Effect of contributing factors on the incidence of non-communicable diseases among adults with common mental health disorders: a systematic review

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Introduction

Worldwide, about 74% of all deaths are attributed to non-communicable diseases (NCDs), of which more than three-quarters occur in low- and middle-income countries (LMICs). 1 NCD prevention and control has mainly focused on the four conditions responsible for the greatest number of deaths. Known as the ‘Big Four’, cardiovascular diseases (CVDs), followed by cancers, chronic respiratory diseases and diabetes, comprise the highest rates of mortality as they contribute to 33.3 million deaths globally. These NCDs are all driven by modifiable behaviours, namely, unhealthy diets, physical inactivity, tobacco smoking and harmful use of alcohol. 2,3 These behavioural determinants are widely recognised as the main entry points for the prevention and control of NCDs. 4

According to the American Psychiatric Association, mental disorders, also called mental illnesses, are classified as any condition characterised by cognitive and emotional disturbances, abnormal behaviours, impaired functioning or any combination of these. 5 The World Health Organization (WHO) identifies depressive disorders and anxiety disorders, including post-traumatic stress disorder (PTSD), as the two main diagnostic categories comprising common mental health disorders. 6 In 2012, the WHO identified three different categories of risks affecting an individual’s mental health, namely, individual attributes and behaviours, social and economic circumstances and environmental factors. 7 Although the WHO called these factors ‘risks to mental health’, they are similar to the social determinants of mental health that were proposed later by Marmot et al. in 2014. 8 At the individual and behavioural level, poor dietary habits9 and smoking10 were found to be associated with depression. In addition, socioeconomic conditions, such as unemployment and financial hardships, predispose individuals to mental illnesses. 7 According to the WHO, the poorest segment of society is often the most susceptible to poor mental health. 11 The stress that accompanies absolute poverty is a powerful risk factor for mental disorders such as depression and substance dependence. 12

Common mental illnesses and NCDs often coexist. Systematic reviews linked cancer with anxiety disorders13,14 and PTSD. 15 Meta-analyses linked diabetes with a range of mental disorders, including depression and PTSD. 16 Across countries, the odds ratios for the association of heart disease with mood disorders, anxiety disorders and alcohol dependence were found to be 2.1, 2.2 and 1.4, respectively. 17

The exact mechanism linking common mental disorders (CMDs) to NCDs remains to be clearly understood. It has been suggested that they influence each other directly through physiological systems (e.g. neuroendocrine system). For instance, depression is shown to provoke a cascade of events in the hypothalamic–pituitary–adrenal axis, which is normally stimulated in response to stress, leading to insulin resistance and eventually to type 2 diabetes. 18 On the other hand, there is evidence associating individual, social and environmental factors, known to be determinants of mental health, with NCDs. Individual factors that were mentioned earlier to be associated with depression, namely, smoking and poor dietary habits, are known to be the main entry points for the prevention and control of NCDs. 4 Poverty, a social determinant of mental health,19 is shown to exert a similar effect on NCD incidence. In a large-scale population cohort in the UK, it was found that the segment of the population who were more socioeconomically deprived had an increased risk for developing heart disease (incidence rate ratio 1.61, 95% CI 1.58 to 1.64) and did so earlier in life than those from the most affluent group (adjusted difference −3.51 years, 95% CI −3.77 to −3.25). Factors such as health inequity, poor access to basic services such as clean water and electricity, political instability, discrimination and armed conflicts are just examples of environmental determinants of mental health7 that were also found to be associated with higher rates of diabetes and CVDs20 and with an increased incidence of cancer. 21

Physical health outcomes for people living with a chronic physical condition are worse in the presence of a mental illness, including a reduced quality of life and a shortened lifespan. Studies show that mental illness contributes to an increased risk of mortality from CVDs and cancer. 22–24 According to the World Mental Health Surveys, an 8–12% increase in mortality, due to smoking, diabetes, history of myocardial infarction and hypertension, was found among people with common mental health disorders. 17 Figures of myocardial infarctions, a NCD complication, are higher among individuals subjected to stressful situations such as armed conflicts or natural disasters compared to pre-emergency figures. 25

Despite the supportive evidence that links CMDs and their determinants to NCDs, and despite the fact that physical health outcomes get worse for people living with a NCD in the presence of a mental illness, the connection between them is usually overlooked in clinical settings and, to a great extent, in country-level policy-making. This has been attributed to factors such as limited government capacities for policy development, weak health systems and insufficient finances, among other factors. 26,27 In 2005, member states in the WHO European Region endorsed the ‘no health without mental health’ statement owing to mental health’s contribution to years of disability. 28 The WHO’s Global Action Plan (2013–20), to reduce the global burden of non-communicable illnesses and preventable mortality, includes mental disorder prevention and control objectives. 29 However, mental and physical disease prevention initiatives remain essentially separate and independent from one another. 30 The current systematic review aims to address the link between mental illness and NCDs by exploring the aforementioned individual, social and environmental determinants associated with both conditions. It aims to answer the research question: ‘What are the individual, social and environmental determinants that lead to the development of NCDs {CVD, cancers, respiratory diseases [including chronic obstructive pulmonary disease (COPD)] and diabetes mellitus [DM]} among patients with common mental health disorders (depression and anxiety disorders including PTSD)?’ Considering these overlapping determinants may be a practical and effective way of providing integrated prevention programmes to reduce the burden of these conditions as well as the associated disability. This review is part of an entire PhD research work on the social determinants of mental health and how they affect population health.

