Smoking may increase the risk of developing diabetes, according to new research by investigators at Wake Forest University School of Medicine and colleagues.

The surprising finding emerged when researchers examined the relationship between smoking and diabetes among participants in a major national study, the Insulin Resistance Atherosclerosis Study (IRAS). They compared the incidence of diabetes after five years among smokers and those who had never smoked.

Twenty-five percent of the participants who smoked and did not have diabetes when the study began had developed diabetes by the five-year follow-up, compared to 14 percent of the participants who had never smoked, according to Capri G. Foy, Ph.D., and her colleagues at the national IRAS coordinating center at the School of Medicine, part of Wake Forest University Baptist Medical Center.

Reporting in the journal Diabetes Care, the researchers found that when the analyses were adjusted to account for other diabetes risk factors, "smokers still exhibited significantly increased incidence of diabetes compared to people who had never smoked," Foy said. "These findings suggest another poor health outcome associated with cigarettes, supporting current surgeon general's warnings against cigarette smoking."

Smoking has long been associated with heart disease, as is diabetes, and Foy noted that diabetes and heart disease share many risk factors.

IRAS focused on a prediabetic condition called insulin resistance, in which increasing amounts of insulin are needed to digest the same amount of glucose, the principal product of the metabolism of carbohydrates.

Other Wake Forest IRAS investigators had reported in the American Heart Association Journal Circulation back in 1996 that insulin resistance is associated with substantially increased atherosclerosis, which involves the buildup of fatty substances, cholesterol, and other substances in the walls of the arteries. The study found that increased thickness of the walls of the carotid artery in the neck suggested that insulin resistance might be an independent risk factor for heart disease.

Since that report, IRAS investigators have been asking other research questions, based on extensive examinations at the start of the study - two four-hour visits scheduled one week apart that directly measured indicators such as glucose tolerance, body mass index (BMI), cholesterol and high blood pressure, comparing those results with a follow-up examination five years later. The initial examinations also included questions about smoking.

Foy said that another strength of IRAS was that it had roughly equal numbers of men and women and roughly equal numbers of African-Americans, Hispanics and whites recruited from Los Angeles and Oakland, Calif., San Antonio, Texas and the San Luis Valley area of Colorado.