Inequalities in somatic comorbidities among immigrants and Norwegians with and without common mental disorders: a national register study

TL;DR

This study finds that immigrants with mental disorders in Norway face higher risks of certain physical health issues compared to Norwegians with similar conditions.

Contribution

The study reveals novel disparities in somatic comorbidity risks among immigrant groups with mental disorders compared to Norwegians.

Findings

Individuals with common mental disorders had higher somatic disease risks regardless of immigrant status.

Non-Western immigrants with mental disorders had higher risks of CVD, hypertension, and diabetes than Norwegians with mental disorders.

Sub-Saharan African immigrants with mental disorders had increased risk for viral hepatitis.

Abstract

Comorbidity between mental disorders and somatic diseases exacerbates health outcomes and contributes to premature mortality. However, differences in this comorbidity among immigrant groups compared to the majority population are unclear. This study aims to examine disparities in the risk relationship between common mental disorders (CMDs) and somatic diseases among the majority population (Norwegians) and various immigrant groups.

This national register study uses information from 3 142 925 residents aged 18+on diagnosed CMDs and selected somatic diseases for years 2008–2016. Poisson regression models were used to study the association between CMD and somatic diseases (i.e., cardiovascular diseases (CVDs), endocrine and metabolic diseases, cancer, and infectious diseases). Differences in risk between Norwegians and immigrant groups were investigated by introducing interaction terms between CMD and immigrant background.

Individuals with CMDs had a higher risk for all somatic diseases compared to those without, regardless of immigrant status. Immigrant groups varied in comorbidity, with those without CMDs showing similar or lower risk compared to Norwegians. However, immigrants with CMDs from non-Western countries (i.e., Eastern Europe, sub-Saharan Africa, South Asia) had a significantly higher probability of developing CVD, hypertension, and diabetes mellitus than Norwegians with CMDs. Additionally, SSA immigrants with CMDs also had a higher risk for viral hepatitis.

Findings suggest that immigrant groups experience varying degrees of comorbidity, which underscores the need for tailored healthcare interventions to address these disparities effectively.

The online version contains supplementary material available at 10.1007/s00127-025-02892-6.

Full text

Introduction

Mental disorders are often associated with comorbid somatic conditions and diseases, both communicable and non-communicable [1, 2], with individuals with mental disorders being at increased risk of at least one somatic disorder requiring treatment [3]. Furthermore, individuals experiencing mental disorder are also more likely to engage in adverse health behaviours [4], and less likely to follow the prescribed treatment [5]. They, thus, have an increased risk of premature mortality, particularly due to somatic conditions [6] and between seven to twenty-four years of life lost when compared to individuals without mental disorder [7, 8]. However, the somatic health of individuals with mental disorders is often neglected [9] and undertreated [10].

Immigrants have increased risk of poor mental health [11–13], though there are differences within the immigrant population. For instance, refugees are at higher risk of common mental health disorders (CMDs) such as anxiety, depression and posttraumatic stress disorder (PTSD) than non-refugees [14]. Immigrants from Middle Eastern and former Yugoslavia countries have both a higher prevalence of self-reported and diagnosed mental disorders [15–17], whereas immigrants from Asia, Sub-Saharan Africa (SSA) and Eastern Europe seem to have lower prevalence [18–20]. There is also research reporting better mental but also somatic health of the immigrant population as compared to majority population in their new country of residency, commonly referred to as the Healthy Immigrant Effect (HIE) [21, 22]. Yet, a recent study from Sweden found this mental health advantage to only apply to immigrants from Western countries [17].

Regarding somatic diseases, research suggests that the burden is higher among some immigrant groups. For example, the prevalence of diabetes mellitus is found to be highest among South Asian immigrants and those from the Middle East and Africa [23, 24]. The risk of cardiovascular diseases (CVDs) is highest among South Asians and Eastern Europeans [24, 25], while overweight/obesity is most common among African and South Asian women, and Eastern European and Middle Eastern men [23, 24, 26]. Hypertension is most prevalent among SSA and South- and Southeast Asians [23, 24].

