Type 1 diabetes has been linked to increased risk of stomach, pancreatic, liver, kidney and endometrial cancers in the new study.
Study coauthor Dr. Stephanie Read, of the Usher Institute of Population Health Sciences & Informatics at the University of Edinburgh in the UK, and colleagues note that previous studies have suggested that people with diabetes have a 20-25% greater risk of cancer than those without diabetes.
However, they point out that the populations of such studies have primarily consisted of people with type 2 diabetes, given that it is the most common form of diabetes, accounting for 90-95% of all cases. This means the association between type 1 diabetes and cancer is less clear.
"It is possible that the relationship between type 1 diabetes and cancer is different from that observed between type 2 diabetes and cancer as a result of differences in the underlying disease characteristics, drug therapies and patterns of risk factors, such as obesity," the authors note.
As such, Dr. Read and colleagues compared cancer incidence among individuals with type 1 diabetes across five countries, with the aim of gaining a better understanding of the link between the two conditions.
Increased risk of site-specific cancers
The researchers identified people with type 1 diabetes - under the age of 40 - using nationwide diabetes registers from five countries: Australia, Denmark, Finland, Scotland and Sweden. They monitored these individuals for cancer incidence, identifying 9,149 cancers across 3.9 million person-years.
The team linked the data with information from national cancer registries in each country, allowing them to compare the cancer incidence of people with type 1 diabetes with that of the general public.
Looking at overall cancer risk, the researchers found that men with type 1 diabetes were at no higher risk than men without the condition. Women with type 1 diabetes, however, were found to have a 7% greater overall cancer risk than women without type 1 diabetes.
When it came to specific cancers, the researchers found that men with type 1 diabetes were 23% more likely to develop stomach cancer, while women with type 1 diabetes were at 78% greater risk for the disease, compared with the general public.
Both men and women with type 1 diabetes were also at greater risk for cancers of the liver (two-fold for men, 55% for women), pancreas (53% for men, 25% for women) and kidney (30% for men, 47% for women), while women also had a 42% increased risk for endometrial cancer.
Lower risk of prostate, breast cancers
However, the researchers also found that women with type 1 diabetes had a 10% reduced risk of breast cancer, while men with type 1 diabetes had a 44% reduced risk of prostate cancer, which the team says may explain why no increased overall cancer risk was identified among men with type 1 diabetes.
Fast facts about type 1 diabetes
- Around 1.25 million children and adults in the US have type 1 diabetes
- Type 1 diabetes is most commonly diagnosed in children and young adults
- The condition is primarily managed through multiple daily injections with insulin pens or syringes or an insulin pump.
While the authors cannot explain exactly why type 1 diabetes was associated with a lower risk of breast and prostate cancers, they have some theories.
Reduced risk of breast cancer among women with type 1 diabetes in the study may be down to the younger cohort, which consisted of fewer postmenopausal women among whom breast cancer is most common.
The lower risk of prostate cancer identified, the researchers speculate, could be down to lower testosterone levels often found among men with type 1 diabetes.
"Higher testosterone levels were previously shown to lead to an increased risk of prostate cancer, while hyperglycemia has also been shown to inhibit testosterone production," the authors explain.
The researchers also found that the risk of cancer was highest in the first year after a diabetes diagnosis; cancer incidence was 2.3 times higher for both men and women in the 12 months after being diagnosed with type 1 diabetes.
The team suggests that this finding is likely down to identification of pre-existing cancers soon after a diabetes diagnosis rather than a result of diabetes itself, noting that cancers are more likely to be detected when a patient is receiving more medical attention.
For most specific cancers, the risk reduced with increasing duration of type 1 diabetes, though the researchers note that the risk of endometrial cancer for women remained elevated for around 18 years after type 1 diabetes diagnosis.
For men, cancer incidence overall fell to a level comparable to that of the general public after around 20 years of having type 1 diabetes. For women, cancer incidence fell to the level of the general population after 5 years of having type 1 diabetes.
Increased cancer risk 'unlikely a result of insulin therapy'
Previous studies have suggested that insulin therapy may be to blame for increased cancer risk for diabetes patients, but Dr. Read and colleagues say their findings indicate this is not the case.
They explain that if insulin treatment was a key risk factor for cancer, their results would have shown a higher cancer incidence for people with type 1 diabetes than those with type 2, given that a significantly smaller number of people with type 2 diabetes are treated with insulin.
"Furthermore, the absence of an association between overall or site-specific cancer risk and increasing duration of diabetes in our study does not support a dose-response relationship between exogenous insulin use and cancer incidence," they add.
Instead, the researchers suggest the observed increased risk of certain cancers among people with type 1 diabetes may be down to shared metabolic deficiencies in people with type 1 and type 2 diabetes, such as high blood glucose levels.
"Hyperglycemia may be a plausible explanation given the identification of a dose-response relationship between glycated hemoglobin levels and the risk of certain cancers," they explain.
While these findings shed light on the association between type 1 diabetes and cancer risk, the authors say the results "do not support changing the policy for screening in persons with type 1 diabetes." They add:
"Similar recommendations for lifestyle approaches to reduce cancer risk such as weight management, increasing physical activity and avoiding smoking apply to persons with type 1 diabetes as for the general population.
Future work should be directed at ascertaining whether the increased incidence of some cancers among persons with type 1 diabetes leads to a raised risk of cancer mortality among persons with type 1 diabetes."
Last month, Medical News Today reported on the development of insulin-producing mini-stomachs that researchers say could offer a new cellular therapy for diabetes.