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Breast cancer risk factors

The key risk factors for invasive breast cancer are discussed on this page. Breast cancer risk is also strongly linked with age.

Around 27% of female breast cancers in the UK are linked to lifestyle.1 Sex hormone exposure is the mechanism by which most lifestyle factors are thought to impact on breast cancer risk.2,3

Meta-analyses and systematic reviews are cited where available, as they provide the best overview of all available research and most take study quality into account. Individual case-control and cohort studies are reported where such aggregated data are lacking. The evidence on this page is for female breast cancer unless otherwise specified.

Breast cancer risk factors overview

The International Agency for Research on Cancer (IARC) evaluates evidence on the carcinogenic risk to humans of a number of exposures including tobacco, alcohol, infections, radiation, occupational exposures, and medications.4 The World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) evaluates evidence for other exposures including diet, overweight and obesity, and physical exercise.5 IARC and WCRF/AICR evaluations are the gold standard in cancer epidemiology. Their conclusions about breast cancer risk factors are shown in Table 4.1.

Table 4.1: IARC and WCRF/AICR Evaluations of Breast Cancer Risk Factors

Increases risk ('sufficient' or 'convincing' evidence) May increase risk ('limited' or 'probable' evidence) Decreases risk ('sufficient' or 'convincing' evidence) May decrease risk ('limited' or 'probable' evidence)
  • Alcoholic beverages
  • Diethylstilbestrol
  • Oestrogen-progestogen contraceptives
  • Oestrogen-progestogen menopausal therapy
  • X radiation and Gamma radiation
  • Body fatnessa
  • Adult attained heighta
  • Digoxin
  • Oestrogen menopausal therapy (hormone replacement therapy)
  • Ethylene oxide
  • Shiftwork involving circadian disruption
  • Tobacco smoking
  • Adult attained heightb
  • Greater birth weightb
  • Abdominal fatnessa
  • Adult weight gaina
  • Total dietary fata
  • Breastfeeding
  • Body fatnessb
  • Physical activity

a Post-menopausal breast cancer only. b Pre-menopausal breast cancer only.

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Age

The risk of breast cancer is strongly related to age.

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Endogenous hormones

Many breast cancer risk factors are associated with higher levels of sex hormones, and it is thought to be by this mechanism that they are linked with breast cancer risk.3 Sex hormones (also known as sex steroids) include oestrogen, progesterone and testosterone.

Post-menopausal

In post-menopausal women, breast cancer risk is around twice as high in those with the highest sex hormone levels (including oestradiol, oestriol, androstenedione and testosterone) compared with the lowest, a pooled analysis of cohort studies showed.6

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Pre-menopausal

In pre-menopausal women, breast cancer risk is not significantly associated with oestrogen levels, a meta-analysis and the EPIC cohort study have shown.7,8 Pre-menopausal breast cancer risk is 56% higher in those with the highest serum testosterone levels compared with the lowest, whilst other sex hormones show no clear association, EPIC showed.8

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IGF-1

Insulin-like growth factor 1 (IGF-1) is positively associated with breast cancer risk in pre- and post-menopausal women, a pooled analysis has shown.9

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Reproductive factors

Some reproductive factors modify sex hormone levels; reduction in overall oestrogen exposure may partly explain the link between reproductive factors and breast cancer risk.

Breastfeeding is classified by WCRF/AICR as protective against breast cancer (Table 4.1).5

Lower parity (having fewer children)

Breast cancer risk decreases by 7% with each live birth, meta- and pooled analyses have shown.10-13 This may be limited to ER/PR positive tumours (risk of ER/PR negative tumours may even increase with parity,55 though this may be tempered by breastfeeding188), and singleton (rather than twin) pregnancies.10-13 The association with parity may vary by tumour types, with the largest risk reduction for mucinous tumours, and a risk increase for medullary tumours, a cohort study indicates.14 Overall, women who have had children have a 30% lower breast cancer risk than nulliparous women.11

Conversely among BRCA1/2 mutation carriers, breast cancer risk is not associated with parity, one meta-analysis showed,164 though another showed a decreased risk with higher parity.213

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Older age at first giving birth

Breast cancer risk increases by 3% for each year older a woman is when she first gives birth, a meta-analysis showed.10 The association may be limited to ER- and PR-positive tumours.12,163

Conversely among BRCA1 mutation carriers, breast cancer risk may be lower in those who are older at first birth, meta-analyses have shown;164,213 among  BRCA2 carriers, breast cancer risk is not associated with age at first birth.164,213

