Mortality from diabetes mellitus, 2004 to 2008: A multiple-cause-of-death analysis

Jungwee Park and Paul A. Peters

Diabetes mellitus is one of the most common chronic diseases in Canada. It occurs when the body is either unable to sufficiently produce or properly use insulin.  Insulin, a hormone secreted by beta cells in the pancreas, enables the cells of the body to absorb sugar from the bloodstream and use it an energy source.Note1 People with type 1 diabetes mellitus produce little or no insulin; in type 2 diabetes mellitus, the pancreas continues to make insulin, but the body develops resistance to its effects, resulting in an insulin deficiency.

In 2008/2009, close to 2.4 million Canadians, about 7% of the population, were living with diagnosed diabetes.Note1 Although many of its complications are associated with mortality, diabetes mellitus itself is not usually reported as the primary cause of death.Note1 For example, it is a risk factor for vascular complications such as coronary heart disease, peripheral vascular disease, kidney disease and nerve damage,Note2 which are often listed as the underlying cause on the death certificate rather than diabetes mellitus.

In accordance with international conventions, a single underlying cause is identified as the disease or injury that initiated the train of events leading directly to death.Note3 But for conditions that are often accompanied by a number of comorbidities, there may be no direct etiologic chain to facilitate identification of a single underlying causeNote4,Note5; instead, a combination of multiple factors results in mortality. Thus, the choice of an underlying cause may obscure the contribution of chronic conditions like diabetes mellitus. To overcome this limitation, multiple cause-of-death statistics that include contributing causes as well as the underlying cause are used.  Such data provide a better understanding of how chronic conditions are associated with mortality.

Recently, Statistics Canada created a dataset that electronically captures underlying and contributing causes recorded on death certificates (see The data). These contributing causes may include conditions recorded as the immediate cause of death, diseases intervening between the underlying and immediate cause, and contributory conditions outside the sequence of conditions leading to death. Based on these new data, this analysis presents a more complete picture of diabetes mellitus as a cause of death from 2004 through 2008.

More likely to be contributing cause

In those five years, a total of 120,050 diabetes-mellitus-related deaths were recorded in Canada (Table 1). They accounted for 10.6% of all deaths that occurred in the period. Diabetes mellitus was the underlying cause of 37,041 of these deaths and a contributing cause for the remaining 83,009. Thus, the disease was more than twice as likely to be coded as a contributing cause rather than as the underlying cause of death.

Crude mortality rates for diabetes mellitus as the underlying cause were stable over the five years at around 24 deaths per 100,000 males and 21 deaths per 100,000 females (Chart 1). However, rates for diabetes mellitus as a contributing cause rose from 50 to 57 deaths per 100,000 males and from  44 to 50 deaths per 100,000 females, an increase of about 15%, compared with a 7% increase in crude mortality rates for causes not related to diabetes mellitus (data not shown).

The upturn in diabetes mellitus as a contributing cause of death may be associated with the increasing prevalence of the disease. Results from the Canadian Community Health Survey show that from 2003 to 2007/2008, the prevalence of diabetes mellitus rose from 4.6% to 5.8% of the population aged 12 or older (from 1.22 to 1.67 million) (data not shown). The growing number of people with the disease may have resulted in more deaths from complications rather than from diabetes mellitus as the underlying cause.Note1

Higher among males

Diabetes mellitus was more likely to be cause of death among males than among females. Over the 2004-to-2008 period, the crude mortality rate for diabetes mellitus as the underlying cause was 23.8 deaths per 100,000 males and 21.7 deaths per 100,000 females. As a contributing cause, the crude rates were 54.7 deaths per 100,000 males and 47.2 deaths per 100,000 females.

Rates rise with age

Regardless of whether diabetes mellitus was coded as a contributing cause or the underlying cause, the average age at death was around 76 (74 or 75 for men; 79 for women) (Table 2).

