Urbanization and prevalence of depression in diabetes

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Original Research

Urbanization and prevalence of depression in diabetes C.H. Lin a,b, Y.Y. Lee c, C.C. Liu d, H.F. Chen e,f, M.C. Ko g,*,h, C.Y. Li c,h a

Department of Psychiatry, Chi-Mei Medical Center, Tainan, Taiwan Department of Special Education, National University of Tainan, Taiwan c Department and Graduate Institute of Public Health, College of Medicine, National Cheng Kung University, Tainan, Taiwan d Bureau of Health Promotion, Department of Health, Executive Yuan, Taiwan e Department of Endocrinology, Far Eastern Memorial Hospital, Panchiao, Taiwan f School of Medicine, Fu-Jen Catholic University, New Taipei City, Taiwan g Department of Surgery, Taipei City Hospital, 145 Zheng Zhou Rd., Taipei 103, Taiwan b

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Article history:

Objectives: To depict recent secular trend (2001e2005) in prevalence of depression among

Received 30 June 2010

diabetic population in Taiwan, and to explore the influences of urbanization on the prev-

Received in revised form

alence of depression.

13 June 2011

Study design: A descriptive correlation study design relating urbanization and prevalence of

Accepted 27 October 2011

depression.

Available online 16 December 2011

Methods: Annual prevalence of depression was calculated as the ratio of number of individuals with depression (ICD-9-CM: 296, 309, or 311) to the size of diabetic population (ICD-

Keywords:

9-CM: 250), which were ascertained from ambulatory care claim data of Taiwan’s National

Co-morbidity

Health Insurance between 2001 and 2005. Multivariate Poisson regression analysis was

Depression

used to assess the secular trend in the prevalence of comorbid depression, and to appraise

Diabetes

the influence of urbanization on prevalence of depression in diabetic patients.

Prevalence

Results: The prevalence of depression among diabetic population increased annually from

Secular trend

22.6/103 in 2001 to 27.0/103 in 2005 with a significantly and linearly rising trend (b ¼ 0.0461,

Taiwan

p < 0.0001). Diabetic population living in urban areas showed the largest increase in prevalence (6.3/103), followed by those from rural areas (5.6/103). Compared to the diabetic patients residing in rural areas, those living in urban areas (RR ¼ 1.28, 95% CI ¼ 1.25e1.31) and those from satellite towns (RR ¼ 1.22, 95% CI ¼ 1.19e1.25) both had significantly increased adjusted RR. Conclusions: There is a significant increasing trend in prevalence of depression among diabetic population in recent years in Taiwan. Diabetic patients from urban areas not only had the greatest prevalence of depression but also showed the largest increase in prevalence during the study period, which highlights a need for managing depression in urban diabetes. ª 2011 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ886 2 2552 3234; fax: þ886 2 2375 8291. E-mail address: [email protected] (M.C. Ko). h M.C. Ko and C.Y. Li contributed to this article equally. 0033-3506/$ e see front matter ª 2011 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.puhe.2011.10.006

