This study found a dose-response relationship between childhood CPA and CSA and diabetes in adulthood. The strongest association with diabetes was observed when both severe and frequent CPA and CSA were reported. CEIPV was not significantly associated with having diabetes in adulthood. Tests for gender and age differences in associations between CM and diabetes yielded non-significant results with the exception of CSA where a negative interaction was observed with age (65 years or older). This observed negative interaction between age and CSA could be due to under-reporting of CSA by older persons or premature mortality among victims of CSA [40].

Very few studies have examined associations between CM and diabetes using large population-based samples. Four studies based on relatively large samples used binary variables to measure CPA and CSA, not taking severity or frequency into consideration [7, 20, 22, 23]. Two of these studies, one based on close to 6000 participants from the US National Comorbidity Survey [20] and the other based on more than 9000 participants from a British birth cohort study [23] did not find significant associations for either CSA or CPA in relation to diabetes. A study based on 21,000 respondents aged 60 or older from Australia [7] found a significant association for CPA but not CSA. Similarly, a study based on more than 18,000 participants from ten countries [22] found a significant association for CPA but not CSA. These others studies did not have the multiple behavioural-specific questions used in the CCHS - MH and in some cases CM was based on a single subjective question (e.g., were you physically abused as a child?).

Similar to our approach, some studies have examined diabetes in relation to CM using abuse measures that account for severity and frequency. Using a large representative sample from the American population, Afifi [16] found a significant association between CPA (defined as responding “sometimes” or more frequent to having been hit so hard it left marks or bruises or caused injury) and diabetes but not with severe physical punishment (defined as responding “sometimes” or more frequent to how often a parent pushed, grabbed, slapped or hit you). Based on data for young adults aged 24–34 years from the National Longitudinal Study of Adolescent Health, Duncan et al. [19] found a significant association between recurrent CSA (> = 3 times) and diabetes for men but not for women and no association between CPA and diabetes for either sex. Results from a study using longitudinal data from the Nurses’ Health Study II [21] found a dose-response relationship between childhood CPA and CSA and incident type 2 diabetes similar to our findings. Mild CPA was not associated with diabetes risk, while moderate and severe CPA were associated with 26 and 54 % higher risk of incident diabetes. Unwanted sexual touching was associated with a 16 % higher risk of incident diabetes, one episode of forced sexual activity with a 34 % higher risk, and more frequent forced sexual activity with 69 % higher risk.

Similar to the study by Rich-Edwards [21], to some extent the associations between CPA and CSA and diabetes in our study were mediated by risk factors for type 2 diabetes. Adult obesity, smoking, and hypertension were important mediators in the association between CPA and diabetes. Controlling for all risk factors for type 2 diabetes simultaneously resulted in a 41 % reduction in the odds of reporting diabetes in relation to severe and frequent CPA and the association was no longer statistically significant. For CSA, there was less of a mediation effect when controlling for type 2 diabetes risk factors. Only adult obesity and smoking resulted in any appreciable reduction in the association between severe and frequent CSA and diabetes.

Another potential pathway that may explain the relationship between CM and type 2 diabetes is via the stress or trauma experienced by victims of CM. Results from clinical studies suggest that stressful experiences in early life result in frequent activation of the hypothalamic-pituitary-adrenal axis [41, 42]. This in turn can result in elevated cortisol levels and have lasting effects on the body’s stress-response system, including a heightened glucocorticoid, norepinephrine, and autonomic response [41, 43]. These changes can lead to insulin resistance, which in turn can cause increases in the blood glucose level, eventually resulting in type 2 diabetes [44]. Some studies have suggested that stressful experiences in general may be associated with the onset of diabetes [35, 36]. However, a meta-analytic review examining associations between adverse psychosocial factors (including stressful events) and diabetes found significant associations with the prognosis of diabetes but not incident diabetes [45]. In our study associations persisted when controlling for current perceived life stress, suggesting a unique association between CM and diabetes as opposed to an association with stress in general.

