A growing body of evidence has demonstrated the bidirectional relationships between SMD, including moderate to severe depression, bipolar disorder, as well as schizophrenia and other psychotic disorders, and physical health conditions including both non-communicable and infectious diseases.
SMD and non-communicable diseases (NCDs)
SMD and the major NCDs, including cardiovascular diseases, diabetes, respiratory illnesses, and cancers, are related in complex ways. From an epidemiological standpoint, mental disorder itself is a well-known risk factor for NCDs; its presence increases the chance that an individual will also suffer from one or more chronic illnesses. Overall, people with SMD have 1.53 times greater risk of cardiovascular disease and 1.85 times greater risk of death due to cardiovascular disease (Correll et al., 2017). People with SMD, particularly those who have had multiple episodes of illness, also have higher rates of diabetes mellitus, with 1.85 times greater risk than the general population (Vancampfort et al., 2016).
The reasons for the high co-morbidity between SMDs and NCDs have been extensively studied. People with SMD are more likely to engage in lifestyle behaviours that contribute to or exacerbate NCDs; that is, poor mental health is associated with the major modifiable risk factors for NCDs including tobacco use, harmful use of alcohol, unhealthy diets, and physical inactivity, which is elaborated further below. Additionally, pathophysiologically, persistent and SMD can affect and in turn, can be affected by stress-related NCDs (Watson et al., 2017), (Kapczinski et al., 2008; Nugent et al., 2015). Furthermore, the iatrogenic effects of medicines used to treat SMDs are linked with increased risk of cardiometabolic diseases. Lastly, individuals with mental disorders are less likely to seek and receive screening and adequate treatment for NCDs, and symptoms may affect adherence to treatment as well as prognosis.
Tobacco consumption (Lasser et al., 2000) is common amongst people with SMD and has been identified as a leading preventable cause of premature mortality in this population. Persons with schizophrenia and bipolar disorder are 5 times and 3 ½ times more likely to smoke currently than the general population, respectively (de Leon and Diaz, 2005),(Jackson et al., 2015). Alcohol use disorders are also common amongst people with SMD, with one study using the national Danish registry finding the comorbidity of alcohol use disorder with schizophrenia, bipolar disorder, and depression to be approxi-mately 35%, 33%, and 23%, respectively. The comorbidity rates of all substance use disorders combined were even higher, with 48%, 40%, and 29% for schizophrenia, bipolar disorder, and depression, respectively (Jørgensen, Nordentoft and Hjorthøj, 2018).
Additionally, people with SMD are more likely to consume unhealthy diets and be physically inactive (Dipasquale et al., 2013) (Jakobsen et al., 2018) (Vancampfort et al., 2017), which can lead to overweight, obesity, diabetes, and cardiovascular diseases. Overall, the risk of being overweight or obese as defined by a body mass index (BMI) of 25 or greater has been shown to be increased 3.4 fold for people with schizophrenia and 3.9 fold for people with bipolar disorder when compared with people without diagnoses of SMD (Gurpegui et al., 2012). When considering obesity alone, as defined by a BMI of 30 or greater, the risk associated with schizophrenia and bipolar disorder jumps to 4.3 fold 4.6 fold, respectively (Gurpegui et al., 2012).
Compounding the risks outlined above, the iatrogenic effects of psychotropic medications frequently used to treat the symptoms of SMD including antipsychotic medication (and to some extent, antidepressants and mood stabilizers) are linked with an increased risk of developing physical health conditions and associated complications (Correll et al., 2015) (De Hert et al., 2011) (Correll et al., 2017). The use of antipsychotic medications has been associated with obesity, insulin resistance, diabetes, myocardial infarctions, atrial fibrillation, stroke, and death (Lieberman et al., 2005) (Henderson et al., 2005) (Chou et al., 2017) (Sacchetti, Turrina and Valsecchi, 2010) (Yang et al., 2018).
SMD and infectious diseases
People with SMD are at greater risk than the general population for exposure to infectious diseases, including HIV, tuberculosis (TB) and chronic hepatitis. In the US, for example, persons with SMD were found to have a 10-fold higher prevalence of HIV (Hughes et al., 2016). In a population-wide study in Sweden, persons with SMD when compared with the general population were found to have approximately 2.6 times greater risk of HIV infection, as well as 2.3 and 6.1 times greater risk of hepatitis B and C infections, respectively (Bauer-Staeb et al., 2017). Further, one country-wide study in Taiwan revealed that persons with schizophrenia have a 1.5 times greater risk for tuberculosis infections than that of the rest of the population (Kuo et al., 2013).
As is seen with NCDs, there is a bi-directionality of the association between SMD and infectious diseases. HIV virus and opportunistic infections associated with AIDS can cause neurological damage, while mental disorders can also arise as a side effect of antiretroviral treatment or from the stigma, stress and socio-economic predicaments associated with the infection and treatment process. There are widespread discriminatory attitudes and behaviours towards people with HIV, TB and Hepatitis B/C in the community where they reside, particularly in developing countries. The psychological distress associated with stigma and discrimination may also trigger or aggravate the symptoms of SMD in affected individuals.