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Introduction

Methods

Although the underlining mechanisms for the relationship between depression and diabetes have not been fully understood,1 comprehensive evidence has shown a clear clustering of depression among diabetic patients.2e4 Depression itself is a debilitating illness that can lead to early death.5 Additionally, previous studies reported that depressive diabetic patients tend to have poor self-care and adherence to medical treatment, leading to unsatisfactory glycated hemoglobin (HbA1c) level and higher likelihood of developing diabetes complications.4 Zhao et al.3 also reported a substantial increase in health care utilization and costs in patients with both diabetes and depressive disorders. Epidemiological evidence shows an increasing prevalence of diabetes in Taiwan. Three different surveys conducted in Taipei city showed that the prevalence rate of diabetes for those aged S40 years was 5.1% in 1970, 7.1% in 1979, and 8.2% in 1986.6 Another study showed that the prevalence of diabetes in Tainan was 9% in 1996.7 In addition to prevalence, incidence of diabetes also increased over time in Taiwan.8 Apart from diabetes, Taiwan’s National Health Insurance (NHI) data also showed that the prevalence rate of depression has increased by a magnitude of 57% from 2001 to 2005.9 Due to a substantial increase in both diabetes and depression, and a possible health care burden and adverse health consequences,10 the presence of concomitant depression symptoms among diabetics may represent a growing health care problem. Previous studies reported that the prevalence rate of concomitant depression among diabetics varied from 6% to 52% with an aggregate estimate of 17.6% by meta-analysis.11 Geographic variation of prevalence rate of depression and diabetes comorbidity may largely depend not only on prevalence of respective risk factors for both diseases, but also on the diagnostic technique for ascertaining both diseases.12 A recent publication indicated that the risk of hip facture was higher in diabetic patients of rural areas than in those from urban regions, suggesting a possible urbanerural difference in quality of care delivered to diabetic patients.13 Given an increased prevalence of diabetes and depression worldwide and limited information on the prevalence of depression and diabetes comorbidity in Taiwan, we conducted this national study to elucidate the recent secular trend of prevalence of depression among diabetic population. In addition, areas with high population densities are characterized, for instance, by higher rates of criminality, mortality, social isolation, air pollution and noise.14 As the extent of these social problems is related to urbanization it is often assumed that rates of psychiatric disorders are also correlated with urbanization. In a recent review, Peen et al. found higher prevalence rates for psychiatric disorders in urban areas than in rural areas, and suggested that urbanization may be taken into account in the allocation of mental health services.15 To explore whether there is an urbanerural difference in depression prevalence in Taiwan, a special interest of this analysis was to assess the influence of urbanization on the prevalence of depression in diabetic patients.

This was a descriptive study that depicted recent secular trend in prevalence of depression among diabetic populations in Taiwan. Data analyzed in this analysis were obtained from Taiwan’s NHI claim files reported from 2001 and 2005. A universal NHI program has been implemented in Taiwan since March 1995. Some 96% of the total Taiwanese population has enrolled in the NHI program,16 and, by the end of 1996, the Bureau of NHI (BNHI) had contracted with 97% of hospitals and clinics throughout the nation.17 The BNHI accumulates all administrative and claims data for Taiwan. The National Health Research Institute (NHRI) cooperates with the BNHI to establish the NHI research database. The NHRI safeguards the privacy and confidentiality of all beneficiaries and transfers the NHI data to researchers after ethical approval has been obtained. The research protocol of this analysis has been ethically approved by the NHRI. To ensure accuracy of the claim files, the BNHI performs expert reviews on a random sample of every 50e100 ambulatory and inpatient claims quarterly,18 and false reports of diagnosis may result in severe penalty from the BNHI.18 For the current analysis, we used all ambulatory care claims (2000e2006). We first identify from diabetic ambulatory care claim records those patients coded with diabetes diagnoses by an ICD-9 CM code of 250. An individual was classified as a diabetic patient if he or she had an initial diabetes diagnosis in each year (2001e2005) and then experienced another one or more diagnoses within the subsequent 12 months. Moreover, the first and last outpatient visits during the 12 month period had to be separated by at least 30 days to avoid accidental inclusion of miscoded patients. The final annual diabetic population therefore was 662,246, 688,403, 702,285, 750,991, and 782,020 from 2001 to 2005 respectively. The diabetic population identified from each year was then linked to ambulatory care claims of the corresponding year, searching for visits due to depression (ICD-9-CM: 296, 309, or 311). A diabetic subject was considered to have depression if he/she had one or more ambulatory care visit for depression in a year. Because data available for analysis in this study were only within the time period of 2001e2005, there is no way of knowing if a diabetic subject with depression was a newly diagnosed depression patient. We therefore used annual prevalence of depression, calculated as the ratio of number of individuals with depression to the size of diabetic population, in this study. We categorized diabetic patients’ living areas into three levels of urbanization according to the classification scheme proposed by Liu et al.19, who considered the following indicators in determining levels of urbanization: population density, proportion of residents with college or higher education, percentage of elderly (>65 years) people, proportion of agriculture workforce, and number of physicians per 105 people. In statistical analyses, we first presented annual overall and sex-specific prevalence of depression during the study period. The annual overall and sex-specific prevalence were then fitted within a Poisson regression model to test the significance of linearity of secular trend. Multivariate Poisson regression analysis was further used to assess the

