Mental disorders are among the leading causes of mental illness and health-related burdens worldwide, with depression being among the most prevalent (GBD, 2022). Holding a 5% prevalence in adults globally, depressive disorders affect approximately 300 million people (WHO, 2017). The term ‘depression’, however, suffers from a lack of conceptual clarity; the World Health Organisation (WHO, 2021) and other major healthcare providers, such as the National Health Service (NHS, 2019), separate the term depression into multiple sub-categories, including clinical depression. Despite its pervasiveness, clarity is often needed to distinguish what depression is and what depression is not.
Within this review of contemporary literature, the mentioning of depression will be a direct referral to Major Depressive Disorder (MDD), also known as clinical depression. MDD is a depressive disorder that derives from research in clinical diagnosis and psychopathology (Fisher, Gonzalez & Polonsky, 2014). In accordance with the current DSM-5 symptom inclusion criteria (American Psychiatric Association, 2013), MDD is characterised by the presence of five or more psycho-physiological changes from the following list of potential symptoms: depressed mood, decreased interest or pleasure in most or all activities in the day, decreased motivation, anhedonia, appetite and/or weight gain, sleep, psychomotor changes, fatigue, guilt or worthlessness, impaired thinking, concentration or decision making and suicidality. These changes are required to last for a minimum of two weeks and must cause impairment in functioning and/or severe emotional distress for a diagnosis of MDD to be made. Although the diagnosis for MDD is symptom-based, popular screening tools such as the Patient Health Questionnaire-2 (PHQ-2), the Patient Health Questionnaire-9 or the International Classification of Diseases (ICD-10) can be used to assess whether individuals meet the criteria for MDD (Kroenke, Spitzer & Williams, 2001). The complex, and often changing, pathophysiological understanding of the neural causes for MDD means that the diagnosis of depression is often based on clinical manifestations, yet with limited objective evidence (Zhang et al., 2018). This subjectivity and broadness of MDD diagnosis mean there is often a challenge, for both general practitioners and the general public, to differentiate between emotional distress and clinical depression (Geraghty, et al., 2019); this emphasises the need to detail what depression is not.
MDD is not undiagnosed feelings of depression (APA, 2000; Gelenberg, 2010; NICE, 2022). Although feelings of depression prior to diagnosis may be MDD or may result in an eventual positive diagnosis for MDD, research suggests that self-report items may overestimate the prevalence of depression (Arias-de le Torre et al., 2020). Experiencing temporary symptoms of MDD irregularly, would not fall under the classification for diagnosis of depression under the DSM-5’s symptom inclusion criteria. The NICE guideline (2022), targeting the treatment and management of depression in adults, emphasizes the importance of the mental health assessment being performed by a mental health practitioner, competent in depression identification. This report identifies that people experience, describe and label their experiences of depression in bespoke ways, adhering to the modern argument that labeling of depression disorders both in literature and within real-world application is often difficult to interpret for validity.
Within the context of depression disorder literature, there is a substantial amount of contemporary literature suggesting that those with MDD experience a lower quality of physical health, such as cardiovascular disease (Correll et al., 2017) and premature mortality rates (Charlson et al., 2014), in comparison to the general population. Despite such effects having transparent links to physical activity and exercise, the use of antidepressants and psychotherapies are still the first choice treatment options for those with MDD (Marasine et al., 2021). However, depression therapies do not take a one-size-fits-all approach and, as a consequence, recommended therapies may not be an effective treatment for some individuals. Research proposes that up to 66% of individuals will not respond to the initial antidepressant prescribed, whereas up to 33% may not respond after multiple attempted interventions (Cain, 2007; Berlim, Fleck & Turecki, 2008). These limitations outline that there is a need for further exploration of alternative interventions, aimed to alleviate the onset and maturation of cardiovascular diseases and depressive symptoms.
The exploration of physical activity and exercise-based interventions has received considerable recognition in recent years, as a consequence of newly emerging findings surrounding the relationship between both factors. Where physical inactivity possesses an elevated risk of depression (Conn, 2010, Schuch, 2019), regular involvement in physical activity has been found to be an effective treatment for mood disorders (Harvey et al., 2018; Schuch et al., 2018). Numerous meta-analyses have explored the efficacy of physical activity interventions and their respective role on depression, with Rebar et al. (2015) concluding that a moderate effect was present regarding exercise-reduced depression. Participation in physical activity, however, can be challenging for those with MDD for numerous reasons.