There is a large body of research investigating the association between mental disorders and in particular, CMDs, and a range of somatic conditions in the general population. CMDs include depression, generalised anxiety disorder, panic disorder, phobias, social anxiety disorder, obsessive-compulsive disorder, and PTSD [27]. A high prevalence of comorbidity was reported between depression and hypertension and metabolic disorders [28]. CMDs also increase the risk of endocrine, and nutritional diseases [28–30], a range of CVDs [28, 31, 32], cancer [3] and infectious diseases [33, 34]. Some of these comorbidities have been ascribed to common risk factors, such as genetic risk factors, limited access and use of healthcare services or poor adherence to medical treatments, stigma, and lifestyle factors (e.g., poor diet, substance abuse, and physical inactivity) that are more common among patients with mental disorders [35, 36]. These risk factors, e.g., limited access and use of healthcare services, poor lifestyle factors and stigma, are also particularly common among immigrants from countries outside of the European Economic Area, North America, Australia and New Zealand [37]. Thus, it would be reasonable to assume that some immigrant groups may be at particularly high risk of comorbidity between mental and somatic disorders.

Despite this, and the known excess risk of somatic comorbidities for individuals with CMDs, there is a lack of research investigating whether comorbidities between CMDs and somatic diseases differ across immigrant groups and the majority population. Only one recent study investigated the incidence of somatic diseases in several immigrant groups, with and without a diagnosis of PTSD and/or depression, and found that immigrants with PTSD/depression had higher rates of somatic disease, particularly infectious, neurological and pulmonary diseases [38]. However, this study only provided a within-group comparison and thus, we still lack knowledge about the differences between the majority population and immigrant groups.

By using national register data, this study aimed to investigate the burden of diagnosed somatic diseases among Norwegians and immigrants with CMDs as well as to examine differences between Norwegians and immigrant groups in the associations between diagnosed CMDs and somatic diseases. We included somatic diseases which have been identified as major comorbid somatic diseases in patients with CMDs, such as CVD, endocrine and metabolic disorders, cancer and infectious diseases [35, 39]. The investigated immigrant groups originate from European Economic Area, North America, Australia and New Zealand (Western countries), Eastern Europe, Middle East/North Africa (MENA), SSA, and South Asia.

Methods

Study design

The study is register-based (covering years 2008–2016), combining sociodemographic information from the National Population Registry and diagnosis information on somatic diseases and CMDs obtained from the Norwegian Patient Registry. Norwegian Patient Registry holds data on all registered diagnoses obtained during contacts with specialist health care services.

Participants

The study participants consisted of Norwegians and immigrants aged 18 years or older who were legal residents in Norway on January 1, 2008 (N = 3 502 467). The Norwegian category refers to individuals born in Norway to two Norwegian-born parents (N = 2 768 685). Immigrants are defined as individuals born abroad to two foreign-born parents (N = 374 240). Those who were registered as deceased (N = 359 542) during the study period (2008–2016) were excluded from analysis. The final sample population consisted of 3 142 925 individuals.

Outcomes

Diagnoses of somatic diseases were received during outpatient and inpatient contacts with specialist healthcare between 2008 and 2016. The diagnoses are based on the International Classification of Diseases, tenth revision (ICD-10). It included CVD, hypertension, diabetes, obesity, metabolic disorders, cancer, and infectious diseases, such as viral hepatitis, influenza and pneumonia and chronic lower respiratory diseases. Table 1 presents the dichotomous outcome variables representing ICD-10 diagnostic codes of somatic diseases.

Table 1ICD-10 codes for outcome variables (somatic diseases)Outcome variablesICD-10 codesCardiovascular diseasesI10-I99Hypertensive diseasesI10-I19 Endocrine and metabolic diseases  Diabetes mellitusE10-E14 ObesityE66 Metabolic disordersE70-E90CancerC00-C96 Infectious diseases  Viral HepatitisB15-B19 Influenza and pneumoniaJ09-J18 Chronic lower respiratory diseasesJ40-J47ICD = International Classification of Diseases

Explanatory variables

CMDs are diagnosed based on ICD-10, chapter V on Mental and behavioural disorders. An individual was coded as having a CMD if they have a diagnosis of depression (F320-F322 & F330-333), generalised anxiety disorder (F411), panic disorder (F410), phobias (F400-409), obsessive-compulsive disorder (F420-F429), or post-traumatic stress disorder (F431). Diagnoses were provided by psychologists or psychiatrists during outpatient and inpatient contacts with the specialist mental health care service between 2008 and 2016.