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independent effect of calendar year, age, sex, and urbanization on depression rate. To account for the possible intercorrelations of data collected from the same diabetic patient, and the possible clustering of data collected from the diabetic subjects living with the same level of urbanization, we performed generalized estimation equation to fit the Poisson regression model, that yields robust standard errors estimated in the model.20 We found that the interaction of calendar year and urbanization was statistically significant (p ¼ 0.004), which justified a stratified trend analysis according to urbanization. Direct standardization method, using the age and sex-specific diabetic population of 2005 as the standard population, was used to calculate age-sex-adjusted prevalence rate of depression.21 All statistical analyses were performed with SAS (version 9.1; SAS Institute, Cary, NC).

Results The annual size of the diabetic population was 662,244, 688,403, 702,286, 750,997, and 782,020 in years from 2001 to 2005, representing an increase of 18.0%. The increase in diabetic population size was more prominent in males (from 321,201 in 2001 to 389,501 in 2005, an increase of 21.3%) than in females (from 341,043 in 2001 to 392,519 in 2005, and increase of 15.1%). The annual age composition and distribution of urbanization of the diabetic population were quite similar in the 5-year period. The elderly diabetics (>¼65 years) constitute some 44% of the patients, followed by those aged 50e64 years (37.77e38.62%), and only 2% of diabetes was those aged 35 or less. Additionally, some 39e40% of the diabetics were from urban areas, and some 32e33% of diabetics were living in rural areas. The data also showed that the size of diabetic population mostly increased in ages 50e64 years (from 250,099 to 301,999, and 20.8%) and in those from rural areas (from 214,567 to 259,787, and 21.1%) (Table 1). The overall prevalence rate of depression during the 5-year study period was estimated at 24.4 per 1,000, with an apparent sex difference (males: 19.5 per 1,000 vs females: 29.1 per 1,000).

And the female to male ratio (some 1.5:1) of prevalence rate appeared to be constant over the study period. The prevalence of depression among diabetic population increased annually from 22.6 per 1,000 in 2001 to 27.0 per 1,000 with a significant and positive rising trend (b ¼ 0.0461). Additionally, the prevalence of depression also increased for both male and female diabetics with a percentage increase of 18.2% (18.1%e21.4%) and 20.8% (26.9%e32.5%), respectively, and the rising trend was marginally significant for men and marginally insignificant for women (Table 2). Table 3 shows the relative prevalence rate of depression in relation to calendar year, urbanization, age, and gender. Compared to that in 2001, the adjusted prevalence rate of depression was significantly (p < 0.0001) elevated in years 2003 to 2005 with a calendar-specific prevalence rate ratio (PRR) of 1.06, 1.12, and 1.20, respectively. The corresponding regression coefficients were 0.0570, 0.1174 and 0.1821, indicating a positive association. Test for linear trend revealed a significant increase in adjusted RR of depression (b ¼ 0.0503, p < 0.0001). Compared to the diabetic patients residing in rural areas, those living in urban areas (b ¼ 0.2434, PRR ¼ 1.28, 95% confidence interval (CI) ¼ 1.25e1.31, p < 0.0001) and satellite towns (b ¼ 0.1961, PRR ¼ 1.22, 95% CI ¼ 1.19e1.25, p < 0.0001) were both significantly and positively associated with an increased adjusted PRR. In contrast compared to the oldest age group (>¼65 years), children and adolescent diabetic population (b ¼ 0.8772, PRR¼0.42, 95% CI¼0.34e0.50, p < 0.0001) and those patients aged 50e64 (b ¼ 0.0827, PRR ¼ 0.92, 95% CI ¼ 0.90e0.94) both had a significantly reduced PRR, indicated by a negative regression coefficients. On the other hand, those aged 20e34 showed a significant but small increased PRR, indicated by a positive regression coefficient, by a magnitude of 7% (b ¼ 0.0707, p ¼ 0.023). Our data showed that female diabetic population had a significantly higher adjusted PRR (b ¼ 0.4091, RR ¼ 1.51, 95% CI ¼ 1.48e1.53) than their male counterparts. Table 4 shows the stratified analysis of annual crude and age-sex-standardized rate of depression according to level of urbanization. The diabetic population living in urban areas showed the largest absolute increase in prevalence (6.3 per