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Younger age at menarche

Breast cancer risk increases by 5% for each year younger at menarche (first menstrual period), a meta-analysis has shown.16 The association is stronger for oestrogen receptor (ER)-positive and progesterone receptor (PR)-positive tumours than for ER- and PR-negative tumours.12 Breast cancer risk may be higher in women whose breast development started at a younger age, a cohort study indicates.182

Among BRCA1 mutation carriers too, breast cancer risk may be higher in those who are younger at menarche, a meta-analysis showed; among BRCA2 carriers, breast cancer risk is not associated with age at menarche.164,213

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Older age at menopause

Breast cancer risk increases by around 3% for each year older at menopause, a meta-analysis has shown.16 Post-menopausal women (natural menopause or induced by surgery) have a lower risk of breast cancer than pre-menopausal women of the same age and childbearing pattern, a meta-analysis showed.16

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Reproductive organ surgery

Breast cancer risk is not associated with tubal sterilisation, a meta-analysis showed.171 Breast cancer risk is 24-41% lower in women who have hysterectomy and oophorectomy before menopause, compared with women who do not have these surgeries, a pooled analysis and case-control study showed.172,173 Hysterectomy and oophorectomy after menopause may be associated with increased breast cancer risk if oestrogen therapy is used after the surgery, a pooled analysis showed.172

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Oral contraceptives (OCs)

Current or recent use of combined oestrogen-progestogen oral contraceptives (OCs) is classified by IARC as a cause of breast cancer (Table 4.1).4 An estimated 1% of female breast cancers in the UK are linked to OCs; because breast cancer risk is generally low in the OC-using population (typically younger women), OC-related risk contributes a relatively small number of additional cases.17

OCs contain synthetic sex hormones, which may explain the link between OC use and breast cancer risk.

Current users of OCs have around 24% higher breast cancer risk compared to never users, a meta-analysis showed.18 However, breast cancers in OC users tend to be less advanced compared with those in OC never-users.18 The relative risk of breast cancer declines after OC cessation, such that 10 years after cessation no excess risk remains.18,19 Breast cancer risk does not appear to increase with longer duration of OC use,18,19 however, younger age at first OC use is associated with a larger increase in breast cancer risk.18

The risk associated with OC use appears to be similar across OC formulations (which have changed considerably over time), family history, BRCA carrier status (though some evidence of no association with OC use in BRCA1/2 mutation carriers), and ethnicity.18,20,213

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Non-oral hormonal contraceptives

Hormonal contraception is also available as injections, implants and patches. There is substantially less evidence on cancer risk associated with these preparations than there is on cancer risk associated with the oral contraceptive pill. Breast cancer risk is increased among users of injectable contraceptives in some studies,21,22 while other studies show no association.23-25 Breast cancer risk does not appear to be increased among users of contraceptive implants.25

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Hormone replacement therapy (HRT)

Current use of combined oestrogen-progestogen hormone replacement therapy (HRT) for menopausal symptoms is classified by IARC as a cause of breast cancer, and use of oestrogen-only HRT is classified as a possible cause of breast cancer (Table 4.1).4 An estimated 3% of female breast cancers in the UK are linked to HRT use.17

HRT contains synthetic sex hormones, which may explain the link between HRT use and breast cancer risk.

Breast cancer risk is 55-100% higher in oestrogen-progestogen HRT (combined HRT) current users versus never users, cohort studies have shown.26,27

Breast cancer risk is also higher in oestrogen-only HRT users, though to a lesser extent than with combined HRT, cohort studies have shown.26,29-33

Breast cancer risk is not associated with past HRT use 5 years or longer ago, cohort studies have shown.26,28 Breast cancer risk among current HRT users increases with duration of use, and with lower body mass index (BMI).26,28-32 Breast cancer risk among HRT users may vary with previous use of OCs, but evidence remains unclear.189-191

Breast cancer risk does not appear to be increased by use of phytoestrogens (plant-derived chemicals used by some women as an alternative to HRT), a meta-analysis showed;168 however the efficacy of phytoestrogens for relieving menopausal symptoms remains unclear.169

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Family history and genetic factors

Hereditary factors explain only around a quarter of breast cancer risk.37 Breast cancer risk is not associated with breast cancer in an adoptive parent, and does not vary with time since the family member was diagnosed, indicating genetic/biological factors rather than environmental factors or increased diagnostic activity underpin familial clustering of breast cancer cases.225,236

