Among the 4572 participants, we identified a total of 1386 current smokers at baseline, 621 previous smokers, 1452 never smokers with reported passive tobacco smoke exposure or cotinine concentration 1-15 ng/ml, and 1113 never smokers with no passive tobacco smoke exposure reported (). The 1386 current smokers smoked a mean of 10 (SD 8.7, range 0-62.5) cigarettes a day. The 1452 never smokers with positive passive smoke were exposed to a mean of 12.6 (SD 18.0) hours of smoke a week. At baseline, the mean age of participants was 25 (SD 3.6) years, 2529 (55%) were women, and 2283 (50%) were African-American. Smokers and never smokers with passive smoke exposure were more likely to be African-American and less likely to be women than were never smokers with no passive smoke exposure (). Current smokers also had lower education, drank more alcohol, and had higher fat intake compared with never smokers with no passive smoke.
Baseline characteristics of 4572 CARDIA participants by baseline (1985-6) tobacco exposure. Values are mean (SD) unless stated otherwise
Incidence of glucose intolerance and smoking or tobacco smoke exposure
In Kaplan-Meier analysis, overall incidence of glucose intolerance was 9.2% (95% confidence interval 8.4% to 10.1%) at 7 years of follow-up, 12.8% (11.9% to 13.9%) at 10 years, and 16.7% (15.5% to 17.8%) at 15 years. Fifteen year Kaplan-Meier incidence of diabetes during follow-up was 3.1% (2.6% to 3.7%). Across categories of tobacco exposure, 15 year incidence of glucose intolerance was greatest among current smokers and lowest among never smokers with no exposure to passive smoke (). Never smokers with exposure to passive tobacco smoke had an intermediate incidence. The risk of previous smokers was statistically similar to that of never smokers with no passive smoke exposure.
Prospective 5, 7, 10, and 15 year incidences (percentages) of glucose intolerance by baseline tobacco exposure: CARDIA study, 1985-2001
In the primary Cox proportional hazards model, current smokers had a higher risk of glucose intolerance than never smokers with no passive smoke exposure, after adjustment for baseline sociodemographic factors (age, sex, race, years of education, family income (year 5)) and for baseline biological and behavioural factors—systolic blood pressure, triglycerides, alcohol consumption, body mass index, and change in smoking (starting for never smokers and previous smokers and stopping for current smokers) collected during follow-up (). Never smokers with positive passive smoke exposure also had a greater risk of developing glucose intolerance, compared with never smokers with no positive passive smoke after adjustment.
Association of smoking and passive tobacco smoke with incidence of glucose intolerance over 15 years of follow-up among 4572 CARDIA participants: unadjusted and adjusted hazard ratios from Cox proportional hazards analysis
In a separate proportional hazards model, the unadjusted point estimate of the risk of development of diabetes was 1.58 (95% confidence interval 0.94 to 2.63) for smokers and 1.40 (0.84 to 2.33) for never smokers with positive passive smoker exposure, compared with never smokers with no passive smoke exposure. Further adjustment for variables as in the primary model did not alter the significance or direction of these results.
Pack years as a marker for amount of tobacco exposure
Consistent with our primary analysis, increasing pack years of smoking over time among the 4572 participants was associated with an increased risk of developing glucose intolerance. After adjustment, for every increase in 10 pack years of smoking the risk of developing glucose intolerance increased by 18% (hazard ratio 1.18, 1.02 to 1.36).
Incidence of glucose intolerance by race-sex subgroups
Incidence of glucose intolerance by tobacco exposure varied among race-sex subgroups (). The association between current smoking and glucose intolerance seemed to be stronger in white people than in black people for both women and men (P < 0.001 for overall interaction with race-sex).
Stratified analysis by race and sex of 15 year Kaplan-Meier incidence and proportional hazards of glucose intolerance by tobacco exposure: CARDIA study, 1985-2001
Impact of potential mediators—waist:hip ratio and baseline insulin
Waist:hip ratio, used as a time dependent variable, was associated with increased risk of developing glucose intolerance. Each increase in the ratio by 0.01 unit was associated with an increase of 8% in risk of glucose intolerance (hazard ratio 1.08, 1.08 to 1.09). Serum insulin (hazard ratio per 1 ng/dl increase 1.04, 1.03 to 1.04) and year 7 C reactive protein (hazard ratio per 10 mg/dl increase 1.08, 1.04 to 1.12) were also associated with increased risk. When waist:hip ratio was added into the primary adjusted model above, the associations of increased incidence of glucose intolerance with current smoking (hazard ratio 1.53, 1.18 to 1.99) and passive smoke exposure among never smokers (1.28, 1.01 to 1.62) were essentially unchanged. When serum insulin and C reactive protein, and subsequently all three variables, were added, the associations were again essentially unchanged, although significance for never smokers with passive smoke exposure was borderline (hazard ratio 1.26, 0.99 to 1.61; P = 0.06).