Obesity   has   a   causal   relationship   with various forms of cancer,  as concluded by the International Agency for Research on Cancer following a comprehensive review of epidemiological studies spanning >30 years ^[1].  In Europe,  obesity accounts for 3%  of cancers in men and 6% of cancers in women ^[2].  The evidence that obesity contributes to   the   development   of   breast   cancer   in postmenopausal   women   is   overwhelming and indisputable ^[3]. 

Obesity is estimated to cause 20%  of all postmenopausal breast cancers   and   27%  of   those   in   women >70 years of age ^[4]. The overall increase in breast cancer risk attributed to obesity ranges 6–19% ^[5]. A 1-point gain in body mass index (BMI)  is estimated to increase the risk of postmenopausal breast cancer by 3% ^[2], and every 5-kg increase in weight increases the relative risk (RR) of developing breast cancer in postmenopausal women by 1.08 ^[6].

In addition to the association between obesity and a higher incidence of breast cancer, various systematic reviews and meta-analyses have demonstrated that obesity at the time of diagnosis is a significant risk factor for a poor prognosis in both pre-  and postmenopausal women ^[7,8].  It has been estimated that up to 50% of postmenopausal breast cancer-related deaths can be attributed to obesity in the US ^[9].

A meta-analysis of   the   English   literature,  including   12 published studies comprising 8029 cases of breast cancer,  found that the overall hazard ratio for death attributed to obesity in breast cancer was 1.56 (95%  confidence interval ^[CI] 1.22–2) ^[10]. Women with breast cancer in the highest quartile for BMI are 2.5 times as likely to die from their disease within 5 years of diagnosis compared with those in the lowest quartile for BMI ^[11]. 

Several large-scale   cohort   studies   and   critical reviews,  such   as   the   US   NHS   (Nurses’ Health Study)  and the Cancer Prevention Study II,  have confirmed the association between obesity and increased deaths from breast cancer ^[12,13].  Obesity is associated with a poor breast cancer outcome even in those with early stage disease ^[14], and obese postmenopausal women with inflammatory breast cancer have been shown to experience a significantly worse outcome compared with similar women of normal weight ^[15].

Amtul R Carmichael, MD
Russells Hall Hospital, Dudley, UK

References