Researchers at the Harvard School of Public Health analyzed dietary-intake data from more than 200,000 men and women in the Health Professionals Follow-Up Study and the Nurses' Health Studies. The participants have been tracked for a decade or more.
The scientists also did a larger analysis, combining their data and that from other published studies to analyze the diets of 442,101 people. About 28,000 of these people developed type 2 diabetes.
The researchers adjusted for the participants' age, weight, physical activity level, smoking, family history of diabetes and other dietary and lifestyle factors. Their findings are published today online in the American Journal of Clinical Nutrition:
•A 2-ounce serving a day of processed meat (hot dog, bacon, salami or bologna) increased the risk of diabetes by 50%.
•A 4-ounce serving a day (the size of a deck of cards) of unprocessed red meat such as hamburger, steak, pork or lamb was associated with a 20% increased risk of diabetes.
•Substituting nuts, whole grains and low-fat dairy such as yogurt for a serving a day of these types of processed or unprocessed meats lowers the risk of developing type 2 diabetes by 16% to 35%, the analysis showed.
"Many previous studies have shown the link between processed meats and diabetes, but this is one of the first (large studies) to show that unprocessed red meat is a significant risk factor," says senior author Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health.
"Clearly, processed meat is much worse than unprocessed meat for raising the risk but unprocessed red meat is not benign," he says. "This is the largest and most convincing data accumulated so far."
Hu says the high amount of sodium and nitrites in processed meats are potential factors that increase diabetes risk.
With red meat, it may be the high amount of heme iron, he says. Although iron helps prevent anemia, many people in the Western world have iron overload, which is a risk factor for diabetes, he says. "There are probably other factors in these meats that contribute to diabetes."
He advises reducing the consumption of these types of meats and incorporating more nuts and low-fat dairy and whole grains into meals.
Previous research has linked eating red meat and processed meat to an increased risk of heart disease and cancer, particularly colorectal cancer.
Registered dietitian Shalene McNeill, a spokeswoman for the National Cattlemen's Beef Association, says, "These are epidemiological studies, and they can't identify cause and effect. They are identifying associations, and what we know from gold-standard research that does look at cause and effect is that higher protein diets that include beef are very effective for helping people manage their weight and balance their blood sugars -- both important factors for reducing your risk of developing diabetes."
Diabetes afflicts more than 25 million adults and children in the USA. Most have type 2 diabetes. The long-term complications of the disease include heart attacks, blindness, kidney failure, nerve damage and amputations.
"Type 2 diabetes has a very strong genetic component, and multiple environmental factors, such as obesity, physical inactivity and poor diet, interact with genetics to increase the risk and accelerate the development of the disease," says Vivian Fonseca, president-elect of medicine and science for the American Diabetes Association and a professor of medicine at Tulane.
"People who are eating a lot of red meat and processed meat may not be eating as much nuts, beans and fish which may be protective. People who eat more of those foods tend to have less diabetes," Fonseca says.