Type 2 Diabetes mellitus (T2DM), characterized by insulin resistance with (or without) islet β-cell dysfunction, is a lifelong lifestyle condition caused by the interaction among various genetic and environmental factors [1]. T2DM has become a major disease endangering the human health worldwide. According to the 10th edition of the IDF Diabetes Atlas, approximately 537 million adults (20–79 years old) worldwide have diabetes in 2021. This figure is estimated to increase to 643 million and 783 million by 2030 and 2045, respectively [2]. China, the country with the largest number of diabetes patients, reported 156 million people with diabetes in 2020. Of these, 90–95% had T2DM [3].
T2DM, an incurable chronic lifelong disease, gives rise to multiple complications and has a long treatment cycle. It not only endangers the physical health of patients, but also adversely affects their mental health. Some common psychological disorders that have been reported in T2DM patients include anxiety [4], depression [4, 5], diabetes-related distress [5–7], fear of hypoglycemia [7, 8], and loneliness, which has been reported recently [9]. Loneliness is a subjective psychological experience caused by the gap between an individual’s desire for communication and actual communication [10]. It is a distressed feeling, which arises when a person’s social needs do not meet the quantity or quality of their social relations [11]. Loneliness is a common affliction in European countries. Approximately 30 million adults in Europe have reported to have experienced loneliness. Approximately one-fifth of Britons [12] and one-third of Americans [13] reported to have experienced loneliness.
At present, the research on loneliness mostly focuses on the elderly [14], adolescents [15], and other normal population. A small number of studies have focused on patients with cancer [16], schizophrenia [17], stroke [18], and other acute and critical conditions. Recent studies have shown that T2DM patients also face varying degrees of loneliness. Approximately 25–53% of patients have reported to suffer from moderate or above loneliness [19]. T2DM, as a chronic lifelong condition, aggravates the sense of alienation and increases the feeling of abandonment in the patients. Consequently, patients feel isolated, which produces a sense of loneliness [20]. Loneliness also increases the risk of cardiovascular diseases, the main cause of death in DM patients [21]. Therefore, severe long-term loneliness increases the risk of death in T2DM patients. In severely lonely people with T2DM, the risk of death may increase by 22–26% compared to that in the normal population [22]. Hence, T2DM may result into loneliness, which adversely affects both the physical and mental health of patients. Therefore, it is highly important to pay attention to and attempt to reduce loneliness in T2DM patients in order to control their condition and improve their physical and mental health.
A growing body of research has been reporting on the protective factors that delay or reduce loneliness. Notably, family function, a concept that reflects the characteristics related to family relations, family intimacy, and adaptability, is being advocated as a protective factor against loneliness. Kim and Baik [23] have stated that the worse the family function, the higher the loneliness level of the elderly. Pan et al [24] studied loneliness in undergraduates and secondary vocational school students independently and reported that their family function is related to loneliness. Johnson et al [25] indicated that a warm harmonious family environment or a strong family cohesion can reduce the risk of loneliness. However, the current research on the association of loneliness with family function mostly focuses on the normal population. In addition, no direct study has been conducted on the relationship between the above two factors in T2DM population. Despite this, some studies have reported that good family function can help TDM patients in regulating their blood glucose fluctuations, strengthening their psychological elasticity, and inhibiting the development of negative emotions such as anxiety and depression [26]. These studies reasonably predicts that a correlation exists between family function and loneliness in T2DM patients. Good family function can help adjust the psychological state of T2DM patients and reduce the degree of loneliness.
Although several articles have proved that good family function has a protective effect on loneliness, the specific path of family function affecting loneliness and the mechanism through which family function affects loneliness in T2DM patients need to be further studied. Existing studies have shown that depression induces loneliness [27]. A significant positive correlation between depression and loneliness in the elderly population has been reported. In addition, depression adversely affects the quality of life of the elderly by partially affecting loneliness [28]. A longitudinal study conducted by Zhou et al. [29] demonstrated that low self-esteem affected depression in adolescents, partly through the mediating effect of loneliness. Good family function can effectively reduce the occurrence or level of depression in postpartum women [30, 31], the elderly [32], and epileptic children [33]. Some studies have also reported that in T2DM patients, good family function can enhance the psychological elasticity of patients, inhibit the generation of negative emotions such as anxiety and depression symptoms, and help regulate the blood glucose fluctuations [26]. Considering the above relationship among family function, depression, and loneliness, we herein aim to verify whether depression has a mediating effect between family function and loneliness in the T2DM population.
Self-perceived burden refers to the patient’s guilt of using the help of a caregiver for daily life activities and the resulting frustration about oneself [34]. McPherson et al. [35] defined self-perceived burden as the physical and psychological burden on caregivers caused by sick individuals’ worry about their own illness and the need to be cared for, which eventually gives rise to a series of negative emotions in patients. A small number of studies have shown that self-perceived burden can affect loneliness. For example, Hill and Frost [36] conducted an investigation of ovarian cancer patients and found that self-perceived burden has an indirect impact on the psychological distress of loneliness. In addition, a survey of adolescents with neurofibromatosis type 1 reported that depression, self-perceived burden, and low levels of social support are predictors of loneliness [27]. Self-perceived burden is common in T2DM patients. Yu et al. [37] investigated 215 patients with diabetes and observed that the self-perceived burden of T2DM patients was at a mild-to-moderate level. A study on patients with diabetic foot showed that 88% of the patients had different degrees of self-perceived burden; the higher the severity of the disease, the heavier the self-perceived burden [38]. The following factors influence the self-perceived burden in T2DM patients: general demographic factors, disease course [39], disease severity factors [40], psychological factors [41], and social support level factors [42]. Although not confirmed in the T2DM population, depressive symptoms are a significant predictor of self-perceived burden in patients with chronic pain [43]. Good family function can significantly reduce the level of self-perceived burden in patients suffering from maintenance hemodialysis [44] and post-breast cancer surgery [45] and in elderly patients with coronary stent implantation [46]. To sum up, we hypothesized that in T2DM population, depression may affect self-perceived burden, which may be a potential mediator between family function and loneliness.
From the above discussion, we can see that the family function, depression, self-perceived burden, and loneliness might be related. The current study examines the series multiple mediating effects of depression and self-perceived burden between family function and loneliness in the T2DM population of China. For this purpose, we propound the following assumptions. Firstly, there is a possible correlation between family function and loneliness in the T2DM patients of China. Secondly, depression and self-perceived burden may mediate this relation between family function and loneliness, respectively. Finally, depression and self-perceived burden may play a series of mediating roles between family function and loneliness.