The incidence risk of type 2 diabetes mellitus in female nurses: a nationwide matched cohort study

Background Diabetes is one of the most common chronic illnesses worldwide. This study was to assess whether the incidence risk of type 2 diabetes mellitus between female nurses and female non-nurses. Methods Study data were obtained from the Longitudinal Health Insurance Research Database, and nurses were sampled from the Registry for medical personnel. Nurses and non-nurses with similar traits and health conditions were selected via 1:1 propensity score matching. A total of 111,670 subjects were selected (55,835 nurses and 55,835 non-nurses). Stages of diabetes development were monitored until December 31, 2009. The Cox proportional hazards model was used to discuss risks and influencing factors related to diabetes. Poisson distribution methods were used to examine the incidence rate of diabetes per 1,000 person-years. Results The propensity matching results show that on average, female nurses who were diagnosed with diabetes were younger compared with the non-nurses (46.98 ± 10.80 vs. 48.31 ± 10.43, p <0.05). However, the results of the Cox proportional hazards model show that the nurses showed a lower risk of developing diabetes compared with the non-nurses (Adj. HR = 0.84, 95 % CI: 0.79–0.90). Factors influencing diabetes development risks among the nurses include advanced age and high Charlson Comorbidity Index levels. Conclusion The low degree of diabetes development among the nurses may be attributable to the fact that nurses possess substantial knowledge on health care and on healthy behaviors. The results of this study can be used as a reference to assess occupational risks facing nursing staff, to prevent diabetes development, and to promote health education.


Background
Nurses work on the front lines in the medical industry and account for 45.2 % of all professional medical personnel in Taiwan [1]. Numerous medical orders and procedures rely on the work of nurses; therefore, nursing care is highly correlated with care quality levels and with patient treatment outcomes, and nurses play a crucial role in the medical industry.
Ongoing care services are essential to the medical industry. Shift work creates certain occupational problems for nurses [2,3]. Relevant studies have shown that in addition to causing an increasing number of accidents [4] and higher rates of disease and cancer development [5], engagement in shift work increases one's likelihood of developing metabolic diseases [6,7]. Previous studies have revealed that individuals working in occupations that involve shift work show higher risks of developing diabetes compared with those who do not work shifts [8][9][10].
Diabetes is one of the most common chronic illnesses worldwide. Statistics [11] show that approximately 366 million people worldwide were diagnosed with diabetes in 2011. This number is estimated to increase to 552 million by 2030, accounting for 4.4 % of the global population. In the United States, research has been conducted on the health status of 67,420 nurses from 1976 to 1996, and the results show that 7,401 (11 %) of the nurses were diagnosed with type 2 diabetes mellitus (DM) [12]. Since 2003, diabetes has ranked third among the top 10 causes of death among women in Taiwan [13]. Nurses are exposed to an occupational environment that may be detrimental to their health, which may affect the quality of medical care. The present study compares type 2 DM development risk levels among female nurses to those of the general female citizen population to further identify influencing factors. These data may serve as a foundation for future discussions on occupational health in relation to health promotion, prevention and protection.

Study design
A retrospective and longitudinal method was adopted in this study, and data were sourced from the National Health Insurance Research Database. To comply with privacy measures, personal information was removed from the collected data.
The Taiwanese government created a compulsory National Health Insurance (NHI) system in 1995, a governmental insurance system that all citizens are mandated to be insured under. Since the end of 2012, the NHI coverage rate reached to as high as 99.85 % [14]. The NHI covers costs for outpatient procedures; emergency procedures; and inpatient prescriptions, treatments, and examinations. The NHI database details all medical claims records for the insured, including treatments for chronic illnesses such as diabetes [15,16].

Data sources and study participants
Data were sourced from the Longitudinal Health Insurance Research Database of 2000 published by the NHI, which included one million population randomly selected to be representative of whole population in Taiwan and a sample of nurses was selected from the nationwide Registry for Medical Personnel published by the NHI. We used female nurses and female non-nurses as the study participants. Nurses were defined as the  [17]. A total of 374,173 participants were selected, 70,675 of whom were nurses and 303,498 of whom were female non-nurses.
To objectively compare diabetes development risks among female nurses and female non-nurses, 1:1 propensity score matching was used to control for selection bias, and participants with similar characteristics and health conditions were selected from the two groups. After propensity score matching, we assigned 55,835 nurses to the observation group and 55,835 non-nurses to the control group ( Table 1). Patterns of diabetes development were monitored until December 31, 2009, covering an average of 9.68 ± 1.06 years (9.72 ± 0.92 and 9.64 ± 1.19 y for nurses and non-nurses, respectively).

