Diabetes and breast cancer are both extremely common conditions in women and may share common risk factors. It is natural to investigate any potential common risk factors and to seek biological clarification and improve prospects for prevention.

Therefore, in order to help clarify the potential association between diabetes, related factors and breast cancer risk, a comprehensive literature review and formal meta-analysis was carried out, planned, conducted and reported following PRISMA guidelines regarding meta-analysis of observational studies. Variables studies in relation to breast cancer risk were adiposity, physical activity, glycaemic load, glycaemic index, diabetes, IGF-1, fasting glucose, fasting insulin and C-peptide, adiponectin and metformin and glargine use among patients with diabetes. For all variables except diabetes and breast cancer, only prospective studies were included in meta-analyses. Summary Relative Risks (SRR) and corresponding 95% Confidence Intervals (CI) were calculated from random effect models.

For breast cancer at all ages, the calculated risks were as follows: diabetes (SRR = 1.27 95% CI (1.16, 1.39); physical activity (SRR = 0.88, 95% CI (0.85, 0.92)); glycaemic load (SRR = 1.06, 95% CI (1.00, 1.12)); glycaemic index (SRR = 1.04, 95% CI (0.99, 1.10)); fasting glucose (SRR = 1.12, 95% CI (1.01, 1.24)); serum insulin (SRR = 1.18, 95% CI (0.75, 1.85)); c-peptide (SRR = 1.29, 95% CI (0.91, 1.82)); adiponectin (SRR = 1.16, 95% CI (0.93, 1.46)); metformin (SRR = 1.00, 95% CI (0.69, 1.46)); and glargine (SRR = 1.11, 95% CI (1.00, 1.24)). An increase of 5 units in Body Mass Index (a weight increase if 14.5 kg in a person 1.70 metres tall) was associated in post-menopausal breast cancer (SRR = 1.12, 95% CI (1.08, 1.16)) but not at pre-menopausal ages (SRR = 0.83, 95% CI (0.72, 0.95)). Serum insulin was associated with breast cancer at post-menopausal ages but not at pre-menopausal ages whereas with c-peptide there was a significant association at pre-menopausal ages but not post-menopausal. For IGF-1, Hodge's Standardised Mean Difference (HSMD) was calculated in cohort studies and there was no significant association with breast cancer at all ages (HSMD = 0.003, 95% CI (−0.059, 0.065)), at post-menopausal ages (HSMD = −0.014, 95% CI (−0.106, 0.077)) or at pre-menopausal ages (HSMD = 0.039, 95% CI (−0.038, 0.117)).

The risk of breast cancer is increased among post-menopausal women who have diabetes. Among those factors related to diabetes, key risk factors for breast cancer appear to be adiposity and lack of physical activity which are both related to the risk of developing diabetes. Action on these lifestyle factors should form the basis of a common prevention strategy. There is a need to re-evaluate potential biological mechanisms to explain the increased risk in post-menopausal women with diabetes.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-13-08.