This review of literature will look to explore the relationship between depression and physical activity within three main concepts: 1) physiological and neurological responses to physical activity, 2) eating habits, physical inactivity and weight gain, and 3) self-esteem and the role of physical activity. Using contemporary literature as a guideline, this review will look to detail how physical activity can benefit individuals with MDD.
Physical activity has been successfully incorporated into the prevention, treatment and rehabilitation of many chronic diseases, with physical activity possessing comparable efficiency in comparison to other depression-reduction options, such as drug therapy and other psychological interventions (Harvey et al., 2018). The benefits of physical activity are not simply identifiable through observation but from investigations into the effect of physical activity on physiological and neurological functioning.
Physical activity supplies the brain with more blood flow from an increase in heart rate, therefore providing more oxygen to the brain (Tomoto et al., 2021). Multiple studies have found that individuals with more well-oxygenated brains are more likely to display reduced symptoms of depression (CADTH, 2014). Hypoxia, being low levels of oxygen within blood tissues, may also explain the relationship between higher rates of depression in those living at altitude (Kanekar et al., 2015). Additionally, Marchi et al. (2020), propose that oxygen-deprived breathing within sedentary hours was associated with atrophy of cortical and subcortical areas known for high sensitivity of oxygen supply – noticeably the hippocampus and the amygdala – two subcortical limbic brain regions commonly associated with depression (Pandya et al., 2012).
In addition to exercise improving the quality of oxygen intake in sedentary hours, physical activity has also been found to increase cognitive function and account for the changes in brain growth factors via the presence of Brain-Derived Neurotrophic Factor (BDNF) pathways (Gourgouvelis et al., 2018; Sleiman et al, 2016). BDNF pathways function collaboratively with neurological growth factors, such as IGF-1 and VEGF (Maass et al., 2016). Where the interaction between BDNF and IGF-1 is to induce the maturation of new neurons, also known as neurogenesis (Nieto-Esteves, Defterali & Vicario-Abejon, 2016), the association with VEGF is to stimulate the growth of new blood cells, being angiogenesis (Shibuya, 2011). Both of these growth factors increase in expression levels post-physical activity (Llorens-Martin, Torres-Aleman & Trejo, 2010). Although changes to BDNF levels are highlighted throughout various cortical regions, BDNF growth and activity are most noticeable within the hippocampus (Liu & Nusslock, 2018; Vitaliano et al., 2022). Studies have outlined that despite BDNF levels in humans elevating in response to acute physical activity with moderate effect (Devenney et al., 2019), participation in regular, chronic exercise interventions significantly increased baseline BDNF levels (Forti et al., 2015; Jimenez-Maldonado et al., 2018). Various studies exploring the role of physical activity have found that mice and rats with depression demonstrate a higher production of BDNF levels and hippocampal neurogenesis when participating in physical activity (Duman, 2008; Sohroforouzani et al., 2022). These studies concluded that elevated levels of BDNF play a role in reducing depressive symptoms.
Multiple studies have outlined that regular participation in physical activity has the potential to promote positive emotions (Li et al., 2022), suppress negative feelings (Li, Yu & Yang, 2021) and induce a euphoric state, known as a ‘runner’s high’ sensation (Sachs, 1980). These ‘feel good’ effects were, until recently, postulated to derive from the upregulation of endorphins – the body’s endogenous opiates. However, recent developments have outlined that, as endorphin molecules are too large to cross the blood-brain barrier, therefore being unable to act on brain receptors (Schoenfeld & Swanson, 2021), another molecule must be responsible for administering the euphoric feelings of the runner’s high, experienced whilst partaking in aerobic exercise. This attention has now been turned to endogenous endocannabinoids (Dietrich, 2004; Heijnen et al., 2016). Endocannabinoids are small enough to diffuse through the blood-brain barrier to act on cannabinoid receptors (Zou & Kumar, 2018) – the same receptor that THC within marijuana binds to. In another study conducted on mice, Fuss et al., (2015) found that aerobic exercise led to possessing higher levels of endocannabinoids, decreased anxious behaviour and higher pain tolerance. Findings like these suggest that participation in physical activity does not simply possess an ability to positively impact symptoms of MDD but has a literal effect on long-term interest and maintenance in exercise. Developing neurological links to an interest in physical activity participation may help battle the ongoing challenge between depression and weight gain.
Significant research has proposed that fluctuations in weight are regularly apparent in those with MDD (Felton et al., 2010), with the DSM-5 noting weight gain or weight loss as common symptoms of depression (American Psychiatric Association, 2013). Despite varying factors contributing towards weight changes in those with depression, including the role of antidepressant medication often including weight gain side effects, the relationship between the effects of eating habits, physical inactivity and consequential weight gain has been heavily discussed.
