KL and Parkinson disease: Evidently there exists a complex interplay between aging and chronic stress in the pathogenesis of PD through mechanisms involving mitochondrial dysfunction and oxidative stress, neuroinflammation, insulin resistance, and vitamin D. It is difficult to extract precise cause and effect in the vicious circle resulting in neurodegeneration; however, there is a case that the hormones cortisol and klotho can contribute to the disease process in a yin-yang manner.