Table 1 e Baseline characteristics of the study diabetic population, 2001e2005. Calendar Year 2001

Sex Male Female Age ＜20 20e34 35e49 50e64 ＞¼65 Urbanization Urban Satellite Rural Total

2002

2003

2004

2005

n

%

n

%

n

%

n

%

n

%

321,201 341,043

48.50 51.50

336,404 351,999

48.87 51.13

344,865 357,421

49.11 50.89

372,226 378,771

49.56 50.44

389,501 392,519

49.81 50.19

2657 12,250 102,106 250,099 295,132

0.40 1.85 15.42 37.77 44.57

2629 12,449 103,444 261,397 308,484

0.38 1.81 15.03 37.97 44.81

2532 12,562 103,740 268,050 315,402

0.36 1.79 14.77 38.17 44.91

2736 14,420 112,315 289,316 332,210

0.36 1.92 14.96 38.52 44.24

2876 14,747 113,972 301,999 348,426

0.37 1.89 14.57 38.62 44.55

268,672 179,005 214,567 662,244

40.57 27.03 32.40 100.00

277,151 186,695 224,557 688,403

40.26 27.12 32.62 100.00

278,527 190,881 232,878 702,286

39.66 27.18 33.16 100.00

294,917 204,722 251,358 750,997

39.27 27.26 33.47 100.00

307,412 214,821 259,787 782,020

39.31 27.47 33.22 100.00

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Table 2 e Overall and sex-specific numbers and prevalence of depression among diabetic population in Taiwan, 2001e2005. Year

Total N

Males

Prevalence

2001 2002 2003 2004 2005

14,985 15,754 16,742 18,988 21,091

Total

87,560

a

22.6 22.9 23.8 25.3 27.0 b ¼ 0.0461 24.4

Females a

N

Prevalence

5828 6193 6622 7468 8352

N

18.1 18.4 19.2 20.1 21.4 b ¼ 0.0429 19.5

34,463

F/M ratio a

Prevalence

9157 9561 10,120 11,520 12,739

26.9 27.2 28.3 30.4 32.5 b ¼ 0.0503 29.1

53,079

1.49 1.48 1.47 1.51 1.52 1.49

a Per 1000.

1000), followed by those from rural areas (5.6 per 1000). The prevalence rate of depression in the diabetic population from satellite towns increased only slightly (1.2 per 1000). The relative change in prevalence rate was also higher in urban (23.7%) and rural (34.3%) areas, and lower (5.7%) in patients living in satellite cities. The urbanization level specific secular trend in standardized prevalence was also presented graphically in Fig. 1.

between. Additionally, urban diabetic patients showed higher concomitant depression prevalence, compared to the patients from lesser urbanized areas. These findings suggested an urgent need for managing depression in urban diabetic patients. Similar to our finding, several US studies also reported an increase in prevalence of concomitant depression among diabetic patients. It increased from 60 per 1000 in 1994,22 180 per 1000 in 1999,4 to 830 per 1000 in 2006.12 The increase in secular trend of concomitant depression among diabetic patients could be partly due to an overall increase in incident depression in Taiwan. A recent population-based cohort study from 1996 to 2003 included a random sample aged 15 years or older (n ¼ 136,045) as a fixed cohort and found that the annual treated incidence for major depressive disorder increased from 1.89 per 1000 to 2.58 per 1000.23 Another possible reason for the increased concomitant depression prevalence was that

Discussion This study demonstrated a significant rising secular trend recently of concomitant depression prevalence in Taiwan’s diabetic population, irrespective of gender. Such increasing trend was more evident in the most and the least urbanized areas than in areas with an intermediate urbanization level in

Table 3 e Adjusted relative prevalence of depression in relation to calendar year, age, and sex. Adjusted estimatea

Crude estimate Coefficient Period 2001 2002 2003 2004 2005 Urbanization Urban Satellite Rural Age ＜20 20e34 35e49 50e64 ＞¼65 Sex Males Females

