International Agency for Research on Cancer (IARC) and World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) classify the role of this risk factor in cancer development.[1,2]
Greater body fatness is associated with higher sex hormone levels (fatty tissue produces more oestrogen), which may partly explain the link between body fatness and breast cancer risk. Abdominal fatness and weight gain during adulthood are thought to be more accurate measures of fatty tissue levels, compared with body mass index (BMI), because BMI includes lean tissue mass. 8% of breast cancer cases in the UK are caused by overweight and obesity.
Breast cancer among post-menopausal women is 12-13% higher in those who are overweight (body mass index [BMI] 25-29.9) and 16-20% higher in those who are obese (BMI 30+), compared with those who are underweight and normal weight (BMI of less than 25), meta-analyses have shown.[6,7] The increased risk may be limited to ER- and PR-positive tumours,[6-8] and women who have never used oestrogen-progestogen hormone replacement therapy.
Breast cancer risk among post-menopausal women is 50% higher in those with the highest waist-to-hip ratio (WHR, a measure of abdominal obesity) versus the lowest WHR, a meta-analysis showed. However the association with WHR or waist circumference probably reflects the effect of BMI, rather than a specific effect of abdominal obesity.[10,11]
Breast cancer risk among post-menopausal women not using HRT is 50% higher in those with the highest levels of adulthood weight gain, versus those with the lowest, a pooled analysis showed. The risk increase is larger for ER- and PR- positive tumours (133% increase) than ER- and PR-negative tumours (34% increase), a meta-analysis showed.
Breast cancer risk among pre-menopausal women is slightly lower in those who are overweight or obese, compared with healthy weight, meta-analyses have shown.[7,8,13,14] However this may be limited to Caucasians and Africans (not Asians), and to ER- and PR-positive tumours.[6-7,13,14]
Breast cancer risk among pre-menopausal women is 79% higher in those with the highest waist-to-hip ratio (WHR) versus those with the lowest, a meta-analysis showed. Breast cancer risk is 8% higher per 0.1 unit WHR increase.
Breast cancer risk among pre-menopausal women is not associated with weight gain during adulthood (since age 20), a pooled-analysis showed. This may vary by hormone receptor status and parity, a case control study indicated. Breast cancer risk among pre-menopausal women may be higher in those who gain weight during middle adulthood (age 40-50) versus those who do not, a cohort study showed.
Breast cancer risk is higher in women with a higher birth weight, with a stronger effect for pre-menopausal breast cancer, meta-analyses have shown.[17-19] Higher in utero oestrogen levels probably explain this association; birth size is not associated with breast cancer risk in women with a male twin, who are also exposed to male sex hormones in utero.
Male breast cancer is 30% higher in men with the highest body mass index (BMI) versus those with the lowest.
UK portrait version shown here. Country versions, cancers caused by other risk factors, and landscape formats are available for free from our cancer risk publications.