What is happening. NR raises cellular NAD+ levels, and resveratrol is a putative SIRT1 activator. Because sirtuins consume NAD+ as a cosubstrate, the two are frequently combined on the theory that NR supplies the NAD+ fuel that resveratrol-stimulated sirtuins require, potentially enhancing downstream effects on mitochondrial biogenesis.
Mechanism. Sirtuins (e.g., SIRT1) require NAD+ as an obligate cosubstrate. NR provides NAD+ substrate while resveratrol may allosterically increase SIRT1 activity, theoretically coupling substrate supply to enzyme activation.
Recommendation. Co-supplementation is generally well tolerated and there is a reasonable mechanistic rationale, but proven additive clinical benefit in humans is lacking. It is fine to take them together; do not expect a confirmed synergistic outcome.