0.0128 0.0527 0.1106 0.1745

PRRb 1.00 1.01 1.05 1.12 1.19

95% CI

p

1.00e1.03 1.03e1.07 1.10e1.14 1.17e1.21

0.1595 <0.0001 <0.0001 <0.0001

Coefficient

p

0.1203 <0.0001 <0.0001 <0.0001

0.2434 0.1961

1.28 1.22 1.00

1.25e1.31 1.19e1.25

<0.0001 <0.0001

0.42 1.07 1.01 0.92 1.00

0.34e0.50 1.01e1.14 0.98e1.04 0.90e0.94

<0.0001 0.023 0.5456 <0.0001

1.48e1.53

<0.0001

1.25 1.20 1.00

1.22e1.28 1.17e1.23

0.8842 0.0453 0.0435 0.0894

0.41 1.05 0.96 0.91 1.00

0.34e0.50 0.98e1.11 0.93e0.98 0.90e0.93

<0.0001 0.1449 0.0017 <0.0001

0.8772 0.0707 0.0085 0.0827

1.46e1.52

<0.0001

0.4091

1.00 1.49

95% CI

1.00 0.0148 1.01 1.00e1.03 0.0570 1.06 1.04e1.08 0.1174 1.12 1.10e1.15 0.1821 1.20 1.18e1.22 Trend test: b ¼ 0.0503, p < 0.0001

0.2202 0.1801

0.3989

PRRb

1.00 1.51

a Estimated from multivariate Poisson regression model with calendar year, age, sex, and urbanization simultaneously included in the regression model. b PRR: prevalence rate ratio.

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Table 4 e Crude and age-sex-adjusted annual prevalence of depression in diabetic population. Level of urbanization

Urban Crude Adjusteda Satellite Crude Adjusted Rural Crude Adjusted

Calendar year

Absolute change (1/103)

Relative change (%)

2001

2002

2003

2004

2005

26.8 26.6

27.5 27.3

28.78 28.6

30.7 30.6

32.9 32.9

6.3

23.7

21.3 21.1

20.7 20.6

21.6 21.4

22.3 22.2

22.3 22.3

1.2

5.7

16.4 16.3

16.9 16.9

17.7 17.7

19.3 19.3

21.9 21.9

5.6

34.3

a Age and sex adjusted prevalence rate.

the symptoms of depression may be present for many years, resulting in a high cumulative prevalence over time. The study by Chien et al.23 reported a cumulative treated prevalence increased from 1.67 per 1,000 to 17.24 per 1,000 in Taiwan over an eight-year period. Our data tended to indicate that prevalence of depression was higher in diabetic patients than in the general population. Chan24 conducted a cohort study between 2000 and 2006 to investigate the bidirectional relationship between diabetes and depression. In the analysis of diabetes in relation to depression onset, Chan included a random sample of 16,957 type 2 diabetic patients who sought ambulatory care in 2000 and the same number of age-sex-matched non-diabetes controls. Over 7 years of follow-up, diabetic patients had a higher incidence density of depression than non-diabetes regardless age and sex. The overall incidence density for diabetic women and their non-diabetic counterparts was 8.06 and 5.87 per 1,000 patientyears. The corresponding figures for men were 6.13 and 4.30

Fig. 1 e Secular trend of age-sex-standardized prevalence of depression in diabetic population according to areas of various urbanization levels.

