Subjects
All patients with schizophrenia were recruited from the inpatient wards of three hospitals in Anhui Province in China between May to December 2018. On the basis of the inclusion criteria and exclusion criteria, 443 inpatients were consecutively enrolled in the study. A total of 333 inpatients completed the study, for a response rate of 75.17%. The data of an additional 4 patients were incomplete and were excluded. Therefore, 329 patients with schizophrenia were ultimately included in this study. All schizophrenia patients were diagnosed by clinicians using the International Classification of Diseases-10th edition (ICD-10). The age range of the patients was 18-75 years and the illness duration was more than 5 years (all were chronic patients [23]). We recruited 175 sex- and age-matched volunteer as the control group for this study at the health check-up centre in hospital. The control group members did not have schizophrenia, and were aged 18 to 75 years old. The schizophrenia patients and the control participants were excluded when the following conditions were present: severe neurological disease; intellectual disability; substance dependence excluding alcohol and tobacco; and pregnancy or lactation. All subjects volunteered to participate in the study; they or their guardians signed an informed consent form.
Definition of the MAO phenotype
Currently, in international studies, MAO is defined in individuals who meet two requirements: metabolic abnormality and obesity [8]. There are two methods for defining metabolic abnormalities: the diagnostic criteria for metabolic syndrome (MS) and insulin resistance/sensitivity cut-off points [8]. To unify the research and facilitate comparisons, the Circulation Research Compendium on Obesity, Diabetes and Cardiovascular Disease recommends using the MS definition for metabolic abnormalities [8]. Regarding the criteria for obesity, we adopted the recommendation of WHO experts for Asian populations [24], that is, body mass index (BMI) ≥ 28 kg/m2 was classified as obesity [25].
In summary, the inclusion criteria for subjects with the MAO phenotype were as follows: (1) meeting 1-4 diagnostic criteria for MS (excluding waist circumference) and (2) BMI ≥ 28 kg/m2. The unified criteria for MS in 2009 [26] were as follows: (1) triglyceride level ≥ 1.69 mmol/L or hypolipidemic drug use; (2) high-density lipoprotein cholesterol (HDL-C) < 1.04 mmol/L for the male sex, HDL-C < 1.29 mmol/L for the female sex, or therapeutic drug use; (3) systolic blood pressure ≥ 130 mmHg and/or diastolic blood pressure ≥ 85 mmHg or antihypertensive drug use; and (4) fasting blood glucose ≥ 5.6 mmol/l or hypoglycaemic drug use. In addition, we selected MHNW patients as the control group according to the following criteria: 0 MS criteria and 18.5 ≤ BMI < 24 kg/m2.
Sociological and clinical characteristics
Data on sex, age, marital status, education level and current smoking status were collected for all subjects. Current smoking behaviour was defined as smoking more than 1 cigarette per day more than 5 days per week in the last two weeks. Height and weight were measured after the subjects took off their shoes and while they were wearing light clothes. The calculation of BMI was calculated using weight (kg) / [height (m)2] [27]. We collected the schizophrenia patients' disease information, including the age at onset, the total course of the disease, the history of accompanying chronic somatic diseases such as hypertension or diabetes, and the types and dose of antipsychotic medications currently in use. All currently used drug doses were converted to chlorpromazine equivalents [28].
Biochemical detection
Fasting blood samples were taken between 6 and 8 am and were sent to the clinical laboratory for examination. Fasting blood glucose was measured by the oxidase method (Meikang Biotechnology Co., Ltd., Zhejiang, China). Triglyceride was measured by the GPO-PAP method (Beijing Lidan Biochemical Co., Ltd., Beijing, China). Cholesterol was measured by the CHOD-POD method (Beijing Lidan Biochemical Co., Ltd., Beijing, China). HDL-C and LDL-C were measured by the terminal method (Neusoft Whitman Biotechnology (Nanjing) Co., Ltd., Jiangsu, China). Fasting plasma insulin and plasma C-peptide levels were both measured by electrochemiluminescence (Roche Diagnostics GmbH, Mannheim, Germany). Glucagon was tested by radioimmunoassay (Beijing North Biotechnology Research Institute Co.,Ltd, Beijing, China). Insulin resistance (IR) refers to low insulin sensitivity and abnormal glucose metabolism. IR was quantitatively calculated by homeostasis model assessment (HOMA), and the formula was HOMA-IR = (fasting blood glucose * fasting plasma insulin) / 22.5 [29]. We defined HOMA-IR values greater than 2.5 as insulin resistance [30].
Scale evaluation
We used the 30-item Positive and Negative Syndrome Scale (PANSS) to evaluate the severity of psychiatric symptoms in patients with schizophrenia [31]. Four trained interviewers administered the PANSS, and the internal consistency coefficient was greater than 0.8. Each item of the PANSS was scored from 1(asymptomatic) to 7 (extremely symptomatic), and the higher the total scores were, the more serious the mental symptoms. We used the five-factor model of the PANSS to evaluate the mental symptoms from multiple dimensions. The five-factor model [32] includes the following: (1) positive factor (P1, P3, P5, G9), (2) negative factor (N1, N2, N3, N4, N6, G7), (3) cognition factor (P2, N5, G11), (4) excited factor (P4, P7, G8, G14), and (5) depressed factor (G2, G3, G6). The six items comprising the PANSS negative factor are blunted affect, emotional withdrawal, poor rapport, passive/apathetic social withdrawal, lack of spontaneity, and motor retardation.
Data analysis
First, we compared the characteristics between the schizophrenia patients and the control group. For continuous variables, the independent samples t-test and Mann-Whitney U test were used as appropriate. For categorical variables, the Chi-square test was used. Second, covariance analysis was conducted to compare the prevalence of MAO and MHNW phenotypes between the two groups after controlling for confounding variables. Sex, age, education level, marital status and smoking status were included as covariance variables. Third, the clinical characteristics of the MAO and MHNW groups of schizophrenia patients were compared. An independent samples t-test, Mann-Whitney U test and Chi-square test were also used as appropriate. Fourth, to control for all possible confounding factors and further compare the mental symptoms in the MAO and MHNW groups, we performed a binary logistic regression with the “Enter” method. The groups (MAO and MHNW) were the dependent variables (MHNW group as the reference group), and the other variables, including sociodemographic and clinical data, the types and dose of antipsychotics, and PANSS factor scores, were simultaneously included as covariates. Fifth, all patients with schizophrenia were divided into MAO and non-MAO groups. The Spearman correlation coefficient was performed to examine the clinical variables related to MAO in schizophrenia patients. All statistical analyses were performed with SPSS 23.0, and a two-tailed P value < 0.05 indicated significance.