PURPOSE:

Diabetes has been associated with increased risk of breast cancer in a number of epidemiologic studies, but its effects on survival among women diagnosed with breast cancer have been examined less frequently. Importantly, prior investigations have rarely considered the influence of factors associated with diabetes such as obesity, age at diabetes diagnosis, duration of diabetes, or diabetes treatments.

METHODS:

We evaluated the effect of self-reported diabetes on breast cancer incidence and mortality in the Long Island Breast Cancer Study Project, which includes 1,447 breast cancer cases and 1,453 controls. Follow-up data for all-cause (n = 395) and 5-year breast cancer-specific mortality (n = 104) through December 2005 were determined for case women from the National Death Index. Adjusted logistic regression and Cox proportional hazards models were used to estimate odds ratios (OR) and hazards ratios (HR), respectively.

RESULTS:

Postmenopausal women with diabetes were at increased risk of developing breast cancer [OR = 1.35; 95 % confidence interval (CI) = 0.99-1.85], as were those who were not of white race regardless of menopausal status [OR = 3.89; 95 % CI = 1.66-9.11]. Among case women, diabetes was associated with a modestly increased risk of death from all causes [HR = 1.65; 95 % CI = 1.18-2.29], an association that was stronger in women who were obese at breast cancer diagnosis [HR = 2.49; 94 % CI = 1.58-3.93]. In analyses restricted to diabetics, there was no statistically significant effect of duration of diabetes or type of treatment on breast cancer incidence or mortality.

CONCLUSIONS:

Our findings suggest that diabetes may increase incidence of breast cancer in older women and non-whites, and mortality due to all causes.