Food and mood often go together and emotional eating has been proposed to be one mechanism linking depression and the subsequent development of weight gain (Konttinen et al., 2019). Abnormal eating behaviours, such as increased food intake in response to depression-induced emotions, are often present in those with MDD (van Strien et al., 2016a). Results from a large study investigating the relationship between daily adversity and eating behaviour found that those who experience daily hassles consumed significantly more high-fat or sugar-heavy foods (O’Connor et al., 2008). These findings were elaborated by a 5-year longitudinal study in the Netherlands, which concluded that depressive symptoms were related to higher emotional eating, which, in turn, increased the likelihood of a lack of physical activity and a greater increase in weight gain (van Strien et al., 2016b). However, a large amount of research does not simply associate depression as the catalyst for obesity but outlines the relationship between depression and weight gain to be bidirectional. Noticeably, Luppino et al., (2010) found that individuals with depression were 58% more likely to become obese than non-depressed individuals, whereas obese individuals had a 55% increased risk of developing depression, in comparison to those of normal weight. Although the side effects of antidepressant medications are likely to demonstrate a strong temporal association with weight gain (Gafoor, Booth & Guilliford, 2018), results like these do not explain what it is these individuals are not doing that can influence obesity levels and the effect this lack of action may have on neurological and psychosocial wellbeing.
Where various factors can contribute towards weight gain in those with MDD, such as the side effects of antidepressant medication and/or an inability to adhere to dietary recommendations, amongst other social and environmental factors, recent literature has outlined that higher physical activity levels can assist in the prevention and treatment of depression.
The lack of adherence to acute physical activity among individuals with MDD, let alone chronic exercise involvement, is alarming and contributes to the high rate of sedentary behaviour with depression (Craft & Perna, 2004; Stranahan, Lee & Mattson, 2008). This lack of physical activity participation may be impacted by depression symptomatology, such as a loss of energy (Achttien et al., 2019). A 2019 study exploring the role of directional associations among motor activity, mood and sleep found that a unidirectional association was present between physical activity and mood level, whereas a bidirectional association was present between physical activity and subjective energy levels (Merikangas et al., 2019). These findings propose that interventions targeting physical activity and energy levels may be more effective than current non-exercise-related interventions aiming to reduce depressed mood or depressive symptoms. A recent meta-analysis of prospective cohort studies also found that lower odds of depression were more likely to be present in those with higher levels of exercise (Schuch et al., 2018). These results are supported by Mammen and Faulkner (2013), who explored physical activity’s relationship with the onset of depression; findings concluded that exercise, regardless of intensity level, was likely to prevent subsequent depression.
Contemporary research on depression and physical activity proposes that although symptoms of depression, such as reduced motivation or amotivation, can affect one’s willingness to participate in physical activity (Cecchini et al., 2017), those with depression must be careful not to fall into the ‘inactivity trap’ (Elfrey & Ziegelstein, 2009), as this may lead to physical inactivity feeding into depressive symptoms, in a hard to escape cycle.
Negative self-representations, such as feelings of worthlessness and incapability, often confer vulnerability to MDD (APA, 2013; Beck, 2009). These self-representations contribute to the formation of “certain attitudes and perception of oneself”, known as self-esteem (Mruk, 2006). Possessing self-esteem is a fundamental psychological factor contributing to quality of life, physical and psychological well-being (Evans, 1997); it is classified as a core indicator of emotional stability and the ability to adjust to daily stressors and life demands (Sonstroem, 1997). Self-esteem literature, however, has only recently begun to provide significant research reinforcing the relationship between physical activity and self-esteem, in support of Fox and Corbin’s (1989) foundational model of self-perceptive factors.
According to research, depression and self-esteem play an intimate role in affecting one another (Korrelboom et al., 2012). Noticeably, Kayani et al. (2018) found that physical activity was identified as being a significant predictor of the enhancement of self-esteem, which, in turn, affected the likelihood of decreased depressive symptoms (Ekeland et al., 2005). Fox and Corbin’s (1989) Physical Self-Perception Profile (PSPP), detailing a three-tier hierarchical model of self-perceptions, provides a foundational insight into the relationship between physical activity and self-esteem. This model outlines that as the body acts as the primary means by which we communicate, this results in an individual’s physical self-worth contributing significantly to one’s global self-esteem. A 2019 study identified 13 systematic reviews and meta-analyses, examining the associations between depression, anxiety and self-esteem in children and adolescents (Dale et al., 2019). Results found that physical activity had positive psychological outcomes for participants, specifically in the deterioration of depressive symptoms and improvements in physical self-worth, a self-esteem domain listed in Fox and Corbin’s (1989) PSPP. This literature was supported by Sani et al. (2016), who proposed that chronic exercise should be encouraged as an intervention aimed at increasing physical activity participation and self-esteem. Increased daily sedentary hours have also been associated with poorer mental health and a decrease in self-esteem (Hoare et al., 2016), further suggesting that physical activity may be a suitable intervention for individuals with MDD who are also experiencing low self-esteem.