Methods

This review was conducted in accordance with a published protocol prepared by the authors and published on PROSPERO with a registration number CRD42019157800. 31

The review follows the latest Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 reporting guidelines. 32

Results

The study selection process is summarised in Figure 1, following PRISMA reporting guidelines. 32 More than 19,000 studies were found in the initial search. A total of 11 studies from 3 countries were included in the final review, with the majority (n = 9) conducted in the USA. All studies were longitudinal prospective studies, presenting outcomes for a total of 71,896 people. Four studies determined the association between depression and cancer, two studies depression with CVD and one study depression with diabetes. One study determined the association between depression and anxiety together with diabetes and another with CVD. No studies were found examining association between CMDs and COPD per se, but two studies addressing asthma were included (one considering depression and the other PTSD). A summary of the included studies is presented in Table 6.

FIGURE 1.

PRISMA flow diagram.

Discussion

Findings from the systematic review suggest that the chronicity of depression and PTSD could be associated with increased risk for cancer and asthma onset, respectively, and therefore treating depression and PTSD could prevent progression into the development of the latter NCDs. Moreover, the high association between depression and CHD suggests the need to target depression as primary prevention to prevent the progression into CHD. On the other hand, factors such as educational attainment, number of cigarettes smoked, female gender, older age, elevated BMI and ethnicity may contribute to an elevated risk for developing NCDs among adults with depression and/or PTSD. Therefore, in order to reduce the incidence of NCDs, mental disorders should be identified and treated, and health and prevention strategies focusing primarily on high-risk groups, where factors listed above are present, can and should be adopted.

Our findings are consistent with cohort studies where older age,61 low educational attainment,62 higher BMI63 and smoking status61 were found to be associated with CVD incidence. In the current review, the study by O’Neil has identified depression as an independent risk factor for CVD onset,46 which is also suggested by other longitudinal studies. 64–66 We, therefore, recommend to include depression as a target for primary prevention of CVD.

Ethnicity was found to be associated with the onset of diabetes. Factors identified earlier as being related to CVD incidence, except smoking, were also reported in this review to be associated with diabetes onset. This is in agreement with findings from the literature that linked older age, higher BMI67,68 and low educational attainment69 with incident diabetes. Our results regarding the type of mental disorder associated with type 2 diabetes incidence contradict findings of the literature, however. While numerous research has demonstrated a bidirectional association between depression and type 2 diabetes,70,71 our review indicates that PTSD, and not depression, was found to increase incident diabetes when the effects of depression and PTSD were studied together. A study by Scherrer et al. found that reducing PTSD symptoms lowered the risk of type 2 diabetes72 among participants with PTSD. In the same study, depression remission contributed to lower diabetes onset among participants diagnosed with comorbid PTSD and depression, and not with depression alone. Therefore, there may be further benefit from reducing PTSD symptoms to lower type 2 diabetes risk in populations that are exposed to potentially traumatic events, such as veterans. For other populations exposed to such events, for example, refugees and the general population affected by adverse events, studies are lacking. Therefore, it is difficult at this point to generalise a recommendation as to target PTSD for the primary prevention of type 2 diabetes.

In relation to cancer incidence, three out of four studies demonstrated an association between depression and incident cancer. This is in line with two previous meta-analyses. 73,74 Although not all studies included in this review showed a significant impact of depression on cancer initiation, some trends emerged regarding the extended duration of depression having an important role in a slowly developing health condition such as cancer. This view is shared by Garssen75 who suggested in 2004 specific risk factors among this population, namely, smoking, BMI, age and alcohol intake. This research also found that behavioural factors, namely smoking76 and alcohol intake,77 were associated with increased cancer onset.

Our review showed a significant association between depression and PTSD with incident asthma. In 2005, a review that explored the interaction between depression and asthma as comorbidities suggested that the respiratory tract (mainly the airways) is greatly reactive to the psychological changes of individuals. 78 The findings of our study suggest that depression could lead to asthma onset via this mechanism. Regarding PTSD, our findings are consistent with those in the literature whereby PTSD was found to be positively associated with asthma onset. 79,80 Our study identified risk factors such as female gender, increased BMI and low educational attainment as being related to asthma onset. Therefore, special attention should be given to such population groups in clinical practice and public health interventions that aim to reduce the incidence of asthma.

