A study involving more than a million people offers some of the best evidence of an association between diabetes (search) and an increased risk of cancer (search).

Researchers have found having elevated fasting blood sugar levels (search) increased the risk of dying from cancers of the pancreas (search) and liver (search), and several other malignancies.

Obesity (search) has been associated with many negative effects on health, including some types of cancer.

Because investigators took into account obesity, which is the biggest risk factor for type 2 diabetes (search), the findings indicate that having diabetes or being at risk for it is an independent risk factor for developing cancers.

The study was conducted in Korea, which has a much lower incidence of obesity and type 2 diabetes than the U.S. Only about a quarter of the population is overweight or obese, compared with more than half of Americans.

The research is reported in the Jan. 12 issue of the Journal of the American Medical Association.

“What this means is that the public health implications of this finding would be much greater for the United States than for Korea,” study author Jonathan Samet, MD, tells WebMD. “Certainly, obesity drives blood sugar and obesity is a known risk factor for certain cancers. So this could be one of the ways in which obesity increases this risk."

One in 31 Cancer Deaths

Approximately 150 million people worldwide have diabetes, but that number is expected to double within the next two decades as the population ages and becomes more obese and less active. While the association between diabetes and heart disease (search) is well established, the role of diabetes in cancer has been less well understood.

In this study, researchers examined the relationship between fasting blood sugar levels and diabetes and the risk of specific cancers in just under 1.3 million Koreans enrolled in a government-run health insurance plan. During 10 years of follow-up, there were 20,566 cancer deaths among the men in the study and 5,907 cancer deaths among women.

After controlling for known cancer risk factors such as smoking and alcohol use, researchers found that the men in the study with the highest fasting blood sugar levels (those greater than 140 mg/dl) were 29 percent more likely to die of cancer than men with the lowest levels (those less than 90 mg/dl). The difference among women with the highest and lowest blood sugar levels was 23 percent.

The association was strongest for pancreatic cancer (search), with high blood sugar and diabetes (defined as a fasting blood sugar greater than 125mg/dl) almost doubling the risk for men and more than doubling the risk for women. An increased risk with high blood sugar or diabetes was also found for colorectal cancer (search) and cancers of the esophagus and liver in men, and cancers of the liver and cervix in women.

The researchers estimated that 848 of the 26,473 total cancer deaths reported during the 10-year period could be attributed to diabetes and high blood sugar levels.

Lowering Risk

The latest population figures suggest that 64 percent of Americans are overweight or obese, defined as having a body mass index (search) (BMI) of 25 or more. Roughly a third of U.S. adults are obese, defined as having a BMI of 30 or more. BMI is determined by calculating height and weight. Being obese is strongly associated with numerous medical conditions and higher death rates.

According to the National Center for Health Statistics rates of obesity have increased from 12.8 percent in 1976-1980 to 22.5 percent in 1988-1994 and 30 percent in 1999-2000.

In an editorial accompanying the study, Kathleen Cooney, MD, and Stephen Gruber, MD, PhD, of the University of Michigan Medical School, wrote if current trends continue, 40 percent of Americans will be classified as obese within five years and type 2 diabetes rates will rise dramatically.

Obesity has long been considered a risk factor for many types of cancer, including postmenopausal breast cancer (search) and cancers of the pancreas, colon, endometrium, esophagus, kidney, gallbladder, and colon.

Cooney and Gruber noted that while the new findings directly implicate diabetes as a risk factor for many of these cancers, they don’t completely explain the obesity/cancer link.

“Several complex physiological changes also result from obesity, including alterations in sex steroid levels,” they wrote.

American Cancer Society epidemiologist Carmen Rodriguez, MD, says the findings strengthen the argument that keeping weight under control is key to preventing disease.

“This study clearly shows that diabetes increases the risk of certain cancers, and being overweight is a big risk factor for diabetes,” Rodriguez tells WebMD. “The message is clear. You can lower your risk for these cancers by controlling your weight and maintaining a healthy lifestyle that includes physical activity.”

By Salynn Boyles, reviewed by Brunilda Nazario, MD

SOURCES: Jee et al. Journal of the American Medical Association, Jan. 12, 2005; vol. 293: pp 194-202. Jonathan M. Samet, MD, MS, Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Md. Kathleen Cooney, MD, departments of internal medicine and urology, University of Michigan Medical School, Ann Arbor, Mich. Carmen Rodriguez, MD, MPH, epidemiologist; and spokeswoman, American Cancer Society, Atlanta.