Figure 1

The association between vitamin C effect and baseline histamine PC ₂₀ level on the common-cold-day. Baseline PC₂₀ level indicates the histamine PC₂₀ level before vitamin C administration on the common-cold day in the Bucca et al. study[25, 26]. Adjusted vitamin C effect indicates that the vitamin C effect of the second day (after recovery at 6 wk) is subtracted from the vitamin C effect on the common-cold-day. For example, participant #9 had a 7.45-fold increase in PC₂₀ level on the common-cold-day and a 1.88-fold increase in PC₂₀ level on the second day. This gives an adjusted vitamin C effect of 3.96 (7.45/1.88). Adding the baseline histamine PC₂₀ level to the null model increased the model fit by χ²(1 df) = 6.2, P = .013. The horizontal dash (−) line indicates the level of vitamin C effect after recovery. The numbers indicate the identification numbers used in Additional file 2. See the Additional file 3 for the calculations of the linear model.