Variables description
In this study, the type 2 diabetes mellitus was defined as a primary or secondary diagnosis with ICD-9-CM: 250 or A-code: A181 and the patients had made 3 or more clinic visits or been hospitalized at least once within 365 days [18], and this study excluded other types DM patients. We categorized residence areas into 7 levels, and level 1 was the most urbanized [19]. We used the modified Charlson Comorbidity Index (CCI) to classify the severity of comorbidity [20]. The CCI involved 17 comorbidities weighted based on severity. Higher score denoted greater comorbidity. Presence of catastrophic illnesses was defined as yes or no. The catastrophic illnesses or injuries were defined by National Health Insurance Administration in Taiwan including 30 categories of major illnesses (e.g., cancer, stroke, hemophilia, type I diabetes, autoimmune diseases, end-stage renal disease etc.) for which patients were exempted from copayment and thus avoided financial hardship [21].

Data analysis
A descriptive analysis was conducted to examine demographical traits of the research population (e.g., age, monthly salary, and CCI) and to classify the participants as nurses and non-nurses. Propensity matching (PS) methods were used, and a Chi-square test was subsequently employed to compare variations in diabetes development risks for the two groups. The Cox proportional hazards model was used to discuss relative risks and influencing factors related to diabetes. Hazard ratios (HRs) and 95 % CIs were derived from Cox proportional hazards models. Poisson distribution methods were employed to examine diabetes incidence rates for 1,000 person-years for the two groups. In this study, statistic significance was set at p < 0.05.

Participants demographics
The data prior to propensity matching revealed significant differences (p <0.05) between age, monthly salary, and CCI levels for the two groups ( After performing the propensity score matching tasks, we did not find any significant differences in terms of age, monthly salaries, residence area urbanization levels, CCI levels, and in other catastrophic illness levels between the two groups (Table 1).

Stratified analysis on the risk of diabetes for the nurses and non-nurses
In the Table 2, the results of the bivariate analysis revealed a lower incidence of diabetes among the nurses (2.68 %) compared with that of the non-nurses (3.13 %), revealing a statistically significant difference (p <0.05). After controlling for other factors, Cox proportional hazards models were used to identify the risk of diabetes for the nurses and non-nurses ( Table 3). The results of the nurses showed a lower risk of developing diabetes compared with the non-nurses (Adj. HR = 0.84, 95 % CI: 0.79-0.90). For age, in the 35-44-and 45-54-year age groups, the nurses exhibited a significantly lower risk of diabetes compared with the non-nurses (Adj. HR: 0.82 and 0.75, respectively). For the income groups of less than NT$ 17,280, NT$ 22,801-NT$ 28,800, and NT$ Regarding urbanization variables, with the exception of participants residing in Levels 4 and 5 urbanization areas, no significant differences in diabetes development risk were found between the nurses and non-nurses (p <0.05). In less urbanized regions, nurses showed lower risks of diabetes development than non-nurses (p <0.05).
An analysis on the comorbidity of other catastrophic illnesses indicated that risks of diabetes development facing those nurses who did not have other catastrophic illnesses were significantly lower than those of nonnurses (Adj. HR: 0.84; p <0.05). While risks of diabetes development among the nurses with catastrophic illnesses were lower than those for the non-nurses, the results were non-significant (p <0.05). The results of the CCI analysis show that the nurses presented significantly lower susceptibility to diabetes development overall compared with the non-nurses (p <0.05).

