The study compares the risk of incident diabetes associated with fasting plasma glucose levels in the normal range, controlling for other risk factors.

We identified 46,578 members of Kaiser Permanente Northwest who had fasting plasma glucose levels less than 100 mg/dL between January 1, 1997, and December 31, 2000, and who did not previously have diabetes or impaired fasting glucose. After assigning subjects to 1 of 4 categories (<85, 85-89, 90-94, or 95-99 mg/dL), we followed them until they developed diabetes, died, or left the health plan, or until April 30, 2007. We used Cox regression analysis to estimate the risk of incident diabetes, adjusted for age, sex, body mass index, blood pressure, lipids, smoking, cardiovascular disease, and hypertension.

Subjects developed diabetes at a rate of less than 1% per year during a mean follow-up of 81.0 months. Each milligram per deciliter of fasting plasma glucose increased diabetes risk by 6% (hazard ratio [HR] 1.06, 95% confidence interval [CI], 1.05-1.07, P < .0001) after controlling for other risk factors. Compared with those with fasting plasma glucose levels less than 85 mg/dL, subjects with glucose levels of 95 to 99 mg/dL were 2.33 times more likely to develop diabetes (HR 2.33; 95% CI, 1.95-2.79; P < .0001). Subjects in the 90 to 94 mg/dL group were 49% more likely to progress to diabetes (HR 1.49; 95% CI, 1.23-1.79; P <.0001). All other risk factors except sex were significantly associated with a diabetes diagnosis.

The strong independent association between the level of normal fasting plasma glucose and the incidence of diabetes after controlling for other risk factors suggests that diabetes risk increases as fasting plasma glucose levels increase, even within the currently accepted normal range.