4.9. Limitations of the Study
MINOR
There are a number of limitations that should be considered in interpreting the current findings. Since this study was designed as an exploratory pharmacokinetic trial, case number calculation was not possible due to a lack of data. Accordingly, a sample size of 12 was considered appropriate as it was established in phase 1 clinical trials. Therefore, it cannot be excluded that the study was not sufficiently powered to detect minor concentration changes. However, the robustness of the data was improved by the cross-over design with placebo control, which can be considered the baseline. Validated analytic results of spermidine, spermine, and putrescine can be estimated as robust but were performed in plasma after protein precipitation. It is unclear whether the substances could be transported intracellularly in blood cells or bound to plasma proteins in relevant amounts, which would have led to an underestimation of the complete amount present in whole blood. It is also possible that there might be some significant effects of spermidine supplementation if taken by special subgroups, in higher doses, or simultaneously with certain foods. Moreover, the intervention phases were rather short (5-day interventions), and it cannot be excluded that a long-term intake of spermidine would show deviating results. The latter seems unlikely, as Soda et al. obtained very similar results on spermine concentrations with similar amounts of supplemented spermidine, even after long-term administration [