Physical activity participation provides those experiencing low self-esteem an opportunity to do something for themselves, consciously challenging their internal perceptions that “I am worthless” or “I am incapable”. Direct physical effort and application towards an individual’s physical self-worth may lead to an increase in their global self-esteem.
This literature review aimed to explore the relationship between physical activity and depression through three main sub-themes: 1) physiological and neurological responses to physical activity, 2) eating habits, physical inactivity and weight gain, and 3) self-esteem and physical activity. The research highlighted within this literature review has provided significant support for the benefits of physical activity for those with MDD.
Whilst exploring the physiological and neurological impact of physical activity, various research provided insight into the benefits of exercise on human’s internal functioning. Increased rates of physical activity supply the blood, thus, the brain with more oxygen (Tomoto et al., 2021), leading to a decrease in depressive symptoms (CADTH, 2014). Research surrounding the role of BDNF pathways (Duman, 2008; Sohroforouzani et al., 2022) outline that these neurological pathways contribute towards reducing depressive symptoms, whilst recent developments in ‘runner’s high’ research have proposed that endocannabinoids may cause physical activity to be just as addictive as marijuana (Fuss et al., 2015). Although the discussed BDNF studies present evidence of the critical importance of these pathways, a major concern is that both studies discussed these effects on rats and mice, not humans. Animal studies may lack reliability and transferability to humans because of their failure to accurately mimic human responses – a potential inadequacy in physical activity and neurological literature (Bracken, 2009).
Positive correlations were displayed between eating habits, a lack of physical activity and depressive symptoms (van Strien et al., 2016a; van Strien et al., 2016b), however, a limitation of these studies was the use of self-report tools to distinguish measurements of height, weight, emotional eating and depressive symptoms. When inputting information into self-report tools for a research study, it is realistic to expect participants to display social desirability bias. If participants were to under- or over-report their depressive symptoms (Jang et al., 2010), body weight and height (Larsen et al., 2008), and levels of emotional eating (van Strien, 1995), these have the potential to significantly alter the results of these studies, therefore impacting research applicability into recommendations for physical activity intervention proposals. This is supported by Vartanian et al. (2004), who found that, when provided a self-report measure, dieters and non-dieters provided inaccurate and untruthful responses about their actual weight.
Recent findings from a large systematic review exploring the role of depression and self-esteem found that physical activity helped to reduce depressive symptoms, whilst increasing physical self-worth (Dale et al., 2019). This significant finding provides support to Fox and Corbin’s (1989) PSPP, which outlines multiple contributing factors to an individual’s levels of self-esteem. These findings, however, are targeted at discovering a deeper understanding of the effects of self-esteem, physical activity and depression within those aged 5-17 years old and do not provide an insight into these effects for an adult population. This is a limitation, as although adolescents may possess the highest prevalence of MDD (14.4%), this fails to acknowledge the age group experiencing the second highest rates of symptoms of MDD (13.8%), being adults aged 18-25 years old (SAMHSA, 2019).
Various factors contributing to physical activity and depressive symptoms were discussed within this review, however, it must be noted that there is a gaping hole within this topic area surrounding the inaccessibility of participation. If access to participation in these studies is limited for those not categorised as western, educated, industrialised, rich and democratic, then there are surely demographics that are not being heard, therefore not being helped. This lack of representation is evident through the sparse amount of depression and physical activity research being conducted in black populations, particularly on Black Brits. Considering, non-Caucasian individuals are more likely to suffer from prolonged, chronic and debilitating depression (Bailey et al., 2019), a recommendation for future research would be to investigate the relationship between physical activity and MDD in the British Black demographic.
In conclusion, this review of literature detailed a neurological cause-and-effect relationship between physical activity reducing depressive symptoms, whilst discussing the relationships between physical activity, body image changes and consequential self-representations. If individuals with MDD are struggling to see the benefits of antidepressants or psychotherapies, then physical activity interventions may be the perfect remedy for MDD remission.
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