Relative factors and the incidence of diabetes among nurses
The analysis results presented in Table 4 show that age and CCI serves as critical factors that influence diabetes development risks among nurses. Regarding age variables, a 10-year period was used as an interval to create age groups. The older nurses presented relatively higher risks of diabetes development compared with the nurses of less than 25 years (Adj. HR: 1.56-16.85). Nurses who were older than 65 years of age showed a 16.85-fold higher risk of developing diabetes compared with nurses who were younger than 25 years of age (95 % CI: 11.47-24.75). CCI significantly influences diabetes development risks among nurses. The analysis results indicate that those nurses with a higher CCI were at a higher risk of developing diabetes than the reference group (CCI = 0). Those nurses with a CCI of ≧10 presented 28.48-fold higher relative risks of diabetes development than the reference group (95 % CI: 15.51-52.29). The research results shown in Table 5 indicate that diabetes incidence rates per 1,000 person-years for the nurses were lower than those of the non-nurses (2.76 ‰ vs. 3.25 ‰). The analysis results on the various variables reveal that with the exception of the ≦25-year age group, the incidence rate of diabetes per 1,000 person-years for nurses of all of the age groups was lower than that of the non-nurses.

Discussion
After the research population was collected from the NHI database, 1:1 propensity score matching was employed. The results of the bivariate analysis shown in Table 2 indicate that the nurses presented a lower risk of diabetes development than the non-nurses (2.68 % vs. 3.13 %), which is consistent with that of the Cox proportional hazards model (Table 3 and Fig. 1; Adj. HR: 0.83, 95 % CI: 0.78-0.89).
Previous studies [8][9][10] have determined that individuals who work in professions that require shift work present higher risks of developing diabetes. After propensity score matching, our nurses cohort and nonnurses control group were similar in terms of demographics, socioeconomic status, and health conditions. However, the nurses showed lower risks of developing diabetes than the non-nurses, and this may be attributed to nurses' medical knowledge and educational training. Furthermore, nurses are assume health promotion and education responsibilities [22] and are thus likely to live healthy lifestyles and to invest in their long-term health. This phenomenon is known as the healthy worker effect [23,24]. Nurses must be in excellent health to achieve effective performance; therefore, nurses are generally healthier than non-nurses. The aforementioned results may be correlated with knowledge, attitudes, and practice (KAP) theory principles [25,26]. Self-care levels depend on patient behaviors, and a patient's knowledge influences his or her attitudes, thereby affecting his or her practices. Acquiring correct knowledge about a disease and adopting positive and proactive attitudes are essential for motivating self-care behaviors, which directly or indirectly affect health outcomes. Nurses are equipped with comprehensive medical knowledge and thus possess a favorable view of personal health and diseases and a positive attitude toward medical care, resulting in superior self-care [27]. However, further study is needed to examine the effect of KAP theory on the nurses' incidence risk of developing diabetes.
The results presented in Tables 3 and 4 show that for the entire sample population, age and CCI variables most heavily influenced diabetes development risks. The older participants presented higher risks of diabetes development than the reference group (<25 y), complementing the results of previous studies [28][29][30], and this shows that glucose tolerance levels decrease with age; therefore, diabetes development risks increase rapidly among individuals who are older than 45 years of age. As shown in Table 5, a rapid increase in the incidence rate of diabetes per 1,000 person-years was observed among both nurses and non-nurses of 45 years of age and older. As shown in Tables 3 and 4, a higher CCI involves an increased relative risk of diabetes development, complementing the results of Huang [31] and Monami et al. [32]. Thus, CCI serves as a critical variable for predicting diabetes development risks.

Study limitations
There are several limitations to our analyses. According to the International Statistical Classification of Diseases (ICD), DM diagnosis is based on an ICD-9 diagnosis code. Therefore, clinical diagnostic data could not be acquired and verified. To optimize the accuracy of this study while compensating for this limitation, diabetes occurrence was identified when patients visited an outpatient department three times or when they were hospitalized more than once within 365 days of receiving a primary or secondary diabetes diagnosis (ICD-9-CM: 250 or A-code: A181) [17]. Some risk factors are not present in analysis. For instance, information on lifestyle, health behaviors and clinical testing data were not available and thus could not be used in variable analyses. So, we used the propensity score matching to control for selection bias. The propensity score adjustment is an important statistical technique to reduce the bias from confounding variables in observational studies and mimic the results of a randomized controlled trial [33]. In addition, the long follow-up and national design provided adequate power.

Conclusion
This study is the first to use a nationwide database to compare risks of diabetes development among female nurses and female non-nurses. The results show that nurses present a lower risk of developing diabetes than non-nurses. This result may be attributable to the fact that nurses possess superior medical knowledge and thus have lower incidence risk of developing diabetes.