Family history

Breast cancer risk is around twice higher in women with one first-degree relative with breast cancer, versus women with no first-degree relatives with the disease, meta- and pooled-analyses have shown.34,35 The risk is higher still with a larger number of affected first-degree relatives, or relatives affected aged under 50.34,35 ER-positive and ER-negative breast cancer risks are associated to a similar extent with family history.36

Over 85% of women with a first-degree relative with breast cancer will never develop breast cancer themselves.34 87% of women with breast cancer have no first-degree relatives with the disease.34

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BRCA1 and BRCA2

BRCA1 and BRCA2 mutations confer a high risk of breast cancer in carriers (high-penetrance). Women with a BRCA1 or BRCA2 mutation have a 45-65% chance of developing breast cancer by age 70.38 BRCA1/2 mutation-carriers have higher breast cancer risk compared with the general population in all age groups.38 BRCA2-negative women with a BRCA2-carrying first-degree relative may also have increased breast cancer risk, a small UK cohort study showed.39 Higher sex hormone levels in BRCA mutation carriers may explain some of the increased risk.40

Breast cancer risk in BRCA mutation carriers may be modified by other factors including family history (breast cancer risk among BRCA2 mutation carriers is 70% higher for each first-degree relative with breast cancer aged 50 or younger, versus BRCA2 mutation carriers with no such family history; breast cancer risk in BRCA1 mutation carriers is not associated with family history),207 previous breast cancer,208 and lifestyle factors (indicated by higher risk in BRCA mutation carriers born post-1950 versus pre-195041).

BRCA1 and BRCA2 mutations are uncommon, though this varies by ethnicity/country of origin.42 They affect an estimated 0.11% and 0.12% of the general population respectively,43 equating to around 1 in 450 women carrying a mutation. Their relative rarity means BRCA1 and BRCA2 mutations probably account for around 2% of all breast cancers overall.43-45 However, they explain around 15-20% of cases with first-degree family history.42

NHS screening for BRCA1 and BRCA2 mutation is available to people who meet eligibility criteria relating to their family history.

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Other breast cancer predisposition genes

The population impact of gene mutations on breast cancer risk depends on both how much the mutation elevates breast cancer risk (penetrance) and how common the mutation is; e.g. a high-penetrance but rare mutation will account for a low proportion of cases overall. The following gene mutations are associated with breast cancer risk:42,46

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Overweight and obesity

Higher body mass index (BMI) is classified by WCRF/AICR as a cause of post-menopausal breast cancer (Table 4.1).5 Abdominal fatness and weight gain during adulthood are classified by WCRF/AICR as probable causes of post-menopausal breast cancer.5

Greater birth weight is classified by WCRF/AICR as a probable cause of pre-menopausal breast cancer (Table 4.1).5 Higher BMI is classified by WCRF/AICR as probably protective against pre-menopausal breast cancer.5 An estimated 9% of female breast cancers in the UK are linked to excess body weight.47

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.48 Abdominal fatness and weight gain during adulthood are thought to be more accurate measures of fatty tissue levels, compared with BMI, because BMI includes lean tissue mass.49

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Post-menopausal

Breast cancer risk among post-menopausal women is 15% higher in those who are overweight (BMI 25-29.9) or obese (BMI 30+), versus healthy weight (BMI 18.5-24.9), a meta-analysis showed.50 The increased risk may be limited to ER- and PR-positive tumours,51,170 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 those with the lowest WHR, a meta-analysis showed.52 However the association with WHR or waist circumference probably reflects the effect of  BMI, rather than a specific effect of abdominal obesity.53,202

Breast cancer 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.54 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.49

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Pre-menopausal

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.51,55,56,170 However this may be limited to Caucasians and Africans (not Asians), and to ER- and PR-positive tumours.50,51,55,56,170

Breast cancer risk among pre-menopausal women is 79% higher in those with the highest WHR versus those with the lowest, a meta-analysis showed.52 Breast cancer risk is 8% higher per 0.1 unit WHR increase.56

Breast cancer risk among pre-menopausal women is not associated with weight gain during adulthood (since age 20), a pooled-analysis showed.54 This may vary by hormone receptor status and parity, a case control study indicated.181 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.216

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.57-59 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.60

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Males

Male breast cancer risk is 30% higher in those with the highest BMI versus those with the lowest, a pooled a pooled analysis showed.187

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Height

Greater adult attained height is classified by IARC as a cause of post-menopausal breast cancer, and a probably cause of pre-menopausal breast cancer (Table 4.1).5

Among post-menopausal women, breast cancer risk is 7-11% higher per 5cm increment in height, a meta-analysis and a pooled analysis showed.61,62 Among pre-menopausal women, no significant association with height was found in these analyses.61,62 The underlying mechanism for the association between height and breast cancer risk is unclear, but it is likely that height is a marker for other exposures (perhaps hormonal) that influence breast cancer risk.