Immigrant background

This variable consisted of Norwegians (individuals born in Norway to two Norwegian-born parents) and immigrants (individuals born abroad with two foreign-born parents) divided into five world region origins: Western countries (Sweden, Denmark, Finland, Island, Greenland/Faroe Islands, Austria, Belgium, Cyprus, France, Germany, Greece, Ireland, Italy, Luxemburg, Malta, Netherlands, Portugal, Spain, UK, Liechtenstein, Switzerland, Monaco, San Marino, Andorra, Guernsey, Isle of Man, Gibraltar, Vatican, Canada, United States, Australia, and New Zealand); Eastern Europe (Albania, Bulgaria, Eastland, Belarus, Croatia, Latavia, Poland, Romania, Lithuania, Moldova, Russia, Slovenia, Ukraine, Bosnia Herzegovina, Slovakia, Czech, Serbia, Montenegro, and Kosovo); MENA (Algeria, Bahrain, Egypt, Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Libya, Morocco, Palestine, Oman, Qatar, Saudi Arabia, Syria, Tunisia, Turkey, United Arab Emirates and Yemen); South Asia (Pakistan, India, Sri Lanka, Bangladesh, Afghanistan, Bhutan, Maldives, and Nepal), and SSA (Angola, Benin, Botswana, Burkina Faso, Burundi, Cape Verde, Cameroon, Central African Republic, Chad, Côte d’Ivoire, Democratic Republic of the Congo, Djibouti, Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Liberia, Madagascar, Malawi, Mali, Mauritania, Mauritius, Mozambique, Namibia, Niger, Nigeria, Republic of the Congo, Rwanda, Swaziland, São Tomé and Príncipe, Senegal, Seychelles, Sierra Leone, Somalia, South Africa, South Sudan, Tanzania, Togo, Uganda, Zambia, and Zimbabwe). The rationale for making this category is crucial for a detailed and nuanced understanding of inequalities in the burden of comorbidities in the immigrant population. The categorisation was based on a combination of geographical and cultural regions, but also with consideration to ensuring a sufficient number of migrants in each category. Migrants from other parts of the world were deemed as too diverse to be grouped together, or too small to be included.

Covariates

Covariates include age (measured continuously), gender (sex at birth: males and females) and household poverty (an index for socioeconomic status (SES)). According to the EU-scale, household poverty is defined as less than 60% of the annual median disposable equivalised household income after adjustment for the number of household members [40]. A dummy variable (0 and 1) was constructed to indicate whether participants were defined as living in a household of poverty in each year. A sum score for all years was calculated with 0 being the lowest score (no years in poverty) and 9 being the highest sum score (in poverty all years in the study). A higher sum score indicates more years with household poverty between 2008 and 2016. This adjustment of household poverty accounts for the cumulative impact of socioeconomic disadvantage on comorbidity outcomes. Given the study’s focus on migrant background, it is particularly relevant, as the socio-economic trajectories across migrants and non-migrants may differ significantly.

Data analysis

First, the characteristics of the study population by immigrant background, and the prevalence of both CMDs and specific somatic diseases were investigated (Table 2). Separate Poisson regression models were then applied to estimate the incidence rate ratios (IRR) of the association between CMD and somatic diseases. We then conducted stepwise modelling where we calculated the risk of each somatic disease by CMD status (Model 1), by immigrant background (Model 2), and an interaction term between CMD status and immigrant background in the Model 3. The interaction term was applied to investigate the differences in the risk of somatic diseases among individuals with and without a CMD by immigrant background. Regression estimates (IRR with 95% confidence intervals (CI)) were adjusted for age, gender, and household poverty, with Norwegians as the reference group (Table 3). The interaction results are presented as marginal probabilities in Table 4. The analyses were performed by means of Stata SE/16.

