NAD and the Ageing Brain: Part Three: The Mechanism in Mice

Abstract

Part Two ended in a null. This part covers the study that explains why the field kept going anyway, and it is a good piece of work.

Stefano Tarantini and colleagues took 24-month-old mice, gave them nicotinamide mononucleotide for two weeks, and measured whether the brain's ability to send blood where it is needed was restored. It was.

The chain from NAD+ to behaviour is measured at every link. It is also entirely in mice, and it uses a different molecule from the one the human trial in Part Five will use.

1.Neurovascular Coupling

When a region of the brain becomes active, local blood flow rises to meet its demand. That coupling is not housekeeping — it is what allows sustained cognitive work, and it is the physical basis of everything functional imaging measures.

It weakens with age. The endothelium lining the smallest cerebral vessels loses its capacity to dilate on demand, and increased oxidative stress is the standard explanation. Impaired coupling is understood to contribute to cognitive decline rather than merely accompany it.

The setting for this is the neurovascular unit — endothelial cells, pericytes, astrocytes, neurons, microglia and a basement membrane, described in the review of the ageing blood–brain barrier by Knox and colleagues in Molecular Psychiatry. It is a coordinated system, and coordination is what degrades.

2.The Experiment

Twenty-four-month-old C57BL/6 mice — old animals — were treated with nicotinamide mononucleotide for two weeks.

Neurovascular coupling was assessed directly: cerebral blood flow responses were recorded by laser Doppler flowmetry while the whiskers on the opposite side were stimulated. This is about as close to measuring the phenomenon itself as an experiment can get.

In aged mice, those responses were significantly impaired, confirming the phenotype the study set out to rescue.

After nicotinamide mononucleotide, the responses were restored. The mechanism was identified rather than assumed: the rescue operated through increased endothelial nitric-oxide-mediated vasodilation.

It was accompanied by improved spatial working memory and gait coordination — behavioural outcomes, not just physiological ones.

And in cultured cerebromicrovascular endothelial cells taken from aged animals, nicotinamide mononucleotide produced sirtuin-dependent improvements in mitochondrial reactive oxygen species production and in bioenergetics, which supplies the cellular step between NAD+ and nitric oxide.

3.Why This Is Better Than Most Preclinical Work

The chain is complete: NAD+ availability, to endothelial mitochondrial function, to nitric oxide, to vasodilation, to blood flow, to behaviour. Each link was measured rather than inferred from the one before it.

The study also used old animals rather than a genetic model of accelerated ageing, which means the phenotype being rescued is ordinary ageing rather than a proxy for it.

This is what justifies a large human trial, and it is why the trial in Part Five exists. A supplement literature built on studies of this quality would be in much better shape than the one we generally document.

4.The Distances That Remain

Three gaps separate this experiment from a person taking a capsule, and none of them is a criticism of the work.

The molecule differs. This study used nicotinamide mononucleotide. The human trial that followed uses nicotinamide riboside. They are neighbouring compounds in the same pathway and the field treats them as interchangeable — which is exactly the substitution our own rules forbid us to wave through. A different chemical form is a different study.

The measurement differs. Whisker-stimulated blood flow recorded by laser Doppler through a mouse skull is a far more direct observation than near-infrared spectroscopy through an adult human scalp.

And the species differs in a way that matters for this specific pathway. Rodent and human NAD+ metabolism are not identical, and Part Two supplies the relevant warning: the same compound family, tested in human muscle, produced nulls across every pooled outcome.

None of that makes the mouse work wrong. It makes it a hypothesis about humans rather than a finding about them, which is what preclinical work is.

Editorial Comment

MindHeaven® makes no claim about NAD+, cerebral blood flow, or neurovascular coupling, and none is authorised in the European Union.

We include this part in full because criticism without acknowledgement is not scepticism, it is a stance. The mouse work here is careful, complete and honestly reported, and the field's persistence with NAD+ precursors is reasonable rather than credulous.

The question is what happens when a mechanism this well described meets a human trial. Part Four covers the one remaining claim with a plausible route to the brain — the circadian one — and Part Five covers the two trials now running.