Association between olfactory function and metabolic syndrome in bipolar disorder patients: a cross-sectional study

Background Olfactory function is closely related to mood and the endocrine system. However, the role of olfactory function in bipolar disorder combined with metabolic syndrome remains unclear. The purpose of this study was to explore the associations among olfactory function, tumor necrosis factor alpha (TNF-alpha), and metabolic syndrome and its components in patients with bipolar disorder. Methods Ninety-six bipolar disorder patients were divided into two groups with and without metabolic syndrome. We also included 46 healthy controls. Olfactory function was assessed using the Sniffin' Sticks test. Blood samples were collected to measure metabolic indicators and serum TNF-alpha levels. Results Significant differences in olfactory function were found among the three groups. Compared with the healthy controls, the bipolar disorder without metabolic syndrome group showed poorer olfactory identification ability (P < 0.001) and the bipolar disorder with metabolic syndrome group showed impaired olfactory sensitivity (P = 0.003) and olfactory identification (P < 0.001). Moreover, the bipolar disorder with metabolic syndrome group had poorer olfactory identification ability than the bipolar disorder without metabolic syndrome group (P = 0.015). Both bipolar disorder groups showed lower TNF-alpha levels than healthy controls. However, there was no significant difference between the two patient groups. Correlation analysis showed that, in the bipolar disorder with metabolic syndrome group, olfactory identification was negatively correlated with systolic blood pressure (r = - 0.424, P = 0.031), and serum TNF-alpha level was negatively correlated with body mass index (BMI; r = - 0.398, P = 0.049), triglyceride (r = - 0.503, P = 0.010), total cholesterol (r = - 0.491, P = 0.013), low-density lipoprotein-cholesterol (r = - 0.491, P = 0.013), and high-density lipoprotein-cholesterol (r = - 0.454, P = 0.023). Conclusions The olfactory identification ability of patients with bipolar disorder is worse than that of healthy controls, and the occurrence of metabolic syndrome will further aggravate the olfactory identification impairment of those patients. Furthermore, there may be a stronger link between serum TNF-alpha level and multiple metabolic indicators in bipolar disorder patients with metabolic syndrome than in bipolar disorder patients without metabolic syndrome.