Obesity is associated with an increased risk of breast cancer, and is also positively associated with tumor size and a higher probability of having positive axillary lymph nodes and faster growing tumors [18–20]. Interestingly, higher energy intake also increases the risk of breast cancer [21]. It is well accepted that obesity increases the risk of developing breast cancer after menopause, and it has even been suggested that up to 50 % of postmenopausal breast cancers are attributable to obesity [22]. However, the degree of increased relative risk and whether or not this also holds true for premenopausal women is contentious. A number of meta-analyses have been performed in recent years examining the effect of BMI on breast cancer with age (Table 2.1). In 2012, Cheraghi et al. performed a meta-analysis of 50 studies, 15 cohort studies and 35 case-control studies involving 2,104,203 and 71,216 participants, respectively [23]. There was no significant effect of BMI on breast cancer risk in premenopausal women, but a direct and significant correlation was observed between BMI and breast cancer risk in postmenopausal women. This was consistent with findings from Bergstrom et al. who demonstrated that a one unit increase in BMI was associated with a 2 % increased risk of developing breast cancer in postmenopausal women [24], a relationship that was not found in the premenopausal group. A number of studies have also described obesity as strongly protective against breast cancer in premenopausal women [25–28]. This has been attributed, at least in part, to a greater number of anovulatory menstrual cycles and hence, decreased lifetime exposure to estrogens. Nevertheless, a study Biglia et al. demonstrated that high BMI was significantly associated with larger sized tumors in both pre- and postmenopausal women [20]. In this case, obese premenopausal women displayed more vascular infiltration and metastasis to axillary lymph nodes compared to healthy weight women. Moreover, a meta-analysis by Pierobon et al. revealed that obesity is a significant risk factor for triple negative breast cancers in pre- but not postmenopausal women [29]. Interestingly, there is also evidence for an association of BRCA1 mutations and BMI with breast cancer risk in premenopausal women (P = 0.045) [30].