## 4 Discussion  
Considering the diversity of existing information on the toxicology of PFAS, we focused on exploring their relationships with metabolic outcomes from epidemiological studies as well as identifying molecular events and adverse outcomes via _in vivo_ and _in vitro_ studies using the AOP-helpFinder tool. Our findings indicated that PFAS exposure could negatively influence all components of metabolic syndrome, either individually or, more likely, simultaneously as a mixture. Only three AOPs presented in AOP-Wiki, AO ID952 hypertension (AOP149), AO ID1343 increased hypertension (AOP226), and AO ID1447 obesity (AOP72), could be connected to various metabolic outcome markers (see Figure 3) that are also associated with PFAS exposure. As illustrated in Figure 3, the main identified events of this study are interconnected with each other, and our collected data show that some KE of existing AOPs can be initiated by PFAS as stressors as well. However, there were several AOPs provided in AOP-wiki that we could not include in this study because they are still under development and are therefore not citable. Our study outcome suggests that PFAS exposure directly influences biomarkers of dyslipidemia, hypertension, and obesity but not insulin resistance. Although concentration differences in PFAS exposure, especially related to the region, sex, and age can vary significantly, we decided to reduce the complexity of the outcomes by omitting such subdivisions in general. For example, we found that studies that reported inverse associations between HOMA-IR and PFAS (mostly for girls) were conducted mainly in children [37; 52] or adolescents [53]. On the other hand, studies that reported associations between elevated PFAS exposure levels and increased HOMA-IR were mostly conducted with adults or elderly individuals [54; 55]. Since adults and elderly individuals are more prone to insulin resistance due to several age-related risk factors compared to children and adolescents, it is inconclusive to what extent these associations are related to the exposure to PFAS. While it was not possible to identify some MIE/KE within the collected data using the AOP-helpFinder, probably due to the selected parameters, it was possible to fill some data gaps with some additional literature research using, e.g., search platforms such as Google Scholar. More recently, for example, one study reported that an exposure to a mixture of PFAS can lower the total brain dopamine concentration in mice as well as the tyrosine hydroxylase transcript level in mice [56], which are KE1450 and KE1452 from AOP72. Obesity, the AO of AOP72, is associated with PFAS exposure, as reported in some epidemiological studies. Tributyltin is the stressor of AOP72, and our collected data indicate that PFAS potentially influences the same molecular pathways, increasing the risk of obesity. Furthermore, although we found that PFAS can increase the intracellular calcium levels (KE1339 of AOP226), it was shown that PFAS could cause a decrease in serotonin transporter activity (KE1320 of AOP226) [57]. While our study indicated some MIE/KE that are associated with PFAS exposure and metabolic outcomes, we also identified some gaps that could be addressed in the future, and we hope that our results can support the knowledge and developmental processes of AOPs. The results from _in vitro_ and _in vivo_ as well as epidemiological studies indicate that PFAS exposure is associated with hypertension, which is the AO of AOP149. However, to the best of our knowledge, it is still unknown if PFAS exposure decreases the expression of guanosine triphosphate cyclohydrolase 1 (GTPCH-1) or if it decreases tetrahydrorobiopterin levels, KE935 and KE934 of AOP149. We also found that some events that are associated with PFAS exposure, e.g., an increase in alanine aminotransferase (ALT), are not included in any AOPs at present; further research could address this.