Women with a higher birth length are at increased breast cancer risk, perhaps with a stronger effect for pre-menopausal breast cancer, a meta-analysis showed.57

Male breast cancer risk is 18% higher in the tallest men versus the shortest, a meta-analysis showed.187

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Breast density

Breast cancer risk is up to 3-5 times higher in women with the most dense breasts (higher percentage non-fatty tissue) compared with the least dense, meta- and pooled analyses have shown.63,64 

Dense breasts are also associated with an increased risk of in situ breast carcinoma.65 Breast density is generally higher in younger, pre-menopausal women with lower BMI and lower parity, but there is also a genetic element.66,67 Sex hormone levels do not appear to explain the association between breast density and breast cancer risk.68,69,184

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Benign breast disease

Some types of benign breast disease are linked with increased breast cancer risk. Among women with benign breast disease, cancer is more common in the breast with the benign disease than in the opposite breast.70,71

Non-proliferative disease (NP)

Breast cancer risk is generally not increased in women in NP. However, breast cancer risk does appear to be increased in women with NP and a first-degree relative with breast cancer, a meta-analysis showed.72

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Proliferative disease without atypia (PDWA)

Breast cancer risk is 44% higher in women with PDWA, compared with women with NP, a meta-analysis showed.72 Breast cancer risk appears to be further increased in women with PDWA and a first-degree relative with breast cancer.72 Breast cancer risk among women with PDWA is higher in those with radial scars, versus those without, a meta-analysis showed.229

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Atypical hyperplasia (AH)

Breast cancer risk is around threefold higher in women with AH, compared with women with NP, a meta-analysis showed.72 Lobular AH is associated with higher breast cancer risk than is ductal AH,72 therefore lobular AH is often grouped with the more advanced conditions lobular carcinoma in situ (LCIS) as 'lobular neoplasia'. Having a first-degree relative with breast cancer does not appear to further increase breast cancer risk in women with AH.72

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In situ breast carcinoma

Women with in situ breast carcinoma have between double and triple the breast cancer risk of the general female population, cohort studies have found.73,74 Among women with in situ breast carcinoma, cancer is similarly common in the breast with the in situ carcinoma as in the opposite breast.74

Ductal carcinoma in situ (DCIS) is considered a necessary precursor for breast cancer, meaning most breast cancer cases are thought to originate as DCIS, however not all DCIS develop into breast cancer.75 LCIS is associated with increased breast cancer risk but is not thought to be necessary for breast cancer to develop.75 DCIS overall is associated with 40-100% increased breast cancer risk, though risk does not appear to be elevated in all DCIS subtypes, US cohort studies indicate.76,77

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Previous cancer

Risk of breast cancer as a second cancer may be associated with radiotherapy treatment for the previous cancer.

Breast cancer

Breast cancer risk is 2-5 times higher in survivors of previous breast cancer, compared with the general population, cohort studies have shown.78-80 Breast cancer risk remains higher than in the general population for 20 years after the primary cancer.79

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Breast cancer risk among survivors of previous breast cancer is 14% higher per 5-unit BMI increase, a meta-analysis showed.82 Breast cancer risk among survivors of ER-positive breast cancer is around 40% lower in those taking tamoxifen for five years, versus those not taking tamoxifen, a meta-analysis showed.83 Breast cancer risk among survivors of ER-negative breast cancer is not associated with tamoxifen use.83

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Breast cancer risk among BRCA1/2-positive women is around twice higher in survivors of previous breast cancer versus BRCA1/2-positive women with no previous breast cancer, a cohort study showed.208 Breast cancer risk among BRCA1/2-positive survivors of previous breast cancer is probably lower in tamoxifen users versus non-users, a systematic review showed.213 Around 30% of breast cancer survivors with Cowden syndrome develop a second breast cancer, a cohort study showed.218

