Our study included adolescent depressive depression, adult depressive depression and late-life depression who had treated with fxed doses of drugs for 2 years or more, and excluded smoking and drinking habits. The indicators we measured contained blood lipid, liver function, renal function, cardiac function, blood inflammation and so on, which could comprehensively understand the related factors between MDD and obesity.
First of all, compared with MDD in different age groups, we found that there was little difference in the incidence of obesity between adolescents and adults, while the incidence of obesity was higher in patients with depression in late life. According to a national epidemiological survey, the results indicated that the prevalence of obesity in urban areas in southern China was 2.8–7.2% from 2010 to 2018, where the subjects in our study came from. This means that the incidence of obesity in patients with depressive disorder treated regularly would increased at different ages.
In addition, the values of TC、TG、LDL、urea、SCr、UA、HbA1c and HsCRP showed an upward trend in the LLD group. In order to further explore the relationship between these serum metabolic indexes and the occurrence of obesity in MDD, we made a correlation analysis between BMI and them, witch showed that: BMI was positively correlated with age, TC, TG, LDL, AST, ALT, LDH, UA, HbA1c and hsCRP, and negatively correlated with HDL. Previous research results suggested that the incidence of obesity increases with age, and MDD is no exception. TC, TG and LDL are the most frequently detected blood lipid indicators in clinic. They were recognized as risk factors in lipid metabolism  . Therefore, it is easy to understand that their increase increases the risk of obesity. In contrast, HDL has the opposite function and its rose was a protective factor for obesity . These were consistent with our experimental results. Clinically, AST and ALT are commonly used to reflect human liver function, and the liver function plays an important role in lipid metabolism. LDH mainly exists in animal tissues such as myocardium and liver. Clinically, it is often used as an auxiliary index for the diagnosis of myocardial infarction and liver disease. Its rise, like AST and ALT, participated in the process of obesity and overweight. SCr is the product of human muscle metabolism. In muscle, creatine forms creatinine slowly mainly through irreversible non enzymatic dehydration reaction, which is then released into the blood, filtered through the glomerulus and discharged with the urine. The detection of SCr is an important index of renal function. Elevated SCr means impairment of renal function. In the longitudinal study of diabetes, the researchers found that obesity was associated with decreased renal function. The results of this study indicated that there were significant differences in the comparison of SCr in different age groups, and SCr in the older group is higher. However, when we did correlation analysis between SCR and BMI, we get negative results, which was inconsistent with our previous results, which might be the impact of the selection of MDD in this study. However, the effect of UA was well recognized, and some people believed that its increase was related to the increase of weight and blood pressure. What’s more, its increase was significantly associated with increased risk of diabetes, especially for overweight people. HbA1c is considered to be a reflection of the average blood glucose level in the body for three months. It was considered to be a risk factor for obesity and diabetes  . HsCRP is an acute phase protein synthesized in the process of inflammatory stimulation such as tissue injury and infection. As a member of inflammatory factors, it participated in the disease process of MDD and was considered to be related to obesity. Therefore, our study verified the previous research and achieved good consistency.
Finally, the results of regression analysis showed that age, AST, ALT and UA were the risk factors for the increase of BMI. To our surprise, conventional blood lipid indexes such as TC, TG, LDL and HDL did not pass the test. As we know that antidepressants are metabolized by liver and kidney, and taking such drugs will often affect liver and kidney function clinically, especially for ALT, AST, UA, etc  . Our results were the same, suggesting that the increase of these indicators is of great significance in the occurrence of obesity in MDD in different age groups.
Unfortunately, in order to obtain a stable blood index of the internal environment of patients with MDD in this study, we selected patients taking drugs regularly for two years or more, and they were also required to rule out serious physical diseases, which caused great difficulties for us to collect elderly samples. At the same time, in order to exclude the influence of smoking and drinking habits, not a few patients were excluded before enrollment, which would cause deviation in our study, but we had to do so.
All in all, we should pay more attention to the monitoring of blood metabolism indicators with the increase of age in MDD, and timely adjust the use of drugs refer to their liver and kidney function, so as to reduce the risk of obesity and improve their quality of life.