In addition to the complexity in conducting longitudinal studies to test the causal association between common mental health disorders and NCDs, there appears to be a challenge in relation to the reporting of health conditions. Epidemiological investigations and population-based studies mainly rely on self-reports to obtain information on health outcomes, such as diabetes, given the high cost and low efficiency of clinical diagnosis. 81,82 A number of studies assessing the accuracy of self-reported data, using test characteristics such as sensitivity (the fraction of people who fit the medical criteria for the outcome and self-report a diagnosis, i.e. true positive) and specificity (the fraction of people who have normal measures and do not report a diagnosis, i.e. true negative), gave mixed results. 83–85 Compared to biochemical measurements, self-reported data on diabetes among Chinese adults ≥ 45 years showed low sensitivity and a moderate overall agreement. Factors associated with accurate self-reported data were female sex, older age, high socioeconomic status, better educational attainment and recent utilisation of medical services. 85 Goldman et al. attributed the lack of accuracy in self-reports to several factors, such as undiagnosed diseases, inability to recall, unwillingness to disclose medical information and lack of accessibility to health services. 84 This could further be exacerbated among individuals facing psychological stressors. Depression, for instance, was found to be associated with barriers deterring individuals from utilising medical services. Factors such as mistrust in health providers and the health system,86 loss of income87 and low motivation to seek medical care88 may all result in underdiagnoses leading to under-reporting. Therefore, the above-mentioned issues that feed into under-reporting should be taken into account when conducting future research on this topic.

Another common challenge pertains to the identification of cases of common mental health disorders. There is no single diagnostic tool for such conditions. For instance, several valid and reliable instruments are used by primary care clinicians and researchers to identify cases of depression and anxiety. 89–91 The selection of the appropriate tool depends on factors such as feasibility, administration time and personnel and interest in measuring the severity of the condition or monitoring the response to therapy. These tools tend to under-report cases compared with the clinical psychiatric examination and hinder the ability to compare results from different studies.

Strengths and limitations

This systematic review provides additional evidence on how common mental health disorders are linked to the onset of the four most common NCDs. The particularity of the evidence that emerged has to do with the type of individual studies: the included longitudinal studies provide valuable evidence for establishing a causal relationship between common mental health disorders and NCDs in this specific direction. However, it should be noted that the utilisation of a longitudinal research design does not completely preclude the possibility of other explanations, such as the influence of third variables. Consequently, it does not definitively demonstrate a causal relationship or enable the formulation of causal claims. 92 Other limitations of the current review should be acknowledged. First, our review did not include studies from the grey literature and was limited to literature published in English language only. Second, measures of depression and anxiety varied among studies, with the majority relying on self-report using different screening tools with various cut-off points. This might have caused a misclassification of exposure and, hence, lessened the association between mental disorders and the incidence of NCDs. Third, measures of covariates were different across studies which makes it difficult to compare their effects on the onset of diseases. And finally, the majority of included studies limited their assessment of mental conditions to one point in time which might have been a transient episode. The association found between chronic depression/PTSD and incident NCDs emphasised the importance of assessing the chronicity of mental illnesses and their impact on health.

Conclusion

This study contributed to the existing global body of evidence regarding the factors that are directly linked to the onset of NCDs. As a result, policy-makers and practitioners can utilise the findings to customise interventions that target these specific factors, with the aim of interrupting the cycle between mental health disorders and other NCDs. Interventions need to target those with common mental health disorders and take into account: age (older age groups are more at risk), sex (females are more at risk), educational level (individuals with ˂12 years of education are more at risk) and smoking status (smokers are more at risk). Furthermore, there is an urgent need to conduct studies to understand the linkages between mental health and NCDs in LMICs, given the absence of evidence in those settings and the increasing burden of NCDs and mental health conditions they are experiencing. Finally, research to evaluate public health interventions and to assess the responsiveness of healthcare systems (mainly at the primary care level) is essential to alleviate the burden of those conditions at population level. In summary, this review adds to the body of evidence that prioritising the social determinants of mental health is crucial for enhancing public health outcomes through the implementation of a preventative health promotion strategy. Through this approach, it becomes possible to address the systemic determinants of health that are often overlooked, which can contribute to the development of disease. 93

Reporting community engagement and involvement: Since this was a review article, there was no patient and public involvement in this study.

Equality, diversity and inclusion: Research around mental and physical health multimorbidity is limited, despite the fact that the global burden of mental illnesses and NCDs is high. This review contributes to the existing global body of evidence regarding the factors that are directly linked to the onset of NCDs. Findings can be utilised to inform interventions for high-risk populations worldwide to prevent development of NCDs.

Additional information

List of abbreviations

BMI

body mass index

CASP

Critical Appraisal Skills Programme

CHD

coronary heart disease

CIDI

Composite International Diagnostic Interview

CMD

common mental disorder

COPD

chronic obstructive pulmonary disease

CVD

cardiovascular disease

DM

diabetes mellitus

ICD

International Classification of Disease

IHD

ischaemic heart disease

MeSH

medical subject heading

NCD

non-communicable disease

PHQ-9

Patient Health Questionnaire-9 item

PICOS

population, intervention/exposure, control, outcome and study design

PRISMA

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

PTSD

post-traumatic stress disorder

SCID

Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders

WHO

World Health Organization

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Appendix 1 MEDLINE search strategy

Appendix 2 Data extraction sheet template