These figures are probably not affected by inclusion of ‘second cancers’ which are actually spread from the first cancer, or by under-recording of second cancers in the same body part as the first (due to international cancer coding rules), but such biases remain possible.79,81

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Other cancers

Breast cancer risk is eight times higher in Hodgkin lymphoma survivors compared with the general population, a meta-analysis has shown; risk is highest among those with Hodgkin lymphoma at a younger age, and those treated with radiotherapy rather than chemotherapy.196

Breast cancer risk is 14% higher in melanoma survivors, a meta-analysis and cohort studies have shown.86,192,193 Breast cancer risk is higher in survivors of lung adenocarcinoma, bowel cancer, uterus cancer, and chronic lymphocytic leukaemia, cohort studies have shown.84,85,192,193 Breast cancer risk is around doubled in childhood cancer survivors.87,88

Breast cancer risk is 24% lower in cervical squamous cell carcinoma survivors, cohort studies have shown.84,85

Breast cancer risk is around 7-14 times higher in children, teenagers and young adults who had any type of cancer in childhood, compared with the general population, a cohort study showed.212

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Ionising radiation

X radiation and gamma radiation are classified by IARC as causes of breast cancer (Table 4.1).4 An estimated 1% of female breast cancers in the UK are linked to radiation exposure, with medical radiation and natural (background) radiation each accounting for around half the cases.89

Radiotherapy

Breast cancer risk is increased after several types previous cancer, with radiotherapy an important factor in this association. Breast cancer risk is nonsignificantly increased in survivors of childhood solid cancer who received radiotherapy, compared with those who did not receive radiotherapy.88 Breast cancer risk is 9-11% higher in women who received radiotherapy for cancer in the opposite breast, compared with women who had surgery alone.90,91

An estimated 11% of the radiotherapy-associated second cancers in the UK each year are breast cancers.92

Diagnostic radiology

Diagnostic radiology involves much lower radiation doses than radiotherapy.

An estimated 0.1% of breast cancers in women under 75 are caused by diagnostic x-rays.93 Breast cancer risk among women with BRCA1/2 mutation may be higher in those who have received diagnostic radiation, but evidence is mixed, a systematic review showed.213

An estimated 0.03-0.06% of breast cancers are caused by receiving a mammogram 3-yearly when aged 47-73.95 Breast cancer risk among women with BRCA1/2 mutation is not associated with receiving mammograms a meta-analysis showed.213

Breast cancer risk is not associated with receiving computed tomography (CT) scans in childhood or adolescence, a cohort study showed.96

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Diet

WCRF/IARC concluded that despite substantial research into dietary exposures, evidence is insufficient (due to quality, consistency or amount) to derive a classification regarding association with breast cancer risk.

Dietary fat

Total dietary fat is classified by WCRF/IACR as a probable cause of post-menopausal breast cancer, based on limited evidence (Table 4.1).5

Breast cancer risk is 13% higher in women with the highest total fat intake compared with the lowest, a meta-analysis showed.101 Risk is also higher in women with the highest saturated fat intake, this meta-analysis and more recent cohort studies have shown;101-104 however this may be restricted to ER-PR-positive tumours.200

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Cadmium

Breast cancer risk is 15% higher in women with the highest dietary cadmium intake compared with the lowest, a meta-analysis showed.194

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Alcohol

Alcoholic beverages are classified by IARC and WCRF/AICR as a cause of breast cancer (Table 4.1).4,5 An estimated 6% of female breast cancers in the UK are linked to alcohol consumption.105

Breast cancer risk is 7-12% higher per unit of alcohol per day, meta-analyses have shown.107-109 Breast cancer risk is 5% higher in light drinkers (up to one alcoholic drink per day, or around 1.5 units) compared with non-drinkers, a meta-analysis showed.110

Breast cancer risk among BRCA1/2 mutation carriers is not associated with alcohol intake, a meta-analysis showed.213

Alcohol consumption is associated with higher levels of sex hormones, which may partly explain the link between alcohol and breast cancer risk.106

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Tobacco

Tobacco smoking is classified by IARC as a probable cause of breast cancer, based on limited evidence (Table 4.1).4

Tobacco smoking is associated with higher levels of sex hormones, which may partly explain the link between tobacco and breast cancer risk.3

