PAPPA and major depressive disorder: Here, hepatocyte growth factor, insulin polypeptides, pregnancy-associated plasma protein-A, and vascular endothelial growth factor were found to be related to depression.139 Other than depression, one cross-sectional aging study found that lower vitamin B6 levels were associated with anxiety.140 These studies exploring the relationship between blood analytes, using various assays with unknown levels of precision to NPS symptoms, and with limited or no replication to date serve to highlight the wide range of possibilities to be explored in future research.