Despite the progress that has been made in treatment and survival, a cancer diagnosis remains one of the most impactful medical events that a person can experience. A new diagnosis of cancer often dominates a person's health concerns and may overshadow other health issues such as chronic illnesses, which may seem less salient in the context of cancer. The relationship between cancer and comorbid chronic illness is a critical area for health research, as an increasing number of cancer patients are living with common chronic illnesses such as type 2 diabetes mellitus (DM),1 and as many as one quarter of cancer patients die from their comorbidities.2

Outcomes for DM and other chronic illnesses are often dependent on patients’ consistent adherence to self-management behaviors, such as medication adherence, diet, and physical activity. Patients’ level of engagement in key DM self-management behaviors is at least partly determined by their beliefs about the severity, chronicity, consequences, and other aspects of the illness. A number of studies have found that certain beliefs about DM are associated with better or poorer adherence to self-management behaviors recommended for DM control in both cross-sectional3 and longitudinal4 investigations. Perceived severity of DM consequences has been related to poorer self-management adherence, whereas greater personal control over DM was associated with better adherence.5 Another study found that greater DM knowledge was positively correlated with better adherence to recommendations for foot care, diet, and glucose testing, whereas more severe perceptions were correlated with poorer diet and exercise adherence.6

Major changes to patients’ health status, such as a new diagnosis with cancer, may affect attitudes toward preexisting conditions such as DM, as the emotional impact of one illness may overshadow another or as patients compare differing perceived risks and consequences. This interplay of illness beliefs about cancer and DM could in turn affect cancer patients’ DM self-management behaviors and outcomes for DM following a cancer diagnosis.

Type 2 DM is a comorbidity of unique concern in the context of cancer due to well-established biological pathways that increase risk of several types of cancer. Adults with DM are more likely to be diagnosed with pancreatic, liver, colorectal, breast, and other cancers and are at greater risk of mortality from these malignancies.7,8 Moreover, there is evidence that DM treatment with insulin is associated with greater risk of cancer.7,9 Poor glycemic control in DM is associated with poorer cancer-related outcomes, and preexisting DM diagnosis has been linked to greater overall mortality risk in patients diagnosed with cancer.9

In turn, there is evidence that the psychological experience of cancer diagnosis and treatment can affect DM control. Based on previous research, there are two competing expectations for how a cancer diagnosis might affect DM beliefs and self-management behaviors. Several studies have characterized a cancer diagnosis as a “teachable moment” for health behavior, based on findings that a cancer diagnosis motivates some individuals to prioritize their health and adopt healthier behaviors such as improved diet and exercise.10, 11, 12 Although these studies have not focused on DM self-management behaviors specifically, if individuals with comorbid DM respond to a cancer diagnosis with these lifestyle changes, their DM management and outcomes could be expected to improve. There is evidence that patients with a history of cancer may perform better on some clinical measures of DM control than patients with no cancer history.13

On the other hand, a cancer diagnosis may have a negative effect on DM beliefs and, therefore, on DM self-management behaviors. The negative mental health effects of a cancer diagnosis have been extensively documented,14,15 and stress and poor mental health are associated with worse self-efficacy, which in turn interferes with effective self-management behaviors and perceived personal control over illness. Depressive symptoms16 and greater perceived stress17 have been linked to poorer self-management self-efficacy in DM and other chronic illnesses. Consistent with this expectation, one large study found that individuals with DM diagnosed with cancer are less engaged in DM care after their cancer diagnosis.18 Furthermore, DM itself can be a source of distress for many patients,19 and the added psychological effect of a cancer diagnosis combined with preexisting chronic illness distress could potentially have critical effects on an individual's perceptions of their health and, therefore, their health decisions.

While there is evidence that cancer treatment has a negative effect on DM self-management behaviors, including exercise, diet, and blood sugar monitoring,19 there remains a gap in knowledge of how this effect develops over the long-term experience of cancer, from initial diagnosis through treatment to remission and maintenance.20 Understanding this longitudinal process could be crucial for providers to better detect and intervene on potentially maladaptive illness beliefs and prevent declines in self-management behaviors during cancer treatment.

To our knowledge, no research has been conducted on the relationship between condition-specific illness perceptions in patients with multiple conditions or on the effect of time from diagnosis on beliefs about cancer and comorbid chronic illness. Given the evidence that emotional representations of illness and illness distress are particularly impactful on self-management behaviors and health outcomes, understanding the effects of newly diagnosed cancer on perceptions of preexisting chronic illness may be critical area of knowledge for primary care and oncology practitioners alike. This study sought to explore how initial diagnosis and treatment of early-stage cancer affect illness beliefs among patients with preexisting DM and whether these beliefs change over time. We examined beliefs about cancer and DM within patients with both conditions and compared DM beliefs between patients with and without cancer to determine whether the experience of cancer diagnosis and treatment affected perceptions of preexisting DM.