Active smoking

Breast cancer risk is 12% higher in current smokers, and 9% higher in former smokers, both compared with never-smokers, a meta-analysis has shown.111 Breast cancer risk increases with amount, duration, and starting age of smoking (particularly starting smoking before first giving birth).111,112,197 The effect of smoking may be limited to pre-menopausal breast cancer and non-obese women,112,113 women without a family history of breast cancer,199 and ER-positive (not triple negative) breast cancer.185,186

Breast cancer risk among BRCA1 mutation carriers is not associated with smoking, a meta-analysis showed; breast cancer risk among BRCA2 mutation carriers is higher in smokers.213

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Environmental tobacco smoke

Exposure to environmental tobacco smoke may not be associated with breast cancer risk, though findings are mixed.114-117,197

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Occupational exposures

Night shift work

Shift work involving circadian disruption is classified by IARC as a probable cause of breast cancer, based on limited evidence (Table 4.1).4 An estimated 4-5% of female breast cancers in the UK are linked to shift work.118,119

Breast cancer risk is not associated with having ever experienced circadian disruption (mainly in night shift workers, but also in flight attendants, those exposed to light at night, and those with sleep deficiency), a meta-analysis of cohort studies showed.234 However, a meta-analysis of case-control studies showed breast cancer risk is 21% higher in women who have ever experienced circadian disruption (defined as above) versus those who have not.234 A high proportion of relevant studies involve flight attendants,234 findings may not apply to other occupational groups.228,234

Breast cancer risk is higher in women with more exposure to artificial light at night, a meta-analysis showed.221

Circadian disruption caused by light at night is linked with lower melatonin levels. Breast cancer risk is lower in women with higher melatonin levels; melatonin reduces circulating oestrogen.120,121 In this way, light at night may partly explain the link between shift work and breast cancer risk, but confounding by other lifestyle factors such as tobacco use, BMI and physical activity is possible.122,123

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Ethylene oxide

Occupational exposure to ethylene oxide is classified by IARC as a probable cause of breast cancer, based on limited evidence (Table 4.1).4 Breast cancer risk is around 2-3 times higher in the most exposed workers compared with the least exposed, cohort studies have shown.148,149

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Medical conditions and treatments

Digoxin

Use of digoxin, a drug used to treat heart failure, is classified by IARC as a probable cause of breast cancer, based on limited evidence (Table 4.1).4 Breast cancer risk is 37% higher in digoxin ever-users, compared with never-users, a meta-analysis showed.195 Breast cancer risk may increase slightly with duration of digoxin use,128 and increased risk may be limited to current (not former) users,129 and ER-positive disease.195

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Diethylstilbestrol (DES)

Use of diethylstilbestrol (DES) is classified by IARC as a cause of breast cancer (Table 4.1).4 Breast cancer risk is 27% higher in women who took DES during pregnancy.130 Breast cancer risk does not appear to be increased in women exposed to DES in utero.57

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Diabetes

Breast cancer risk is 10-23% higher in diabetic women, compared with non-diabetic women, meta-analyses have shown.137-139 The association may be limited to post-menopausal women and those with Type II diabetes.138

Breast cancer risk among diabetic women may vary by treatment type, with lower risk in pioglitazone or metformin users, meta-analyses have shown.140-143

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Benign thyroid diseases

Breast cancer risk is around three times higher in women with autoimmune thyroiditis, a meta-analysis showed.144 Breast cancer risk is not associated with hyperthyroidism or hypothyroidism.144

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Bone mineral density

Breast cancer risk is 62-82% higher in women with the highest bone mineral density, versus those with the lowest, a meta-analysis showed.165

High bone mineral density is associated with higher lifetime oestrogen exposure,167 which may partly explain the link between bone mineral density and breast cancer risk. Some breast cancer treatments can affect bone mineral density.

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Factors shown to decrease or have no effect on breast cancer risk

Decrease risk

Breastfeeding is classified by WCRF/AICR as protective against breast cancer (Table 4.1).5 An estimated 3% of female breast cancers in the UK are linked to women breastfeeding each of their children for fewer than six months.15

Among mothers, breast cancer risk is 4% lower for every 12 months of breastfeeding, a meta-analysis showed.10

Among BRCA1 mutation carriers too, breast cancer risk may be lower in those who breastfeed for at least 1-2 years, a meta-analysis showed; among BRCA2 carriers, breast cancer risk is not associated with breastfeeding.164,213

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Physical activity (of all types) is classified by WCRF/AICR as probably protective against breast cancer, though the evidence is stronger for post-menopausal than pre-menopausal cancer (Table 4.1).5 An estimated 3% of female breast cancers in the UK are linked to inadequate physical activity (less than 150 minutes moderate physical activity per week).97

Higher levels of physical activity are associated with lower levels of sex hormones, which may partly explain the link between physical activity and breast cancer risk.

