Individual oxidative stress and inflammation responses to vitamin C supplementation: Aggregated sets of n-of-1 trials

Background Personalized antioxidant supplementation is promoted to optimize redox balance and inflammation profile. Objective To quantify the short-term effects of vitamin C supplementation on redox and inflammatory outcome measures and explore the potential for supplement response heterogeneity in participants with vitamin C inadequacy through aggregated sets of multi-cycle n -of-1 trials. Methods Eight healthy young males (age 25.56 ± 3.15 years, body mass 68.24 ± 9.70 kg) completed four supplementation (vitamin C 1g) and four placebo trials administered on repeated occasions in randomized sequences following a 1-month run-in period. Vitamin C, F 2 -isoprostanes, interleukin-6, and tumor necrosis factor-α were assessed as primary outcomes. Separate within-participant linear mixed-effects modelling and meta-analytic models estimated replicate-averaged treatment effects and person-by-treatment response variation to vitamin C supplementation. Results Supplementation resulted in a statistically significant increase in plasma vitamin C of 20.6 μmol/L (95% confidence interval [CI]: 16.8 to 24.5). This mean treatment effect was lower than our selected clinically important threshold of 23 μmol/L. Vitamin C supplementation reduced F 2 -isoprostanes by 25.9 pg/mL (CI: 22.2 to 29.6 pg/mL), interleukin-6 by 1.2 pg/mL (CI: 0.7 to 1.7 pg/mL), and tumor necrosis factor-α by 0.5 pg/mL (CI: 0.2 to 0.9 pg/mL). The participant-by-treatment variance component from linear mixed-effects modelling was not statistically significant for all outcomes ( P >0.05), agreeing with the small τ-statistics for all outcomes. Shrinkage-adjusted estimates also showed strong shrinkage toward the mean, indicating that the observed response variation mainly reflected random within-person cycle-to-cycle variability rather than true inter-individual variability. Conclusions Replicate-averaged treatment effects of vitamin C supplementation on our study outcomes were statistically significant, but heterogenous treatment effects were not detected between participants with baseline inadequacy. Cycle-to-cycle within-participant variation was larger than the observed inter-individual variability for each primary outcome response, suggesting that, if clinically relevant, "average treatment" may suffice for people prone to vitamin C inadequacy. Clinical Trial Registry Open Science Framework (osf.io/e567r). Ethics ERC-009/2024; #83059/2024.