Human whole-blood NAD⁺ levels do not vary with age or lifestyle interventions

Nicotinamide adenine dinucleotide (NAD⁺) levels in blood and tissues are widely proposed to decline with age, yet evidence in human blood is inconsistent. Using a rigorously validated ultra-high-performance liquid chromatography coupled with high-resolution mass spectrometry system that accounts for real-world analytical variability, we quantify NAD⁺ across seven independent human cohorts. We find that whole-blood NAD⁺ levels remain remarkably stable with age and across lifestyle interventions, but change in response to nicotinamide riboside supplementation, as expected. Our results challenge the utility of blood NAD⁺ levels as a biomarker of ageing or lifestyle factors.