This review article examined nicotinamide mononucleotide (NMN) supplementation and the metabolic variability observed in response to NMN treatment across different populations and conditions. The review found that individual differences in NAD+ metabolism, age, genetic factors, and baseline metabolic status significantly influence how people respond to NMN supplementation, with implications for personalizing potential therapeutic applications. **Evidence level: Review article** (synthesizes existing literature rather than presenting original human clinical or experimental data).