Breast cancer risk is around 25% lower in the most active women compared with the least.98 Breast cancer risk decreases by 5% for every 2 hours per week increment in recreational activity (moderate and vigorous), a meta-analysis showed.99 Light-intensity activity may be insufficient to reduce breast cancer risk, a case-control study indicated.100 Breast cancer risk is 17% higher in the most sedentary women versus the least, a meta-analysis showed.235

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Body fatness is classified by WCRF/AICR as probably protective against pre-menopausal breast cancer, though it increases the risk of post-menopausal breast cancer (Table 4.1).5

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Breast cancer risk is lower in people with coeliac disease compared with the general population, cohort studies have shown, however mechanisms are unclear.145,227

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Breast cancer risk may be lower among women whose mothers had pre-eclampsia, perhaps due to lower levels of in utero oestrogen exposure, but evidence remains unclear.57,146,147

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Breast cancer risk is 12-25% lower in women who regularly use aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs), meta-analyses have shown.131-133 However, no such effect was found for NSAID use in a recent large prospective cohort study of post-menopausal women.134 NSAID use is associated with lower oestradiol levels, which may partly explain the link between NSAIDs and breast cancer risk.135

section reviewed 02/04/14
section updated 02/04/14

Breast cancer risk is lower in people with higher consumption of the following, meta- and pooled analyses or systematic reviews have shown:

  • Fruit and vegetables.153 
  • Dietary fibre (at least 25g/per day).154 
  • Some carotenoids (may be limited to current smokers).155 
  • Lignans (post-menopausal women).156 
  • Soya-based foods (Asian populations only).157,162,198 
  • Flavonols and flavones (post-menopausal women).158 
  • Marine omega-3 polyunsaturated fatty acids (PUFA).159
  • Mushrooms (post-menopausal women).209
  • Coffee (post-menopausal women214 and BRCA1 mutation carriers213,214).

section reviewed 01/05/14
section updated 29/08/14

Breast cancer risk among women with osteoporosis (low bone mineral density) may be lower in those using bisphosphonates versus those not, but evidence from observational studies and RCTs is conflicting.166,233

No effect

WCRF/AICR make no judgment on the association between pre-menopausal breast cancer risk and intake of cereals (grains) and their products; dietary fibre; potatoes; vegetables; fruits; pulses (legumes); soya and soya products; meat; poultry; fish; eggs; milk and dairy products; fats and oils; total fat (no judgment for pre-menopausal breast cancer, but increased risk for post-menopausal breast cancer); vegetable fat; fatty acid composition, trans-fatty acids (no judgment for pre-menopausal breast cancer); cholesterol; sugar (sucrose); other sugars (no judgment for pre-menopausal breast cancer); sugary foods and drinks; coffee; tea; carbohydrate; starch; glycaemic index; protein; vitamin A; riboflavin; vitamin B6; folate; vitamin B12; vitamin C; vitamin D; vitamin E; calcium; iron; selenium; carotenoids; isoflavones; dichlorodiphenyldichloroethylene; dichlorodiphenyltrichloroethane; dieldrin; hexachlorobenzene; hexachlorocyclohexane; trans-nonachlor; polychlorinated biphenyls; dietary patterns; culturally defined diets; adult weight gain (no judgment for pre-menopausal breast cancer, but increased risk for post-menopausal breast cancer); birth weight (no judgment for post-menopausal breast cancer, but increased risk for pre-menopausal breast cancer); birth length (no judgment for post-menopausal breast cancer); energy intake; and being breastfed, due to limited evidence.5

section reviewed 13/06/14
section updated 13/06/14

Breast cancer risk is not associated with the following factors, meta- and pooled analyses or systematic reviews have shown:

  • Pregnancy loss (spontaneous or induced).150
  • Systemic sclerosis.151
  • Pregnancy-induced hypertension in mother.57,146
  • Undergoing in vitro fertilisation treatment involving ovarian stimulation203,204 (though some evidence of higher risk with multiple clomiphene citrate cycles205,206).
  • Polycystic ovary syndrome.211
  • Gestational diabetes (some evidence of higher risk in case-control studies).223
  • Parkinson’s disease230 (some evidence of higher risk231).
  • Migraine.237
  • