DPP4 and depressive disorder: Previous studies have reported higher levels of DPP4 activity in patients with diabetes, obesity and depression (Zheng et al., 2014, 2016a; Nargis and Chakrabarti, 2018), in addition, diabetic and depressive patients both exhibited lower levels of BDNF in the circulation (Krabbe et al., 2007; Molendijk et al., 2014), therefore, whether DBR could be used as a novel risk biomarker for MCI in patients with diabetes, obesity and depression remains to be clarified in further research.