This compound was also identified in the dust samples, without being statistically significantly different (Figure ). Interestingly, opposite trends were observed, which suggests that biological samples do not always reflect environmental exposures. A previous study reported decreased levels of pipecolic acid in plasma samples from pre-PD individuals, suggesting that this may be indicative of microbiota-gutbrain axis (MGBA) dysregulation . Pipecolic acid can be produced by intestinal bacteria and cross the BBB , where it can act as a neurotransmitter, modulating the uptake of γ-aminobutyric acid (GABA) by brain neurons , . Moreover, pipecolic acid may originate from the breakdown of lysine in human mitochondria and peroxisomes, which are organelles involved in many roles including β-oxidation metabolism, and ROS homeostasis , . Dysfunction of these organelles contributes to the aging process and neurodegenerative diseases, including PD .