However, it is not clear whether higher urinary BDNF concentrations reflect higher or lower systemic BDNF levels, and even more complex, both low and high serum BDNF levels have been linked to altered neurobehavior, with lower levels associated with anxiety and depression [96,97], while high levels are linked to ADHD and autism spectrum disorders [98,99]. Here, BDNF is linked to attention deficit-hyperactivity disorder.