Type 2 diabetes and obesity in midlife and breast cancer risk in the Reykjavik cohort

Purpose: As obesity and type 2 diabetes (T2D) have been increasing worldwide, we investigated their association with breast cancer incidence in the Reykjavik Study.

Methods: During 1968-1996, approximately 10,000 women (mean age = 53 ± 9 years) completed questionnaires and donated blood samples. T2D status was classified according to self-report (n = 140) and glucose levels (n = 154) at cohort entry. A linkage with the Icelandic Cancer Registry provided breast cancer incidence through 2015. Cox regression with age as time metric and adjusted for known confounders was applied to obtain hazard ratios (HR) and 95% confidence intervals (CI).

Results: Of 9,606 participants, 294 (3.1%) were classified as T2D cases at cohort entry while 728 (7.8%) women were diagnosed with breast cancer during 28.4 ± 11.6 years of follow-up. No significant association of T2D (HR 0.95; 95% CI 0.56-1.53) with breast cancer incidence was detected except among the small number of women with advanced breast cancer (HR 3.30; 95% CI 1.13-9.62). Breast cancer incidence was elevated among overweight/obese women without (HR 1.18; 95% CI 1.01-1.37) and with T2D (HR 1.35; 95% CI 0.79-2.31). Height also predicted higher breast cancer incidence (HR 1.03; 95% CI 1.02-1.05). All findings were confirmed in women of the AGES-Reykjavik sub-cohort (n = 3,103) who returned for an exam during 2002-2006. With a 10% T2D prevalence and 93 incident breast cancer cases, the HR for T2D was 1.18 (95% CI 0.62-2.27).

Conclusions: These findings in a population with low T2D incidence suggest that the presence of T2D does not confer additional breast cancer risk and confirm the importance of height and excess body weight as breast cancer risk factors.