per 1,000 patient-years. After adjusting for potential confounders, diabetic patients were found to experience a 1.43 fold increase in risk of depression onset, and such increased risk was not significantly modified by sex (women: 1.48; men: 1.39). Nonetheless, interpretation of a higher incidence of depression in diabetes should be cautious as it is likely that diabetics regularly access health care, compared to those without a longstanding illness, and so are more likely to have comorbidities diagnosed. Although we noted a significant rising secular trend in prevalence of depression among diabetic patients, the prevalence rate (22.6 per 1,000 to 27.0 per 1,000 in 2005) noted in Taiwan were much lower than those reported from some community studies of other nations such as Iraq (520 per 1,000),25 Korea (320 per 1,000),26 China (230 per 1,000),27 and the Netherlands (170 per 1,000).28 Comparisons made across studies are very difficult, even it’s not impossible, to interpret, mainly due to dissimilarities in patients’ demographic characteristics and in methods of ascertaining depressive patients. Nonetheless, previous studies had reported that depression may goes undiagnosed and untreated among some diabetic patients.29,30 And our study relied solely on the information of claim data, which might lead to an underestimation of concomitant depression among diabetic population in Taiwan. This study demonstrated that urbanization not only posed significant and positive influence on the prevalence of concomitant depression but also played a role of significant effect-modifier that modified the secular trend of prevalence of concomitant depression. Diabetic patients living in urban areas were found to not only have the greatest prevalence of concomitant depression, but also have the largest absolute increase in prevalence of concomitant depression. Previous studies showed a substantial geographic variation (some 15 fold) in prevalence rate of depression in the 41 US states and territories.12 Previous studies also disclosed that adults living in urban environments were at higher risks of both depression31,32 and general psychological distress.33,34 Additionally living in areas with a higher population density was also found to be associated with depression and anxiety as compared to those living in less populous regions.35 A higher prevalence of depression in urban general population could also due to the fact that more migrants can be seen in cities than in countryside. Several studies have indicated higher levels of psychiatric disorders among migrants to cities than

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among native residents.36 Nonetheless, the apparent urbanerural difference in prevalence of concomitant depression noted in our study and in others could also be artificial, at least to some extent. Previous studies noted that rural living has been associated with stigmatized attitudes toward mental health care and reduced willingness to seek help for mental health care,37 fewer visits to a mental health specialist,38,39 lower concordance with guidelines on treatment,38 and increased risk of suicide.40 All these might have made rural diabetic patients less likely to have ambulatory care visits, resulting in under diagnosis of depression in rural diabetes. The substantial increase in prevalence of concomitant depression noted in urban areas was likely due to a higher incidence of depression in urban residents. A recent national study23 reported a higher incidence of treated major depressive disorder among those who lived in urban areas of Taiwan between 1996 and 2003. Although rural areas were associated with the lowest prevalence of concomitant depression, our study clearly demonstrated that the largest relative increase (34.3%) in prevalence was noted in rural areas. A nationwide “share care” program was implemented in Taiwan in 1996, aiming to providing comprehensive and continual ambulatory care services to diabetic patients.41 Research has indicated that the “share care” model has effectively upgraded the care accountability and significantly improved the quality of diabetic medical care systems in Taiwan.42 Since community interventions usually appear to be more effective among people of lower socioeconomic backgrounds,43 the greatest relative increase in prevalence of concomitant depression among rural diabetics could be a consequence of the “share care program” that has drastically increased availability and accessibility of medical care delivered to rural diabetic patients in Taiwan. Our study noted that urban diabetics had nearly 1.3 fold prevalence of depression onset than their rural counterparts. Blazer et al.44 studied the urban/rural differences in the prevalence of psychiatric disorders from a community survey of 3,921 adults. Major depression disorders were found to be twice as frequent in the urban areas in the adjusted analysis. The increased prevalence (2 fold) of depression noted in the urban general population was greater than the figure (1.3 fold) noted in our diabetic population, suggesting diabetes might attenuate, at least to some extent, the possible relationship between urban living and depression prevalence. In addition to urbanization, we also noted that both gender and age were also significant predictors for concomitant depression in diabetic patients. Previous findings reported that female diabetic patients were 1.5e2.0 times more likely than male diabetics to suffer from comorbid depression.11 Our study likewise noted a 2 fold prevalence of depression in female diabetics. In the studies of general populations, Blazer et al. found that women were 2.7 fold more likely than men to suffer from major depressive episodes.44 Additionally, using Taiwan’s NHI data, Chien et al.23 reported that women had nearly 2 fold risk of developing depression as compared to men in the general population. The above information tended to suggest that the sex difference in risk of depression is somewhat lower in diabetic patients than in the general populations. One possible interpretation is that female patients with diabetes tended to report more depressive symptoms than male patients did and were more likely than male patients to seek social support.45

