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How NMN crosses
into a cell —
the two models under study.
An earlier article explained why the body works in precursors rather than the finished molecule. That poses a sharper question, and a genuinely unsettled one: once NMN is present, by what route does it actually cross into a cell? Researchers describe two main models — and which applies, and where, is still being worked out. This is a look at an open question, told honestly.
I
A sharper question —
by what route does NMN get in?
A previous article set out why the body works in precursors rather than in finished NAD+: cells take in smaller forms and assemble the cofactor internally. That answers the general question of why precursors exist. It leaves a more specific one untouched — and it turns out to be one of the genuinely open questions in the field. Once a molecule of NMN is present outside a cell, by what exact route does it cross the membrane and reach the inside?
It is worth being clear about what kind of question this is. It is not a question about whether NMN reaches cells, and it is not a question about delivery to the body as a whole — that broader subject of how compounds reach cells is a separate topic. This is the narrower, more technical matter of the final step: the molecular route across the cell membrane itself. And on that specific point, the research has put forward two different models that have not been fully reconciled.
Both models are worth understanding precisely because the honest answer is that the question is not closed. Active science often looks like this — a well-studied molecule whose finer mechanics are still being mapped, with different research groups reporting different findings. Rather than smooth that over, the more useful approach is to lay out the two proposals plainly, note where they agree, and be candid about what remains unsettled. What is not in dispute is the destination, where the enzyme NMNAT completes the molecule into NAD+ inside the cell.
The question is not
whether NMN reaches cells.
It is the narrower one of
by what route it crosses
the membrane itself.
Two Models, One Open Question
What the research has
proposed so far.
Two accounts of how NMN crosses the cell membrane, plus the honest state of the question. All biology described here is drawn from independent research that did not involve any specific Codeage product.
One model proposes that cells take up NMN directly, through a dedicated membrane transporter identified in the research as SLC12A8. In this account the molecule crosses into the cell intact, without changing form first, and is then completed toward NAD+ inside. The proposal drew considerable interest when it was first described. It also remains debated — some research groups have reported difficulty reproducing aspects of it — which is one of the reasons the broader question has stayed open rather than settled.
A second model holds that NMN is acted upon just outside the cell before it enters: a phosphate group is removed, converting it into nicotinamide riboside (NR), which then crosses the membrane and is converted back toward NMN and NAD+ once inside. In this account the molecule changes form on the way in rather than entering whole. The two precursors and how they relate are examined on their own terms in the NMN and NR article; here they appear simply as two stages of one proposed route.
The two models are not necessarily mutually exclusive — it is possible that different routes operate in different tissues, or under different conditions, and that the balance between them varies. Sorting this out is difficult: the methods used to track a small molecule across a membrane are demanding, and findings have not all pointed the same way. The candid position is that the exact route, and how much each model applies where, is an area researchers are still actively working through.
II
Where the models agree —
and why that matters more.
For all the difference between entering whole and entering after a change of form, the two models agree on the parts that matter most. Both begin with a precursor outside the cell and end with the material for NAD+ inside it. Both rely on the cell's internal machinery to carry out the final assembly. The disagreement is about the middle — the precise mechanics of the crossing — not about the start or the destination. It is a debate over the route, not over whether the journey is completed.
Keeping that distinction in view guards against a common error: treating an unsettled mechanistic detail as though it cast doubt on the whole picture. The broad outline described across this series — that the body works in precursors, takes them in, and builds NAD+ within the cell — does not stand or fall on which uptake model proves dominant. The open question is a refinement within an established frame, the kind of detail that active research exists to resolve, not a crack in the foundation beneath it.
It is also why this is presented as an open question rather than a settled fact. There is more authority, not less, in saying plainly what is known and what is still being worked out. The mechanics of how a single small molecule crosses a membrane are genuinely intricate, and the literature reflects that intricacy. Describing it accurately means resisting the temptation to choose a winner the evidence has not yet chosen.
The Two Routes, Side by Side
Two ways in,
one place they meet.
The proposed routes laid alongside each other — entering whole, entering after a change, and the point where both arrive.
Route A · Direct
Across intact, via a transporter
In the first model, NMN crosses the membrane unchanged, carried by the proposed SLC12A8 transporter, and reaches the inside in the same form it started. This is the more direct of the two routes as described — and the more debated.
Route B · Converted
A change of form on the way
In the second model, NMN is converted to nicotinamide riboside just outside the cell, that form crosses the membrane, and it is converted back inside. The molecule arrives by way of an intermediate step rather than entering whole.
Both · Convergence
The same inside destination
Whichever route applies, the cell ends up with the material it needs to complete NAD+ internally, through NMNAT and the salvage machinery. The two models differ on the path in; they agree on where it leads.
The Question in Brief
What is proposed,
and what is shared.
2
Main models proposed for how NMN crosses the cell membrane
A direct route through a proposed transporter, and a route in which the molecule is converted before it enters. Both appear in the research, and neither has fully displaced the other. Research describing these models was conducted independently and did not involve any specific Codeage product.
1
Shared destination — the material for NAD+, inside the cell
However the crossing happens, both models end in the same place: the cell holds what it needs to complete NAD+ within, using its own enzymes. The agreement on the endpoint is far broader than the disagreement on the route.
1
Open question — which route operates, and where
Whether one model dominates, or both apply in different tissues and conditions, is still being worked out. It is a detail under active study, sitting within a broader picture that does not depend on its resolution.
III
An honest open question,
inside a settled frame.
A series that has followed NMN through the roads that supply NAD+, the enzymes that act on it, the balance the cell reads, and the reason the body works in precursors arrives, here, at one of the points where the science is still in motion. How exactly NMN crosses into a cell — directly through a proposed transporter, or by way of a converted form — is genuinely unsettled. Two models stand, they agree on far more than they dispute, and the question of which operates where remains open.
Presenting it this way is the accurate thing to do, and it keeps NMN's place properly framed. The molecule's position in the pathway — a precursor one step from NAD+ — is not altered by which uptake model proves dominant. As with much of NAD+ biology, the finer mechanics here are still being mapped, and the account given reflects a field that continues to fill in its own picture. The open question concerns the route across a membrane, not the larger architecture the route sits within.
Holding both the certainty and the uncertainty at once is one expression of Cellular Longevity — Pillar 03 of The Longevity Code, the dimension of the system built around NAD+ biology and the science of how cells sustain themselves across time. To follow a question to the edge of what is known, and to say so plainly, is part of taking the science seriously.
Two models stand.
They agree on the destination.
The route between is the part
the research
is still working out.
Codeage · Pillar 03 · Cellular Longevity
Built for the
cellular long game.
Cellular Longevity is Pillar 03 of The Longevity Code — the dimension of the system built around NAD+ biology, mitochondrial health, and the science of cellular aging.
Explore Cellular Longevity →Research and studies referenced throughout this article were conducted independently and did not involve any Codeage products. Statements have not been evaluated by the FDA. Codeage products are not intended to diagnose, treat, cure, or prevent any disease.
