A muscle NAD+ enzyme fell with inactivity and rose with resistance training
In short
In healthy mice given chemotherapy, NAD+ was 14% lower in atrophying muscle after two weeks of cisplatin (p=0.047), while groups without atrophy kept their levels. The most responsive marker was the NAD+ biosynthetic enzyme Nrk2, which fell 84–93% across every regimen where muscle was being lost (p < 0.001). A cross-study analysis pointed the same way in humans — inactivity lowered the enzyme and resistance training raised it. The muscle NAD+ pathway tracks the direction muscle mass is moving.
NAD+ has been one of the best-selling words in the supplement aisle for years. This study did not test a supplement; it looked the other way down the pathway — what happens to NAD+ metabolism while muscle is being lost. Healthy mice received common chemotherapy regimens: folfiri or cisplatin for two weeks, folfiri or folfox for five, with vehicle-treated controls, and NAD+ metabolites were measured in skeletal muscle and liver.
What moved together with muscle loss?
- Muscle NAD+ — in atrophying muscle, −14% after two weeks of cisplatin (p=0.047) and −18% after five weeks of folfiri (p=0.069). In the non-atrophic groups (two-week folfiri, five-week folfox) levels were preserved.
- Nrk2 — the most responsive NAD+ biosynthetic enzyme: −93% with cisplatin, −84% with folfiri, −92% with folfox, all at p < 0.001.
- Liver NAD+ — only fell with prolonged treatment: −20% after five weeks of folfiri (p=0.013) and −15% with folfox (p=0.043).
The split matters. NAD+ loss did not appear in every treated group — it appeared in the groups whose muscle was actually shrinking. It travelled with the state of wasting rather than with the drug itself.
What is left for a lifter?
The authors then pooled published rodent cachexia datasets with human inactivity studies. The direction was consistent: muscle Nrk2 fell early and reliably wherever muscle was being lost, and rose with cachexia-targeted interventions and with resistance training in humans. That makes the enzyme less a lagging readout of muscle mass than something that moves alongside it.
So the sentence to take from this paper is not "buy a precursor". It is that training pushes this pathway upward, and a stretch without training takes down more than muscle size — the metabolic machinery underneath goes with it. That is usually what a two- or three-week gap for an injury or a work trip looks like when the Muscle Index reads as a decline rather than a plateau. How many rest days to take and what fades first once supervision ends are two faces of the same question.
The core experiment is chemotherapy given to healthy mice, and the human portion is a reanalysis of existing datasets. It did not test NAD+ precursor supplements, and it cannot settle whether anyone needs one.
Frequently asked questions
Does this study show NAD+ supplements work?
No. It examined whether chemotherapy disrupts tissue NAD+ metabolism. The effect of NAD+ precursor supplementation was not tested.
What is Nrk2?
It is one of the enzymes that build NAD+. In this study it fell 84–93% across every chemotherapy model with ongoing muscle loss, making it the marker most responsive to changes in muscle mass.
How is resistance training related to the NAD+ pathway?
In a pooled analysis of existing datasets, human inactivity lowered muscle Nrk2 expression while resistance training raised it — the same direction as muscle mass itself.
How far did muscle NAD+ actually fall?
In atrophying muscle it was 14% lower after two weeks of cisplatin (p=0.047) and 18% lower after five weeks of folfiri (p=0.069). Groups without atrophy kept their levels.
Does this transfer directly to humans?
Not directly. The experiment is a mouse model, and the human component is a reanalysis of published data. It indicates a direction rather than a conclusion.
Source: PubMed