In this Swedish multicentre study of depression in 1027 participants with newly diagnosed T2D, younger age, women, previous depression, anxiety, antidepressant use, obesity (both BMI ≥ 30 and ≥ 40 kg/m2), smoking, and physical inactivity, were associated with current depression. There were distinct differences between younger and older participants. Younger participants (< 60 years) had significantly higher prevalence of current and previous depression, anxiety, antidepressant use, obesity (both BMI ≥ 30 and ≥ 40 kg/m2), high HbA1c (> 64 mmol/mol), smoking, and physical inactivity, than older participants. There were also sex differences with higher prevalence of both current and previous depression, anxiety, antidepressant use, obesity (both BMI ≥ 30 and ≥ 40 kg/m2), and smoking in the younger women, but higher prevalence of HbA1c > 64 mmol/mol (> 8%) in the younger men. In the younger women, previous depression, anxiety, antidepressant use, and severe obesity were associated with current depression. In the younger men, physical inactivity in addition to previous depression, anxiety, and antidepressant use, were associated with depression. In the older participants, the prevalence of current depression did not differ between women and men, but the older women had significantly higher prevalence of previous depression, antidepressant use, and obesity (both BMI ≥ 30 and ≥ 40 kg/m2) than the older men.
To our knowledge, the prevalence of depression at the time of diagnosis of T2D has not previously been explored in Sweden. According to previous research, depression is a risk factor for incident T2D [2, 3] and for cardiovascular and all-cause mortality . Depression as a risk factor for incident T2D seems to be as important as smoking and physical inactivity . The presence of anxiety in depressed patients seems to further increase the risk of incident T2D . In this study the association between depression and anxiety was very robust in all subgroups. The prevalence of current depression (12%) in all participants was the same as the prevalence of depression in the Swedish population study (11.7%), where depression was defined as in the present study (HADS-D ≥ 8) . However, the prevalence of current depression in the younger women (< 60 years) (31%) with newly diagnosed T2D was 2.6 times higher than for younger women in the Swedish population study (Swedish women: 18–84 years 11.2%, 18–64 years 11.9%, ≥ 64 years 9,4%) , while the younger men with newly diagnosed T2D had the same prevalence of depression (12%) as younger men in the Swedish population study (Swedish men: 18–84 years 12.3%, 18–64 years 12.2%, ≥ 64 years 13.2%) . The prevalence of current depression in the older participants (≥ 60 years) (both men and women 9%) was approximately the same as for older women in the Swedish population study, but lower for the older men with T2D than in the Swedish population . On the other hand, a large proportion of the older participants, particularly the women, reported being depressed previously. In another Swedish study, the prevalence of depression (HADS-D ≥ 8), was 13% in patients hospitalized due to coronary heart disease and 4% in healthy controls . Thus, the younger women with newly diagnosed T2D had 2.4 times higher prevalence of depression (31%) than patients with coronary heart disease (13%), and 7.8 times higher prevalence of depression than the healthy controls (4%). In a Swedish study of depression (HADS-D ≥ 8) in patients with T1D aged 18–59 years, the prevalence of depression in the women was 11% and 10% in the men , rendering a depression prevalence 2.8 times higher in the younger women and 1.2 higher in the younger men with newly diagnosed T2D in this study.
The prevalence of antidepressant use in our study was higher for younger women (4 times higher), older women (3.3 times higher), younger men (2.5 times higher), and for older men (1.4 times higher) than in the Swedish population (Swedish women/men: 18–84 years 9.8%/men 5.3%), 18–64 years 9.0%/4.8%, ≥ 64 years 10.1%/5.6%) . Younger T2D patients had also higher prevalence of antidepressant use than T1D patients aged 18–59 years (8%) [24, 25].
Obesity is another major contributor to both incident T2D [9, 10, 14, 26] and to CVD and mortality . The total prevalence of obesity (BMI ≥ 30 kg/m2) in the Swedish general population during the period 2016–2017 was 16.6% (women 14.5%, men 18.1%) . In our study the prevalence of obesity (BMI ≥ 30 kg/m2) was higher both in depressed and non-depressed participants compared to the Swedish general population. The highest prevalence of obesity was demonstrated in the depressed younger women (85%), 5.9 times higher than for women in the Swedish population (14.5%) . The demonstrated association between obesity and depression in this study of T2D differs from findings in patients with T1D, where no association between depression and obesity was found .
