Framingham, MA - A new analysis of the Framingham Offspring Study shows that the incidence of type 2 diabetes has doubled in the US over the past 30 years [1].

Most, but not all, of the increase occurred in people who were obese, lead author Dr Caroline S Fox (National Heart, Lung, and Blood Institute's Framingham Heart Study, MA) told heartwire. The research is published online June 19, 2006 in Circulation.

Although there are existing data on the prevalence of diabetes, information on incidence rates is sparse, Fox and colleagues state. "To the best of our knowledge, these data are among the first to show trends in incidence rates of diabetes in a community-based, long-term epidemiological study."

They analyzed data from 3104 participants (ages 40 to 55, of whom around 50% were women) in the Framingham Offspring Study who were diabetes-free at baseline. All participants received a routine physical during three time periods: the 1970s (1971-1975), the 1980s (1979-1983), and the 1990s (1987-1991). They were followed for eight years to track new cases of type 2 diabetes, which was defined as fasting plasma glucose >7.0 mmol/L or treatment with insulin or a hypoglycemic agent.

The odds of developing diabetes increased by 40% from the 1970s to 1980s and then doubled between the 1970s and 1990s. Analysis by gender revealed an 84% increase in diabetes incidence among women during the 1990s compared with the 1970s. In men, diabetes more than doubled during the 1990s compared with the 1970s.

Odds ratio (OR) of incident diabetes during the 1980s and 1990s compared with the 1970s

Fox told heartwire that pinpointing the incidence helps obtain a sense of the magnitude of the problem. "Prevalence data provide a snapshot of the burden at any one time, but incidence—the estimate of new cases in a given year—provides us with an assessment of how new cases accrue over time."

As people have been living longer with diabetes, the prevalence has gone up, but there was little prior information about whether the actual incidence of diabetes had changed, she notes.

Most, but not all, of the increase in diabetes occurred in people who were obese—those with a body-mass index (BMI) of 30 or more. Adjustment for BMI reduced the odds of incident diabetes by around 30%, but the temporal effect remained significant, suggesting that other factors were also playing a role.

One explanation might be that BMI is not the best overall measure of adiposity to reflect diabetes risk, Fox and colleagues note.

"Measures of central adiposity, specifically waist circumference and visceral adipose tissue compartments, are more closely related to diabetes risk," they explain. Unfortunately, measures of waist circumference were not available for the earlier decades of the study. Other reasons for the increased incidence of diabetes could be poor diet and/or lack of exercise, but neither of these was assessed in this study.

Fox says the findings show that the proportion of obese people with diabetes continues to rise, and it is not known how this will affect the morbidity and mortality associated with diabetes.

Her team plans to continue to monitor the new cases of diabetes in this data set. "Incidence rates are more helpful for the continued surveillance of diabetes as obesity rates continue to rise," she concludes.