Having type 1 diabetes may raise the risk of some cancers, but lower the risk of others, a new study suggests.
Insulin treatment not a risk
A higher risk was seen for cancers of the stomach, liver, pancreas, endometrium, ovary and kidneys. But a reduced risk was seen for prostate and breast cancers, researchers reported.
In type 1 diabetes, the body cannot produce the hormone insulin, which helps regulate blood sugar levels. People with type 1 diabetes must be treated with insulin to survive.
The good news from this study, said researcher Sarah Wild, is that it doesn't seem that insulin treatment is responsible for an increased risk of some cancers, which has been a concern.
Read: Diabetes, cancer link strengthens
"This pattern of cancer risk [seen in the study] is similar to that seen for people with type 2 diabetes and people who are overweight," said Wild, a professor of epidemiology at the University of Edinburgh in the United Kingdom. "This suggests that insulin treatment for type 1 diabetes does not itself increase risk of cancer."
And, Wild pointed out, the new findings only show an association between type 1 diabetes and an increased risk for cancer, not that type 1 diabetes is a direct cause of the increased risk.
Another diabetes expert said it's not clear if there is a cause-and-effect relationship between type 1 diabetes and cancer risk.
"We need to take their [the authors of the new study] findings with caution," said Dr Joel Zonszein, director of the Clinical Diabetes Centre at Montefiore Medical Centre in New York City. "Patients with diabetes have enough problems, and they don't need cancer on top of their complications."
Read: Can pre-diabetes raise cancer risk?
He said the lack of a plausible reason why type 1 diabetes might cause an increase in certain cancers makes him suspect that there could be another explanation for the study findings.
It's possible, Zonszein said, that patients in this study were misdiagnosed or misclassified in the national registries used for this study. Some of them may have had type 2 diabetes rather than type 1 diabetes, he suggested. This confusion may have occurred because all the patients the researchers looked at were taking insulin, he said.
The report findings were published in the journal Diabetologia.
For the study, Wild and her colleagues collected data from national registries on more than 9,000 cancers among people with type 1 diabetes from Australia, Denmark, Finland, Scotland and Sweden. They compared people with type 1 diabetes to people in the general population of each country.
Read: Diabetics have higher risk of death from cancer
Looking at all cancers combined, Wild's team didn't find an increase in cancer risk for men with type 1 diabetes. However, women with type 1 diabetes had a 7 percent increased cancer risk, the study authors said.
Many cancers are rare
The lack of overall cancer risk among men with type 1 diabetes was mostly due to an apparent 44 percent decreased risk of prostate cancer, Wild said.
When data for sex-specific cancers – such as prostate and breast cancers -– were removed from the analysis, an increased cancer risk was seen in both men and women with type 1 diabetes. That increased risk was 15 percent for men and 17 percent for women, the researchers said.
Type 1 diabetes was linked to a 23 percent higher risk of stomach cancer for men and a 78 percent higher risk for women, the study found. For liver cancer, the risk for men with type 1 diabetes was doubled, while it was 55 percent higher for women, the study authors said.
However, women with type 1 diabetes were 10 percent less likely to develop breast cancer, Wild said.
Read: DNA connection between diabetes and blood cancer
Because many of these cancers are rare, the actual risk is slight, Wild added.
Also, the risk for cancer appeared to be highest shortly after diagnosis of type 1 diabetes, she said. During the first year after a diabetes diagnosis, the cancer risk was more than doubled for both men and women. The longer someone had type 1 diabetes, the lower the odds of cancer diagnosis, she said.
Lifestyle changes reduce cancer risk
After about 20 years, the cancer risk dropped to that of the general population for men. For women, it took only five years for the cancer risk to drop to almost normal, the study found.
The reported elevated cancer risk soon after type 1 diabetes diagnosis may owe to the detection of pre-existing cancers, the researchers suggested.
Although type 1 diabetes hasn't been linked to lifestyle factors like type 2 diabetes has, Wild noted that lifestyle changes might help reduce cancer risk.
"Lifestyle changes to reduce cancer risk – such as avoiding smoking, [and improving] weight management and physical activity – are important for people with type 1 diabetes, particularly because several of the cancers are ones where these factors affect risk," Wild said.
What is diabetes?
Symptoms of diabetes
Treating type 1 diabetes