Loneliness is a common issue among the older adults; it refers to the unpleasant subjective emotional experience caused by an individual’s perceived lack of social relationships and emotions [1]. Scholars have studied loneliness as a disease, and severe loneliness is accompanied with some diseases or manifested through disease behavior [2]. For example, accompanying symptoms include decreased quality of life, increased risk of mental disorders (such as depression, anxiety, and physical diseases like hypertension), and increased correlation with mortality and suicidal ideation [3, 4]. Clinical studies indicated that decreased adaptability due to increased body vulnerability is a significant cause of loneliness among older adults individuals [5]. Therefore, seeking ways to enhance the physiological reserves and body resistance of the older adults is of great importance in reducing their loneliness and promoting successful aging.
Loneliness is divided into social loneliness and emotional loneliness, which can occur simultaneously or independently [2]. Social loneliness arises from social isolation, resulting from a lack of connection and integration into society [6–8]. Studies have shown that the activity of daily living (ADL) in older adults individuals has a significant influence on their experience of loneliness [9]. Insufficient ADL can reduce their social interactions and limit participation in social activities, leading to an increase in loneliness due to social isolation [10, 11].
Our literature review revealed that frailty was one of the main factors affecting the daily activities of older adults individuals [12]. Frailty refers to a dynamic state in which individual body vulnerability and disease susceptibility risk increase due to the decline in physiological reserves and body resistance [13]. The development of frailty may lead to decreased stress capacity and poor health status. Meanwhile, frailty is also a reversible process in the early stage, and early identification and active intervention can delay older adults people from entering a state of disability [14]. The World Health Organization notes that frailty is a better indicator of the quality of life and functional status of older adults people than age [15].
The existing frailty theories are mostly based on medical models, in which the integral conceptual model of frailty lays an important theoretical foundation for the study of frailty in sociology and psychology [16]. The model considers frailty to involve physical, social, and psychological frailty, and their combined effect is a significant cause of various adverse outcomes (such as disability and mental abnormalities). Through interventions, frailty can be corrected to varying degrees. These interventions include physical exercise, nutrition, cognitive training, psychological support, and socio-economic support [17–19]. Physical exercise is the preferred option for preventing and treating frailty in the older adults [20]. The combination of resistance and balance exercise in physical exercise forms can improve the lower limb muscle strength and balance ability of frail older adults people. In addition, moderate aerobic exercise can alleviate negative emotions in the older adults, enhance self-confidence and self-efficacy, and help reduce psychological frailty in the older adults [21]. Physical exercise can also create opportunities and space for older adults people to socialize and improve interpersonal relationships, reduce social frailty, and promote mental health [22, 23].
Research has found a negative correlation between frailty and physical exercise but a positive correlation between frailty and loneliness in the older adults [24]. However, there is no research to explain the relationship between physical exercise and frailty and loneliness of the older adults from the perspective of frailty intervention through physical exercise. On the basis of previous research, this article proposed that older adults people in a state of physiological frailty may develop a risk of negative self-awareness and separation of interpersonal relationships due to excessive concern about body vulnerability; older adults people with social frailty often lack interpersonal communication and social support; psychological frailty is related to the negative emotions of the older adults, and the combined effects of these risk factors will lead to the occurrence of depression, loneliness, and other adverse consequences in the older adults [25]. Physical exercise, as a means of health promotion intervention, can enhance the individual’s physiological reserves and body resistance, reduce the occurrence of physical and psychological frailty, increase the interpersonal communication of the older adults, and relieve their social frailty and the resulting loneliness [26, 27]. Therefore, this study proposes the following hypothesis: physical exercise significantly negatively predicts loneliness in the older adults through the mediating effect of frailty.
Depression and loneliness often occur simultaneously, with a correlation coefficient between 0.4 and 0.6 [28, 29]. The core symptoms of depression include low mood, loss of pleasure, and reduced energy [30]. Depression in the older adults is due to physiological and social causes. With older age, a series of changes in biochemical substances and brain structure can occur in the older adults. This process is often accompanied with changes in individual personality characteristics, such as isolation, passivity, dependence, and stubbornness, which make the older adults population’s ability to withstand setbacks decrease significantly, leading to the occurrence of depression and other psychological problems [31, 32]. Stress and challenges in the living environment are also important factors causing depression in the older adults. Given that the older adults are more likely to experience the impact of life events such as retirement, empty nest, loss of friends and relatives, disabilities, and illnesses, their social relationships are reduced and they are more likely to feel emotionally lonely than their counterparts [33]. Many studies have reported that a high depressive state is an important factor leading to loneliness in the older adults, but there are also views that loneliness may exacerbate the depressive symptoms in the older adults [34, 35]. Therefore, depression and loneliness in the older adults may have an interactive relationship.
Good physical exercise habits have a protective effect on individuals facing negative emotions such as depression and anxiety [36, 37]. The cognitive behavior hypothesis suggests that physical exercise can promote positive thinking and emotions in individuals, and it has a resistance effect on negative emotions such as depression, loneliness, and anxiety. The cardiovascular hypothesis suggests that consistently engaging in physical exercise can improve the physical abilities and cardiovascular function of the older adults, thereby reducing negative emotions such as depression caused by physiological aging [38, 39]. The social interaction hypothesis suggests that communicating and interacting with friends or family during physical exercise can help reduce the occurrence of depression and loneliness [40]. In summary, there is extensive direct physiological and psychological evidence for the effect of physical exercise on depression. With the release of negative emotions and the formation of positive perception, the perceived loneliness of the older adults is believed to be reduced accordingly. Therefore, hypothesis H2 is proposed: Physical exercise significantly negatively predicts loneliness in the older adults through the mediating effect of depression.
Frailty and depression may share similar physiological mechanisms [41]. Studies have shown that physical frailty and depressed older adults people exhibit elevated levels of inflammatory cytokines such as changes in interleukin-6 (IL-6), C-reactive protein (CRP), and tumor necrosis factor-α (TNF-α) [42, 43]. Additionally, vascular depression is characterized by slowness, fatigue, and muscle weakness, which are symptoms that are considered early warning signs of frailty [44–46]. Physical exercise and physical activity have been proven to be key drivers affecting chronic inflammation. Even in the oldest older adults, physical exercise can improve physiological mechanisms such as mitochondrial dysfunction, cardiovascular disease, myokine secretion, autophagy, oxidative damage, and IGF-1 signaling pathway, thereby improving physical and psychological levels and reducing the burden of non-communicable diseases and premature overall mortality [47, 48].
Previous studies reported that frailty is an indicator for predicting depression in the older adults, as depression in the older adults is typically characterized by a relative lack of emotional symptoms and more autonomic nervous symptoms, known as "depression without sadness" or "implicit depression" [49, 50]. In addition, the theory of frailty identity crisis suggests that frail older adults individuals may perceive a transition from complete self-care to limited self-care abilities [51], such as long-term physical dysfunction and weight loss. These changes may influence the older adults’s perception of their own health status, becoming a source of stress for them and potentially leading to the onset of depression.
In summary, the sequential effect of frailty and depression may have adverse effects on the social experience and social activity ability of the older adults, thereby increasing their risk of social isolation and loneliness. Physical exercise, as a means of health promotion intervention, can improve the social frailty of the older adults through interpersonal interactions during physical activities, alleviate the worries and negative emotions arising from physiological frailty, reduce depression levels, and holistically improve their loneliness [52]. Therefore, we propose Hypothesis 3: Physical exercise significantly negatively predicts older adults loneliness through the sequential mediating roles of frailty and depression.