What exactly does blood NAD+ tell us?

In science, it is important to consider alternative explanations and to recognize novelty even when a topic seems well described. In the NAD field, one striking observation is that NAD+ in blood does not seem to fall with age in healthy people, while NAD+ in some tissues such as muscle does decline. This raises an important question: what exactly does blood NAD+ tell us?

If blood is viewed simply as a collection of cells, the answer may seem limited. However, if it is viewed as a communication interface within a cooperative network of tissues that exchange metabolites and metabolic signals, blood NAD+ may have broader significance. Erythrocytes are not merely oxygen carriers; they are metabolically active cells with a particularly strong capacity for NAD+ synthesis from nicotinic acid through the Preiss–Handler pathway. This suggests that these cells contain not only high NAD+ levels, but also elevated concentrations of intermediates in the NAD+ synthesis pathway, which could be biologically important.

In healthy, non-supplemented individuals, blood NAD+ falls within a relatively broad physiological range, from 20 µM to 36 µM. After supplementation, it can rise to as high as 100 µM and reach saturation. This suggests that blood may act as a buffer for NAD+ and as a reservoir of bioavailable NAD+ precursors for the rest of the body. A similar principle is known for vitamin B9 distribution in the body. The liver converts dietary folate into methyl-tetrahydrofolate and releases it into circulation as the basic active form of vitamin B9 for use by other tissues. By analogy, blood may likewise help distribute NAD+-related intermediates to peripheral tissues. Although the mechanism of this transfer remains to be established, there are indications that it exists.

From this perspective, blood NAD+ may reflect more than the status of a single organ. It may represent the balance between supply and demand for the active forms of vitamin B3 across the body. Values within the physiological range suggest that this buffering system is working well. Levels at the lower end of the range, or below it, may indicate increased demand and reduced buffering capacity.

It also raises an important caution. Because NR and NMN can increase NAD+ levels, they are sometimes taken regularly even without a clear medical need. But if blood NAD+ has a normal physiological range, that range must matter first for blood itself, even if we do not yet know why it is set where it is. Just as pain is not simply an inconvenience but an essential warning signal, NAD+ levels may also reflect the body’s metabolic state in a way that should not be ignored. Raising them without a real need may not be harmless. To use an analogy, would anyone take painkillers in advance just to prevent any possible pain? Pain is not only something unpleasant; it is a vital signal that helps the body detect dysfunction early. Blocking that signal would risk missing the first signs of disease. The same principle may apply to blood NAD+ levels: they may reveal increased demand by showing how strongly the blood’s NAD+ buffering capacity is being used.

In conclusion, blood NAD+ may be more than a marker of one tissue’s NAD+ status. It may function as an indicator of whole-body metabolic balance and buffering capacity. Monitoring blood NAD+ could therefore help guide decisions about vitamin B3 supplementation and identify situations in which NAD+ demand is increased.