A recent placebo controlled trial tried to figure out effects of NR in humans undergoing exercise.
The study setup:
participants were supplied with 1 gram of NR or placebo for 1 week
One hour of endurance exercise was carried out after 7 days and metabolic parameters were assessed directly afterwards and 3 hours later. This involved muscle biopsies (so taking muscle tissues samples of the participants) - while invasive for the participants, this gives a much better picture of the impact on real humans than studies with tissues in a petri-dish or mice. The study can be found using tjese links:
Nicotinamide riboside supplementation does not alter whole‐body or skeletal muscle metabolic responses to a single bout of endurance exercise
https://physoc.onlin...0.1113/JP280825
https://www.biorxiv....446v1.full-text
Their findings, quote:
"There was no effect of NR supplementation on substrate utilisation at rest or during exercise or on skeletal muscle mitochondrial respiration."
"NR supplementation did not increase skeletal muscle NAD+ concentration, but it did increase the concentration of deaminated NAD+ precursors nicotinic acid riboside (NAR) and nicotinic acid mononucleotide (NAM) and methylated nicotinamide breakdown products (Me2PY and Me4PY), demonstrating the skeletal muscle bioavailability of NR supplementation."
This could point to different interpretations:
- 1 week is not sufficient to develop the necessary adaptions
- 1 gram is insufficient to increase NAD+ sufficiently
- it doesn't work (for example because of feedback reduction in endogenous NAD+ cycling)