Diabetes is already linked to a number of complications, but emerging evidence suggests an increased risk of cancer can be added to that list.
A new study found that women with diabetes had an 8% increased risk of developing cancer generally, while men with diabetes had a 9% higher risk when rates of prostate cancer were excluded from the calculation.
The risk of dying from a cancer was also higher in people with diabetes - 11% greater for women and 17% higher in men.
Diabetes and cancer in men
"We used a prospective cohort to evaluate the relationship between diabetes and cancer risk," said the study's lead author, Gabriel Lai, a cancer prevention fellow at the US National Cancer Institute. "Diabetes was associated with an 8% increase in cancer risk in women, and there was a similar pattern in men, except for prostate cancer," said Lai.
For reasons that remain unclear, diabetes was actually associated with a lower incidence of prostate cancer in men, the study found. When rates of prostate cancer were included in the mix, diabetic men's odds for cancer generally were reduced by 4%.
But once the statistics on prostate tumours were factored out, men with diabetes were found to have a 9% higher risk for cancer overall, compared to non-diabetic men.
The study included data from the National Institutes of Health-AARP Diet and Health Study that included 295,287 men and 199,665 women from eight states (California, Florida, Georgia, Louisiana, Michigan, North Carolina, New Jersey and Pennsylvania).
Diabetes was self-reported by the study participants, but they didn't note whether or not they had type 1 or type 2 diabetes. However, Lai said that in this population, the majority would have type 2, the more common form. The researchers also were unable to evaluate diabetes management or different medications, as this data wasn't included in the initial study.
After 11 years of follow-up, 55,888 men and 26,364 women had developed cancer.
Liver cancer higher in diabetics
The risk of liver cancer was increased more than two-fold in people with diabetes, according to the study. The risk of cancer of the rectum was increased by 28% in people with diabetes, and the risk of colon cancer was increased by 15%.
In men, the risk of pancreatic and bladder cancers was increased in those with diabetes. In women, stomach, anus and uterine cancer risk was greater in those with diabetes.
No association was found between lung, skin and other cancers and diabetes in this study.
Lai said it's not clear what the mechanism behind these increased risks might be, but said there are numerous possibilities. "It's important that more studies are done," he added.
Dr Joel Zonszein, director of the clinical diabetes centre at Montefiore Medical Center in New York City, agreed that there could be many causes. "There are so many risk factors for cancer. Is it the way they eat, inactivity, socioeconomics? We really don't know what the cause is," he said.
But, he added, "Those with diabetes need to be aware that they are at a higher risk of certain cancers, and they have to be screening for cancer. Also, modifiable risk factors, like smoking, should be stopped."
Lifestyle changes help diabetics avoid cancer
Lai recommended lifestyle modifications, such as eating a healthful diet and exercising, to prevent diabetes and cancer. "There are a lot of risk factors that are very similar among the two diseases. Maybe avoiding diabetes may be even better for avoiding cancer risk. In general, there are a number of benefits in avoiding diabetes, including the possibility of reducing morbidity and mortality in cancer," he said.
Another study, also scheduled for presentation at the AACR meeting, linked metabolic syndrome to an increased risk of liver cancer. Someone with metabolic syndrome has three or more of the following conditions: high blood pressure, elevated waist circumference, high triglycerides, increased fasting blood sugar levels and low HDL (good) cholesterol levels.
People with this condition are known to have an increased risk of heart disease, but the current study, which analysed more than 4,000 people with liver cancer and compared them to nearly 200,000 people without cancer, found that people with metabolic syndrome were even more likely to develop liver cancer.
The study found that 37.1% of people with a type of liver cancer called hepatocellular carcinoma had metabolic syndrome, while only 17.1% of those without liver cancer had metabolic syndrome. Nearly 30% of people with another type of liver cancer called intrahepatic carcinoma had metabolic syndrome.
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