The crude mortality rate for diabetes mellitus as the underlying cause rose from 0.8 deaths per 100,000 people younger than 45 to 394.1 deaths per 100,000 people aged 85 or older. As a contributing cause, the crude mortality rate rose from 0.9 deaths per 100,000 people younger than 45 to 835.8 deaths per 100,000 people aged 85 or older. The percentage of deaths that were related to diabetes increased from less than 3% among people younger than 45 to 13% at ages 65 to 84, and then fell to 9% of deaths at age 85 or older (data not shown).

Higher income/Lower rates

Crude mortality rates for diabetes mellitus varied by neighbourhood income.Note7 The highest rates were in the lowest income quintile neighbourhoods (31.4 deaths per 100,000 population when diabetes was the underlying cause;  67.1 deaths per 100,000 when it was a contributing cause);  the lowest rates were in the highest income quintile neighbourhoods (17.2 and 39.2, respectively).

Geographic differences

Differences in provincial/territorial crude mortality rates for diabetes mellitus were substantial. For instance, the rate in Newfoundland was 45 deaths per 100,000 population, compared with 13 deaths per 100,000 population in Alberta. Rates tended to be relatively low in Prince Edward Island, Quebec, Alberta, British Columbia, Yukon and Northwest Territories. This geographic distribution of mortality reflects the provincial/territorial prevalence of diagnosed diabetes mellitus.Note1

Rarely single cause

Diabetes mellitus was rarely the single cause of death. It was the only cause for just 1.4% of the deaths reported over the 2004-to-2008 period. By contrast, the percentage of all deaths with a single cause during this period was 16%.

When diabetes mellitus was identified as the underlying cause, an average of 3.2 other causes were recorded on the death certificate. For deaths overall, the average number of contributing causes was 2.2.

When diabetes mellitus was a contributing cause, on average, 5 conditions were listed on the death certificate (an underlying cause and 3 other contributing causes). The average number of causes on death certificates overall was about 3.

Co-existing causes

Whether diabetes mellitus was the underlying cause or a contributing cause, cardiovascular diseases were the most common co-existing conditions listed on death certificates.  Nine out of ten deaths with diabetes mellitus identified as the underlying cause, also had cardiovascular diseases as a contributing cause (Table 3). Among these cardiovascular diseases, ischemic heart disease was reported most often (55%), followed by other cardiovascular diseases (52%), hypertensive heart disease (31%), and cerebrovascular disease (22%) (data not shown). Other contributing causes frequently reported for deaths from diabetes mellitus were genito-urinary diseases, neuropsychiatric conditions, respiratory diseases, endocrine disorders, respiratory infections, and digestive disorders.

When diabetes mellitus was a contributing cause, cardiovascular diseases were most often the underlying cause of death (45%). Diabetes mellitus was also commonly listed as a contributing cause in deaths from malignant neoplasms.

The close association between diabetes mellitus and specific diseases may reflect common risk factors. Diabetes mellitus and cardiovascular diseases share risk factors such as obesity, physical inactivity and poor nutrition. As well, the presence of diabetes mellitus is known to magnify the effects of other risk factors for diseases of the circulatory system, such as hypertension and smoking.Note2

Conclusion

Cause-of-death data are typically based on the underlying cause, not on other causes that contribute to the death. However, single-cause mortality statistics may obscure the contribution of chronic conditions like diabetes mellitus, which is rarely an underlying cause of death.

Based on the multiple-cause death file, this study found that from 2004 to 2008, diabetes mellitus was more than twice as likely to be coded as a contributing cause rather than the underlying cause of death. Diabetes-mellitus-related mortality rates were higher among males, older individuals, and residents of lower-income neighbourhoods. When diabetes mellitus was included on a death certificate, cardiovascular diseases were listed most often as either the underlying or a contributing cause of death.

Future research on diabetes-mellitus-related deaths will be enhanced as multiple-cause-of-death data become available for longer periods. Also, analytical power can be increased by linking these data to sources that provide additional information about the deceased.

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