Many aging-related biomarkers are associated with both frailty and depressive disorder, such as neuroinflammation and altered microglia activation in the brain [343] and with increased CRP, interleukin 6 (IL-6), neutrophil gelatinase-associated lipocalin (NGAL), shortened leukocyte telomere length (LTL) [336], lower levels of vitamin D [335], and higher oxidative stress [344,345] in both cerebrospinal fluid (CSF) and blood [346,347]. Here, IL6 is linked to depressive disorder.