Table 2. Characteristics of the study population, by region of originTotalN (%)NorwegianN (%)WesternN (%)Eastern EuropeN (%)Middle East/ North AfricaN (%)Sub-Saharan AfricaN (%)South AsiaN (%)N (%)3 197 1822 768 685 (86.6)144 019 (4.9)158 487 (4.9)48 514 (1.5)38 228 (1.2)38 228 (1.2)Male1 558 504 (49.6)1 365 480 (49.3)79 248 (55.0)101 376 (63.9)29 399 (60.6)22 567 (57.5)21 828 (57.1)Female1 584 421 (50.4)1 403 205 (50.7)64 771 (45.0)57 111 (36.1)19 115 (39.4)16 682 (42.5)16 400 (42.9)Age, Mean (SD)46.5 (16.4)46.6 (16.4)39.8 (15.1)34.1 (11.2)36.9 (11.5)33.4 (10.7)38.0 (12.7)Household poverty index, Mean (SD)0.5 (1.3)0.4 (1.2)0.2 (1.0)0.3 (1.2)1.2 (2.1)1.1 (2.1)0.8 (1.8)Common mental disorders202 682 (6.4)181 013 (6.5)6123 (4.2)5285 (3.3)5908 (12.2)1292 (3.3)2162 (5.6)Cardiovascular diseases798 857 (25.4)746 617 (26.9)21 290 (14.8)13 834 (8.7)7202 (14.8)3864 (9.8)6280 (16.4)Hypertensive diseases372 579 (11.9)351 053 (12.7)9222 (6.4)5118 (3.2)2331 (4.9)1567 (3.9)3086 (8.1) Endocrine and metabolic disorders Diabetes mellitus173 423 (5.5)155 237 (5.6)4101 (2.8)2813 (1.8)3039 (6.3)1980 (5.0)4638 (12.1)Obesity68 583 (2.2)63 577 (2.3)1605 (1.1)1138 (0.7)1061 (2.2)486 (1.2)720 (1.8)Metabolic disorders203 629 (6.5)191 277 (6.9)4966 (3.4)2649 (1.7)1781 (3.7)952 (2.4)1761 (4.6)Cancer264 012 (8.4)252 154 (9.1)6977 (4.8)2587 (1.6)993 (2.1)530 (1.4)741 (1.9) Infectious diseases Viral Hepatitis15 976 (0.5)11 463 (0.4)350 (0.2)1004 (0.6)518 (1.1)1221 (3.1)505 (1.3)Influenza and pneumonia110 975 (3.5)103 733 (3.8)2686 (1.9)1448 (0.9)1089 (2.2)853 (2.2)1080 (2.8)Chronic lower respiratory diseases177 476 (5.7)166 333 (6.0)4383 (3.0)1859 (1.2)1794 (3.7)808 (2.1)1683 (4.4)SD– standard deviation; N– number

Table 3. Incidence risk ratio (IRR) with 95% confidence intervals (95% CI) of somatic diseases. Adjusted for age, gender and household povertyCardiovascular diseasesHypertensive diseasesDiabetes mellitusObesityMetabolic disordersCancerViral hepatitisInfluenza and pneumoniaChronic lower respiratory diseasesIRR95% CIIRR95% CIIRR95% CIIRR95% CIIRR95% CIIRR95% CIIRR95% CIIRR95% CIIRR95% CI Model 1 CMD (ref.no)1.41.39–1.421.41.36–1.401.451.42–1.472.432.38–2.482.021.99–2.041.151.13–1.173.032.91–3.162.041.99–2.081.831.80–1.84 Model 2

Immigrant background Norwegians (ref.)111111111Western0.730.72–0.740.740.73–0.760.670.65–0.690.470.45–0.500.670.65–0.690.760.74–0.780.610.54–0.670.690.66–0.720.640.62–0.66Eastern Europe0.610.59–0.620.630.61–0.650.590.57–0.610.310.29–0.330.470.45–0.490.410.40–0.431.421.33–1.520.520.49–0.540.320.31–0.34Middle East/North Africa0.920.89–0.940.820.78–0.851.791.73–1.850.910.85–0.960.840.81–0.880.480.45–0.511.581.45–1.740.970.92–1.040.870.83–0.91Sub-Saharan Africa0.740.71–0.760.880.84–0.931.811.72–1.870.530.49–0.580.710.67–0.760.390.36–0.425.515.18–5.861.261.18–1.350.580.54–0.62South Asia0.950.93–0.971.231.19–1.273.343.25–3.430.810.76–0.881.050.98–1.090.400.37–0.432.582.36–2.831.221.15–1.291.020.98–1.07IRR– Incidence risk ratio; 95% CI– 95% confidence interval; CMD– Common mental disorders