Strengths and limitations

A major strength of this study is the large representative sample of Canadian adults. Also, the array of variables collected in the 2012 CCHS - MH made it possible to examine the mediating effects of several risk factors for type 2 diabetes and to control the potentially confounding effects of numerous socio-demographic factors when examining associations between CM and diabetes. In addition, the 2012 CCHS - MH included several CM questions making it possible to examine three types of CM in relation to diabetes as well as to consider the severity and frequency of maltreatment. Furthermore, the CCHS - MH CM items are behaviorally-specific and thus are likely to have higher validity and reliability than broad and subjectively defined items [46–49].

This study has some limitations that should be considered when interpreting results. All information collected was based on self-reports. A review of the literature on the validity of adult retrospective reports of adverse childhood experiences indicates that the rate of false negatives can be substantial, and that false positive reports are rare [47]. A study examining the psychometric properties of the CEVQ items concluded that it is a reliable and valid instrument with considerable agreement between self-reported CPA and CSA (including severe forms) and independent reports from clinicians [29]. Although the use of the behaviorally-specific CM items used in the 2012 CCHS - MH may have reduced the rate of false negatives in this study, the assessed types of CM may still have been underestimated due to recall bias. As well, it is possible that individuals who experienced CM who currently perceive themselves as being in good health are less likely to report the maltreatment. For CSA, items that separated attempted forced sexual activity from actual forced sexual activity would have allowed a more complete analysis of CSA severity.

In the 2012 CCHS - MH, respondents were asked to report on long-term health conditions lasting six months or more and that had been diagnosed by a health professional. Although misreporting could introduce bias, validity studies have found high agreement between self-reported diabetes and medical records [50, 51]. No information was collected about the specific type of diabetes (type 1, type 2, or gestational diabetes). However, previous studies have shown that that 90–95 % of diabetes cases in Canada are type 2 [3]. Finally, it is possible that some respondents may have had diabetes that had not yet been diagnosed by a health care professional. A study based on plasma glucose readings using data from the 2007 to 2009 Canadian Health Measures Survey found that 0.9 % of the Canadian population aged 6 or older had undiagnosed diabetes, representing 20 % of all cases of diabetes [3].

It is unknown how these limitations of the diabetes and CM measures might influence associations. Furthermore, it is possible that use of more objective measures of the risk factors for type 2 diabetes (e.g., measured BMI) might result in further attenuations in associations between CPA/CSA and diabetes. As well, family history of diabetes and abnormal lipid profile were not measured and could account for some residual variance.

The cross-sectional nature of the data precludes establishing the temporal order of events and conclusions regarding the causal nature of associations. However, a study comparing the associations between CM and adverse health outcomes in adulthood concluded that retrospective and prospective studies yield similar results [52]. When testing for mediation, it is assumed that the mediation variable is in the pathway between CM and the diagnoses of diabetes. This may not always be the case. For example, the association between depression and diabetes is complex; some studies have found depression is associated with incident type 2 diabetes [12, 13], while others have found that type 2 diabetes precedes depression [12, 53]. If the latter is true, it would be inappropriate to consider depression as a mediating variable.

Finally, the degree to which findings in this study may be attributable to unmeasured factors such as childhood socioeconomic status and other childhood family adversities such as neglect, emotional abuse, and parental mental and substance abuse disorders is unknown. However, analysis of the data from the Nurses’ Health Study II considered several early childhood covariates as potential confounders (including birth weight, parental history of diabetes, and parental education and occupation) and the observed associations between CM and diabetes persisted [21].

Implications

Diabetes is the sixth leading cause of death in Canada [54] and reduces the health-related quality of life for those living with the disease [55]. Individuals with diabetes are at risk for a number of long-term and life-threatening complications including heart disease, stroke, blindness, kidney disease, and lower-limb amputation [56]. Based on the results from this study, associations between CM and diabetes were the strongest for repeated and severe incidents of childhood CPA and CSA. Failure to consider severity and frequency of CPA and CSA may limit our understanding of the importance of CM as a risk factor for diabetes. Early intervention is critical to reduce the risk that people who have experienced CM will develop this debilitating disease.