’d put a repeatable functional improvement at the centre of the next comparison: can a blood response reliably accompany it in a well-defined group of people? The walking and muscle findings give that question somewhere concrete to go. --- An independent field notebook on whether peptide findings keep their shape when the analytic lens shifts—not a clinic, a shop, or medical advice.