Table 4. Predictive margins based on the interaction analysis for CMD and somatic diseases. Adjusted for age, gender and household povertyNorwegianWesternEastern EuropeMiddle East/ North AfricaSub-Saharan AfricaSouth AsiaPr95% CIPr95% CIPr95% CIPr95% CIPr95% CIPr95% CI Cardiovascular diseases  No CMD0.250.25–0.250.180.18–0.190.140.14–0.150.220.22–0.230.180.18 − 0.170.230.23–0.24 CMD0.340.34–0.350.290.28–0.300.400.38–0.420.340.32–0.360.330.29–0.370.410.38–0.44 Hypertensive diseases  No CMD0.120.12–0.120.080.08–0.090.070.06–0.070.100.09–0.100.100.09–0.110.140.14–0.15 CMD0.160.15–0.160.130.12–0.140.180.17–0.200.120.11–0.140.180.14–0.210.250.21–0.27 Diabetes mellitus  No CMD0.050.05–0.050.030.03–0.040.030.03–0.030.100.09–0.100.090.09–0.100.170.16–0.17 CMD0.070.07–0.070.060.05–0.060.090.08–0.100.120.11–0.130.170.13–0.190.250.23–0.27 Obesity  No CMD0.020.02–0.020.010.01–0.010.010.01–0.010.020.02–0.020.010.01–0.010.020.01-0.0 CMD0.050.04–0.050.030.03–0.040.030.02–0.030.030.03–0.040.030.02–0.040.040.03–0.05 Metabolic disorders  No CMD0.060.06–0.060.040.04–0.040.030.03–0.030.050.05–0.060.040.04–0.050.060.06–0.07 CMD0.120.12–0.130.090.08–0.100.100.09–0.110.100.09–0.110.120.09–0.140.130.12–0.15 Cancer  No CMD0.080.08–0.090.060.06–0.070.030.03–0.030.040.03–0.040.030.03–0.030.030.03–0.04 CMD0.090.09–0.100.090.08–0.090.070.06–0.080.060.05–0.070.060.04–0.080.050.04–0.07 Vital Hepatitis  No CMD< 0.01< 0.01–0.01< 0.01< 0.01–0.010.010.01–0.010.01< 0.01–0.010.020.02–0.020.010.01–0.01 CMD0.010.01–0.010.010.01–0.010.020.01–0.020.010.01–0.010.030.03–0.040.020.01–0.03 Influenza and pneumonia  No CMD0.080.08–0.090.060.06–0.070.030.03–0.030.040.03–0.040.030.03–0.030.030.03–0.04 CMD0.090.09–0.100.090.08–0.090.070.06–0.080.060.05–0.070.060.04–0.080.050.04–0.07 Chronic lower respiratory diseases  No CMD0.080.08–0.090.060.06–0.070.030.03–0.030.040.03–0.040.030.03–0.030.030.03–0.04 CMD0.090.09–0.100.090.08–0.090.070.06–0.080.060.05–0.070.060.04–0.080.050.04–0.07Pr = Predictive probability; 95% CI– 95% confidence interval; CMD– Common mental disorders

Results

Characteristics of the study population

The descriptive summary of the study population, by immigrant background, is presented in Table 2. From January 1, 2008, to December 31, 2016, 6.4% (N = 202 682) of the study population had a CMD diagnosis. The highest percentage with a diagnosed CMD originated from MENA (12.2%) and lowest percentage from Eastern Europe and SSA (3.3%). For the most studied somatic diseases, the general pattern was that immigrants had a lower prevalence than, or comparable prevalence to the Norwegians. There were, however, some marked exceptions. Diabetes mellitus was more prevalent among South Asian immigrants compared to Norwegians (12.1% vs. 5.6%). Further, viral hepatitis was more common among SSA immigrants than Norwegians (3.1% vs. 0.4%).