Furthermore, smoking is a major risk factor for both incident T2D , and for CVD and mortality (27). In 2016, the prevalence of smoking was 9% in the Swedish population , which can be compared to 13% in all participants, and 20% in the depressed and 12% in the non-depressed participants, with the highest prevalence in the younger depressed participants (29%).
Physical inactivity is another important contributor to both incident T2D [9, 10] and cardiovascular and all-cause mortality . The prevalence of physical inactivity (less than 30 min of moderate physical activity once a week) was high in the participants (31%) compared to Swedish T1D patients (13%) . Physical inactivity was particularly common in the young depressed participants in our study (52%). We have no data from the Swedish general population of the prevalence of physical inactivity defined as in our study.
As T2D usually has a long pre-detection period of 3–7 years , and as we do not know for how long the participants in our study had experienced depressive symptoms or used antidepressants, we cannot determine whether depression preceded or succeeded the onset of T2D. Due to the cross-sectional design, no causality can be drawn from our results.
Clinically, as well as in further research, it is probably important to detect depression and explore potential underlying conditions in patients with newly diagnosed T2D, particularly in young persons, in order to provide optimal treatment. Underlying conditions could be either somatic, psychological and/or social. Increased cortisol secretion has been demonstrated in both depressed and obese patients [6, 7, 30, 31], and cortisol secreting tumours may induce obesity and T2D . Weight stigma and discrimination are linked to both depression and to binge eating disorder , which has a high life-time risk for developing T2D . Post-traumatic stress disorder (PTSD) has also been linked to depression , obesity , and T2D . Attention deficit/hyperactivity disorder (ADHD) is another disorder previously associated with depression , obesity , and T2D . Additionally, in further research, exploration of shared endocrine and inflammatory disturbances in patients with depression, obesity and T2D would be of interest. We intend to explore the food habits of these participants in a separate article.
One strength of our study is the multicentre recruitment from a large number of health care units in both urban and rural areas in two separate Swedish regions during two years. The clinical diagnoses of T2D were confirmed as all included participants had remaining insulin secretion without immunological signs of autoimmunity . By including questions about previous depression and the use of antidepressants, we could show that depression was not just a reaction to the T2D diagnosis. Since previous research indicated increasing prevalence of severe obesity , two levels of obesity were reported. Relevant variables were included as depression previously has been linked to weight gain and obesity, particularly in atypical depression [6, 7], incident T2D [2,3,4], high HbA1c levels , physical inactivity , smoking , coronary heart disease [1, 17], stroke , and all-cause mortality . Age is relevant as incident T2D increases with age , but younger-onset T2D is particularly harmful with increased mortality . The increased prevalence of depression, obesity, smoking, and physical inactivity, may all be explanatory factors to the increased mortality previously demonstrated in younger-onset T2D . Age divided into 7 age-groups made it possible to study the details of the distribution of depression and obesity, and also facilitated determining a suitable cut-off for further analyses. The prevalence of depression declined until the age-group 60–69 years, which indicated that the age of 60 years was a suitable cut-off level for the age analysis.
A major limitation to our study was the high number of non-respondents to the question “Do you take antidepressant medication?” Other limitations were that there was no information regarding type of antidepressants and duration of use. Also, current depression was not confirmed by a structured interview. Yet, HADS-D has shown high validity for assessing depressive symptoms both at an individual and a collective level . HADS does not include symptoms that could be signs of a somatic disease accompanied by weight changes , and is extensively used in research [8, 17,18,19,20, 24, 25]. Patients with inadequate knowledge of Swedish were excluded, as we could not guarantee the quality of the translation of HADS into other languages. This exclusion criteria probably contributed to underrepresentation of immigrants. According to recent research, first generation immigrants constitute about 21% of newly diagnosed T2D in Sweden , compared to 12% in this study. This is important as nationwide Swedish studies have shown increased risk of depression in immigrants . Though the total number of included patients was quite high, it is still a limitation that just 29% of patients with newly diagnosed clinical T2D were included. However, the percentage might have been higher if only serologically verified T2D had been included in the total number. The reasons for non-participation were not registered systematically, but several health care units have reported they lacked time to include all new patients due to staff shortage. Other reasons for non-participation were that patients did not wish to participate, their T2D diagnoses were not serologically verified, or their HADS questionnaires were not adequately completed.