The effect of Benign Thyroid Disease, Diabetes Mellitus and Physical Activity on Breast Cancer Risk

Abstract

The aetiology of breast cancer has been the topic of numerous observational studies with benign thyroid diseases, diabetes mellitus, physical activity and weight loss associated with altered breast cancer risk. However, there has been a degree of inconsistency in the results of the primary studies, with a consensus on the exact nature of the association between breast cancer and these four risk factors yet to be determined. The aim of this study was to complete a meta-analysis of primary observational studies clarifying the risk of breast cancer associated with benign thyroid diseases, diabetes mellitus, physical activity and weight loss. A comprehensive database search was completed with MEDLINE, EMBASE, PubMed, Current Contents Connect and Google Scholar searched with additional cross checking of reference lists. Inclusion criteria depended on the risk factor investigated but general requirements included the use of an internal control group and reporting of both an odds ratio, relative risk or hazards ratio and 95% confidence interval. Collated data was assessed for heterogeneity and a pooled odds ratio calculated. The results of this study identified an increased risk of breast cancer associated with type 2 diabetes mellitus while both physical activity and weight loss were found to be protective in terms of breast cancer risk. These findings are in keeping with the “lifestyle” approach to breast cancer risk. While sub group analyses were completed wherever possible, there were insufficient studies to analyse the risk of breast cancer associated with graves disease and type 1 diabetes mellitus. In conclusion, type 2 diabetes mellitus and autoimmune thyroiditis are associated with an increased risk of breast cancer while physical activity and weight loss was found to be protective. Prospective longitudinal studies of high quality are required to further explore these risk factors.