FormalPara Key Points

  • Cancer patients commonly live with additional chronic conditions, which may affect their prognosis and outcomes.

  • The international literature suggests that Indigenous peoples with cancer are more likely to have concomitant comorbidity than non-Indigenous peoples.

  • Diabetes, cardiovascular disease, and respiratory disease are the most common comorbid conditions among Indigenous peoples with cancer.

  • Diabetes and cancer commonly co-occur in Māori in Aotearoa New Zealand. Higher rates of co-occurrence are observed for Māori compared to non-Māori for most cancer types, particularly for gastrointestinal, endocrine, and obesity-related cancers.

People who are diagnosed with cancer commonly have additional coexisting chronic conditions, referred to as comorbidity [1]. Comorbidity has been associated with elevated excess mortality in cancer populations [2, 3]. Preexisting comorbidity may affect treatment options, decisions, and tolerance. Both existing and new comorbidities that develop during and after cancer treatment can impact opportunities to participate in clinical trials. As such, the personalization and optimization of cancer care requires a careful consideration of an individual’s comorbidity burden [1, 2].

Generally, Indigenous people with cancer have a higher prevalence and increased severity of preexisting comorbidity compared to non-Indigenous people with cancer. The measured prevalence of comorbidity is likely to depend on the population, clinical features, comorbidity measures, and data sources. In Australia, the reported prevalence derived from hospital admission data of having at least one comorbidity ranges from 10% in Aboriginal and Torres Strait Islander women diagnosed with breast cancer [4] to 61% in Aboriginal and Torres Strait Islander people diagnosed with any cancer type [5]. When using medical chart data, the estimates range from 37% in Aboriginal and Torres Strait Islander people diagnosed with head and neck cancers [6] to 60% in Aboriginal and Torres Strait Islander people diagnosed with non-small cell lung cancer [7].

In Aotearoa New Zealand, the prevalence of having at least one comorbidity ranges from 10% among Māori women diagnosed with cervical cancer [8] to 63% among Māori diagnosed with stomach cancer [9]. In the United States, 52% of American Indian and Alaska Native peoples with cancer had at least one comorbidity [10].

The most prevalent coexisting comorbid conditions in Indigenous populations are respiratory disease (range: 6–74%) [11, 12], cardiovascular disease (7–52%) [12], hypertension (11–47%) [13], and diabetes (12–45%) [6]. Generally, these conditions were more prevalent in Indigenous than non-Indigenous cancer populations. Survival inequalities between Indigenous and non-Indigenous cancer patients have been shown as partly due to the underlying elevated comorbidity burden in both Australia [11] and Aotearoa New Zealand [14]. Inequalities in intermediate factors, such as time from diagnosis to treatment, may also be affected by the presence of comorbidity, as shown in the United States [10].

The Co-occurrence of Diabetes and Cancer for Indigenous Māori in Aotearoa New Zealand

Indigenous Māori in Aotearoa New Zealand (NZ) are around 20% more likely to be diagnosed with cancer but nearly twice as likely to die from cancer in comparison with non-Indigenous New Zealanders [15]. In addition to experiencing a disproportionate burden of cancer, Māori and Pacific peoples are also more likely to develop diabetes. The prevalence of diabetes mellitus is increasing in NZ by 7% per year and is approximately three times higher among Māori and Pacific people than in European New Zealanders [16].

The co-occurrence of diabetes and cancer arises from a combination of factors:

  • Shared risk factors: obesity and physical inactivity are key risk factors for both diabetes and cancer. The majority of diabetes cases can be attributed to obesity [17], while around 25% of postmenopausal breast cancers and 20% of both uterine and colon cancers can be attributed to obesity [18].

  • People with diabetes are more likely to get some cancers: diabetes is linked to an increased risk of multiple cancers due to a combination of shared risk factors, insulin-resistance-related cell proliferation and dysfunction in programmed cell death, and chronic inflammation [19].

  • People with some cancers are more likely to get diabetes: in the other direction, those who develop cancer are more likely to also develop diabetes than those who do not develop cancer. For example, there is an increased risk of diabetes development among colorectal cancer patients [20].

  • Both conditions occur commonly: aside from being directly linked, the co-occurrence of diabetes and cancer is also related to both conditions being common—wherein an individual may have both conditions, without the two being linked at all.

The co-occurrence of cancer and diabetes has important consequences for cancer outcomes. The presence of diabetes can lead to delays in cancer diagnosis [21], increased risk of developing metastases [22], and reduced likelihood of receiving aggressive curative cancer treatment [23]. An Australian study showed that Indigenous Australians with cancer and diabetes were 40% more likely to die compared with Indigenous Australians with cancer but no diabetes (adj. hazard ratio: 1.4, 95% CI 1.1–1.8)—but found little evidence for differences by diabetes status for non-Indigenous patients (adj. hazard ratio: 0.8, 95% CI 0.5–1.2) [24].

A recent study in NZ examined differences in the rate of diabetes and cancer co-occurrence, both within and between ethnic groups [25]. To do this, national-level data on diabetes prevalence were extracted from New Zealand’s Virtual Diabetes Register, and national-level data on cancer registrations were extracted from the New Zealand Cancer Registry. Nearly five million individuals over 44 million person-years were used to describe the rate of diabetes and cancer co-occurrence, with an emphasis on comparing rates between Indigenous Māori and the majority European population.

The investigation found that the rate of cancer (including the majority of individual cancers) was highest for those with diabetes compared to those without diabetes—but more importantly, the rate of cancer and diabetes co-occurrence was much higher among Indigenous Māori compared to other ethnic groups (age-standardized rate (ASR): Māori 1304/100,000 person-years (PY); Europeans 1165/100,000; Pacific 949/100,000; other Asians 670/100,000; South Asians 474/100,000) [25]. When the study directly compared ethnic groups, only Māori with diabetes had a higher rate of cancer than Europeans with diabetes (rate ratio (RR): 1.12, 95% CI 1.08–1.16).

For Māori, the rate of cancer was higher for those with diabetes for the majority of cancer types (Fig. 48.1). The highest rate of cancer and diabetes co-occurrence for Māori was found for uterine (RR: 2.94, 95% CI 2.50–3.46), liver (RR 2.32, 95% CI 1.93–2.80), thyroid/endocrine (RR 2.10, 95% CI 1.64–2.68), gallbladder/biliary (RR 2.04, 95% CI 1.47–2.83), pancreatic (RR 1.97, 95% CI 1.62–2.40), kidney (RR 1.93, 95% CI 1.59–2.34), and stomach (RR 1.91, 95% CI 1.60–2.29) cancers.

Fig. 48.1
A forest plot between 24 different types of cancers and age-adjusted R R with 95% C I. The bars for prostate, cervix, and melanoma have a lower rate among those with diabetes, while uterus and liver have a higher rate among those with diabetes.

Forest plot comparing rates of individual cancers among Māori with diabetes compared to Māori without diabetes. (Source: Gurney et al. [25])

Full size image

As such, gastrointestinal, endocrine, and obesity-related cancers formed the bulk of those cancers that co-occurred most commonly among Māori. This finding strongly reinforces the need to prevent those risk factors that are shared between diabetes and cancer. It also reinforces the need for a multidisciplinary approach to the care of both diabetes and cancer, particularly for Indigenous populations who experience a disproportionate burden of both conditions.

Conclusion

The heightened risk of comorbidity in Indigenous peoples with cancer seems to partially explain elevated excess mortality. Diabetes, cardiovascular disease, and respiratory disease are particularly prevalent. Culturally responsive strategies for the prevention and early detection of these conditions after a cancer diagnosis may help address the persistent gaps in cancer-specific and overall survival experienced by Indigenous peoples.