Study co-author Dr. Theodore C. Friedman of Charles R. Drew University, Los Angeles, and colleagues used serum cotinine levels -- measures of exposure to tobacco smoke -- to verify passive smoking.
The researchers examined data from more than 6,300 adults who participated from 2001 to 2006 in the National Health and Nutrition Examination Survey, a nationally representative sample of the U.S. population.
Smokers, who made up 25 percent of the sample, were defined for the study as having a serum cotinine level greater than 3 nanograms per milliliter. Non-smokers, 41 percent of the sample, were those who answered "no" to the question "Do you smoke cigarettes?" and who had a cotinine level below 0.05 ng/mL.
Participants who answered "no" to the question of smoking, but whose cotinine level was above 0.05 ng/mL were defined as secondhand "smokers," or 34 percent.
After factoring for age, sex, race, alcohol consumption and physical activity, the researchers found secondhand smokers, when compared with non-smokers, had a higher measure of insulin resistance, a condition that can lead to type 2 diabetes. They also had higher levels of fasting blood sugar and a higher hemoglobin A1c, a measure of blood sugar control.
"This finding also showed that the association between secondhand smoke and type 2 diabetes was not due to obesity," Friedman said in a statement.
The findings were presented at The Endocrine Society's 94th annual meeting in Houston.