COVID‑19 induces profound shifts in host metabolism and immune regulation. Most notably, depletion of NAD⁺ and dysregulation of tryptophan metabolism via the KP. NAD⁺ is essential for cellular energy, DNA repair, and innate antiviral responses, but during SARS‑CoV‑2 infection, NAD⁺ levels drop due to increased PARP activity and inflammation-associated oxidative stress. At the same time, pro‑inflammatory cytokines (e.g., IFN‑γ, IL‑6) activate indoleamine 2,3‑dioxygenase (IDO), diverting tryptophan into the KP. This shift leads to both reduced serotonin synthesis and accumulation of immunomodulatory metabolites like kynurenine and quinolinic acid — a hallmark seen in COVID‑19 and associated with worse outcomes and cognitive impairment (See 2022 August Featured Article). By starving cells of NAD⁺ and tryptophan while flooding the system with downstream KP metabolites, SARS‑CoV‑2 effectively undermines cellular resilience, promotes immune dysregulation, and plausibly contributes to persistent fatigue, neurocognitive effects, and prolonged recovery.
In a recent Nature Metabolism study, Schreiber et al. report findings from the COVit-2 trial, a large, randomized, double-blind, placebo-controlled study involving 900 symptomatic outpatients with PCR-confirmed COVID-19. Participants received either 1 g/day nicotinamide (500 mg immediate-release + 500 mg ileocolonic-targeted CICR-NAM [Setamer®]) or placebo for four weeks. Adverse events, predominantly mild, were reported in 70.8% of the nicotinamide group and 65.7% of the placebo group, mostly reflecting the natural course of early infection.
By week 2, 57.6% of the nicotinamide group showed recovery from physical performance decline, compared to 42.6% in the placebo group — a clinically meaningful improvement (number needed to treat = 7). Stool microbiome analyses further revealed functional shifts in microbial metabolism among nicotinamide-treated participants, including reduced tryptophan and NAD⁺ salvage biosynthesis pathways, suggesting relief from infection-induced microbial compensation. These changes, primarily at the functional rather than taxonomic level, indicate that nicotinamide may restore microbiome metabolic output toward a healthier, pre-infection state through both direct gut exposure and improved host–microbe crosstalk.
These findings support the hypothesis that NAD⁺ replenishment via nicotinamide may enhance resilience against viral-induced immunometabolic stress. Given the known role of the KP in neurobehavioral outcomes, modulation of this pathway may also benefit cognitive and psychiatric sequelae of infection. Longitudinal studies integrating KP metabolite profiling with symptom trajectories are warranted to inform post-COVID care and management of long COVID (see other related posts of KP and long COVID in 2024 April/May; 2023 Oct, Nov and 2022 Aug featured articles).