Differences in the risk of somatic diseases between Norwegians and immigrants

Table 3 presents the results of Poisson regression models for the IRR (95% CI) for each somatic disease, categorized by CMD status and immigrant background, with Norwegians as the reference group. Estimates were adjusted for age, gender, and household poverty. Model 1 revealed that individuals with a CMD had a significantly higher risk for all somatic diseases compared to those without a CMD. In Model 2, immigrants from Western and Eastern European countries showed lower or comparable risk for all investigated somatic diseases compared to Norwegians, except for viral hepatitis, which was significantly higher among Eastern European immigrants. Immigrants from MENA, SSA, and South Asia exhibited significantly higher risks of diabetes mellitus and viral hepatitis. Moreover, immigrants from SSA and South Asia also had a significantly higher risk of influenza and pneumonia. Notably, this risk of cancer was particularly low for immigrants from MENA, SSA and South Asia compared with Norwegians.

To investigate the difference in the risk of comorbidity between CMDs and somatic diseases by immigrant background, an interaction term between CMD status and immigrant background was added to Model 3. We found significant interactions between CMD status and immigrant background for many somatic diseases (see supplementary Table 1). Interaction terms are presented as marginal probabilities in Table 4 illustrating the marginal probabilities of somatic diseases among Norwegians and immigrants with and without CMDs. These interaction findings are also presented in the forest plots for those with and without CMDs separately (see supplementary Figs. 1 and 2).

The probability of most somatic diseases was significantly higher among individuals with a CMD, regardless of immigrant status. Some immigrant groups with a CMD demonstrated a notably higher probability of CVD, hypertension, and diabetes mellitus. Specifically, South Asian and Eastern European immigrants with a CMD had significantly higher rates of CVD, while Eastern European, SSA, and South Asian immigrants with a CMD exhibited significantly higher rates of hypertension. Additionally, Eastern European, MENA, SSA, and South Asian immigrants with a CMD had significantly higher rates of diabetes. Moreover, SSA immigrants with a CMD showed a significantly higher probability of viral hepatitis. The majority of differences in the probability of these somatic diseases between Norwegians and immigrant groups without a CMD were either non-significant or lower.

Discussion

In this study, we examined disparities in the risk of somatic diseases based on immigrant background and the comorbidity between CMDs and somatic diseases diagnosed in specialist healthcare between 2008 and 2016 in Norway. Immigrants displayed either similar or significantly lower risk compared to Norwegians for the majority of examined somatic diseases. However, immigrants from MENA, SSA, and South Asia exhibited an elevated risk of diabetes mellitus and infectious diseases (viral hepatitis or influenza and pneumonia). This higher risk among these immigrants for some of these diseases has been documented in previous research (e.g., 23, 24, 25).

Particular interest was paid to potential disparities in comorbidity between Norwegians and different immigrant groups. Through interaction analyses and results presented as predictive margins, we observed significant differences in the likelihood of CVD, hypertension, diabetes mellitus, and viral hepatitis based on CMD status and immigrant background. Relative to Norwegians, the disparities in the likelihood of CVD, hypertension, or diabetes mellitus were more pronounced among Eastern European, SSA, and South Asian immigrants with a CMD than among those without.

Potential reasons for the increased risk of these somatic diseases among immigrants from non-Western countries with CMDs include poor lifestyle choices, such as unhealthy dietary habits and physical inactivity [37] and low health literacy, which may be more prevalent among these immigrants compared to other immigrant groups [41, 42]. Individuals with a CMD may also encounter greater challenges in navigating the healthcare system, leading to delayed access to services and potentially exacerbated health outcomes upon seeking care. Norway has a universal healthcare system that covers both primary and specialist healthcare services. However, user fees (egenandel) are required for consultations, diagnostic tests, and certain treatments until an annual cost ceiling (frikort) is reached, after which services become free for the rest of the year. While this system ensures affordability for most residents, immigrants may face challenges such as limited awareness of their healthcare rights, administrative hurdles, and financial constraints before qualifying for exemption. These factors can contribute to disparities in healthcare access and utilization among immigrant populations. Additionally, immigrants from non-Western countries have poorer living conditions than the general population [43], and the acculturation stress [44] and stigma [45] associated with mental disorders may be particularly pronounced, resulting in heightened risk for comorbidities and illness severity.