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Nonetheless, it is still unclear whether the sex difference in prevalence of comorbid depression may reflect an underlying biological mechanism or it is merely an artefact arising from the fact that it is more socially acceptable for women to report feelings of depression.46 Concerning the age-depression relationship, a recent Taiwanese study23 noted that compared to those aged 24 or less a higher incidence of treated major depressive disorder was detected in the adult groups with a hazard ratio increasing from 1.28 (aged 25e44 years) to 1.97 (65 years or older). Unlike the clear tendency that incidence and prevalence of depression increased with age noted in the general population,23,24 our study did not note apparent variation in age-specific prevalence of depression, except that child and adolescent diabetics had apparent reduced prevalence of concomitant depression, as compared to adult diabetics. There was little variation in age-specific prevalence of concomitant depression in adult diabetics aged 20 years or more. Certain factors such as smoking47 and visual impairment48 were found to be associated with depression, and these factors are believed to be more prevalent in adult diabetic patients than in child and adolescent patients, which might explain, at least to some extent, the age-difference in prevalence of concomitant depression in diabetes. This study has the following strengths. First, it included a large sample that is population-based and is highly representative, leaving little room for selection bias. The large sample size also has made it possible for making urbanization-, age-, and sex-specific analysis without comprising the required sample size. Second, we not only assessed the role of urbanization on the prevalence of concomitant depression but also evaluated whether urbanization may modify the secular trend in prevalence of concomitant depression in diabetic patients. This way of doing may have avoided overlooking the diabetes-depression comorbid problem in rural areas. There are also limitations to this study. First, as we relied solely on claim data that do not cover those who went un-diagnosed the prevalence rate noted in this study could be underestimated. But it would have little influence on the secular trend in prevalence. In addition, sole reliance on claim data could also lead to disease misclassification of depression. However, such disease misclassification should not be substantial as expert chart reviews have been periodically conducted by the BNHI.18 Moreover, such disease misclassification is believed to be nondifferential, which usually tends to underrate the association of diabetes and depression, and should not be a valid argument for the findings observed in this study. Second, due to non-specific diagnostic information of the NHI claim data, we were unable to differentiate between Type 1 and Type 2 diabetes, which limits the specific interpretation of the influence of age on comorbid depression. Despite that, a meta-analysis that included 20 studies showed that the prevalence rates of comorbid depression were statistically similar for Type 1 and Type 2 diabetes.2 Third, a number of potential confounders that may play some roles in the relationship between urbanization and depression. These potential confounders included certain social factors, such as migration and population density. Besides, in a recent review, Peen et al. found higher prevalence rates for psychiatric disorders in urban areas than in rural areas, and concluded that adjustment for confounders

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had limited impact on urbanerural difference in psychiatric disorders found, which shows that urbanerural differences in prevalence rates are only partly explained by population characteristics.15 Last, diabetics are more likely to regularly access health care, compared to those without a longterm illness, and so are more likely to have comorbidities diagnosed. Thus, the prevalence of depression observed in the non-diabetic population might have been under-estimated. In conclusion, this national population-based analysis demonstrated an increase in comorbid depression prevalence in diabetic patients of Taiwan in recent years. Such increase was urbanization dependent, diabetic patients from more and least urbanized areas showed the greatest increase. Additionally, prevalence of comorbid depression increased with level of urbanization, diabetic patients from urban areas were at the greatest risk of comorbid depression. Findings of this study highlight an urgent need for managing depression in urban diabetics and a need to impede effectively the rising trend in comorbid depression in rural diabetes. We also call for interventions to reduce the incidence of diabetes, which would in turn reduce the overall numbers of those with diabetes and depression.

Ethical approval Access to the National Health Insurance data has been ethically approved by the National Health Research Institute.

Competing interests The authors declare no competing interests concerning this study.

Funding This study was supported by a grant from Taiwan’s National Scientific Council (NSC98-2314-B-227-001-MY2).

Acknowledgments This study was supported by a grant from Taiwan’s National Scientific Council (NSC98-2314-B-227-001-MY2). The interpretation and conclusions contained herein do not represent those of BNHI, Department of Health or NHRI.

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