The relative differences in risk of somatic diseases between Norwegians and most immigrant groups without a CMD were, however, lower. This finding is in accordance with other studies suggesting that immigrants have lower odds of multimorbidity than non-immigrant populations [46, 47]. Alternatively, the general lower risk of somatic diseases among immigrants without a CMD may be a result of screening practices among some immigrant groups (e.g., cancer), poorer help-seeking behaviours for both mental and somatic diseases, help-seeking at later stages [48–51], or better health compared to the Norwegians (i.e. HIE) [52]. Moreover, immigrants are less likely than Norwegians to use specialist healthcare help for mental disorders [53–55]. It’s imperative to recognize that out-of-pocket payment for health services may serve as a barrier for certain immigrants with CMDs. Thus, there may be more undiagnosed CMDs among immigrants than Norwegians, which could underestimate some of the inequalities in the risk of somatic diseases among those without a CMD.

Limitations

First, the methodology applied to study the association between CMDs, and the risk of somatic comorbidity does not allow us to determine the causal path. As both CMDs and somatic disorders could be diagnosed at any time of the study follow-up, we cannot say which was the first. We did not consider whether individuals had multiple mental disorders and/or multiple somatic diseases either. Second, the dataset did not allow identification of individuals who emigrated during the follow-up period. If many immigrants migrate home when they experience serious illness (mental or somatic), as suggested by the Salmon bias hypothesis [56], we may be somewhat underestimating the risk of CMDs and somatic disorders as well as the level of comorbidities. However, this would only have a limited impact on the results, particularly for non-Western immigrants as the out-migration for this group is very low [57]. Third, as reported in previous research, immigrants may be at increased risk of some but not necessarily all types of CMD. Thus, focus on more specific diagnosis could provide an even more nuanced result. However, the use of specific CMD as well as a range of somatic diseases could threaten the statistical power of the analyses. Furthermore, it is important to reflect that underdiagnoses of some diseases may be present. This may particularly apply to CMDs among immigrants, who appear less likely to use mental healthcare [53, 58, 59]. Lastly, we recognize that significant diversity within the five broad world regions may influence our results, potentially masking important differences among migrants. As a result, this categorization is not optimal. The within-group variations may be substantial, and a more detailed classification—such as specific measures of ethnicity or country of origin—is needed to better understand how and why these groups may differ.

This study, however, has several strengths, such as the methodological advantages that the use of Norwegian Patient Registry brings. First, the coverage of healthcare services and the quality of health records in Norway is considered to be high, which facilitates representativeness and reduces selection bias [60]. Second, a larger study population brings adequate statistical power to detect the heterogeneity in the clinical epidemiology of some relatively rare somatic diseases across several immigrant groups with and without mental disorders. Further, all diagnoses were based on clinically set diagnosis from specialist care, and not self-reported information thus reducing the risk of recall bias.

Conclusions

In conclusion, this study on the comorbidity between CMDs and various somatic disorders among immigrants and Norwegians is of great importance as it highlights which groups may be at greatest risk of comorbid disorders and related consequences. Although the differences in risk of comorbidity between mental and somatic diseases were, to a large extent, non-significant or slightly lower among immigrants than Norwegians, there are groups that show higher risk. Further, the largest relative differences between Norwegians and the studied immigrant groups were found among those with a CMD. This was highly marked for immigrants from non-Western countries. Thus, the healthcare system should give greater attention to individuals with CMD. In particular, targeted interventions addressing early screening among psychiatric patients in groups that have a higher risk of somatic diseases should be given a priority. An additional crucial clinical implication is the necessity to ensure access to regular examinations by general practitioners for individuals at elevated risk of somatic disorders. Future research should aim to better understand the mechanisms behind these inequalities in order to find out if the differences in mental and somatic comorbidities may be a result of specific risk factors, barriers regarding healthcare help-seeking among immigrants, underdiagnoses or better health of immigrants than of Norwegians.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary Material 1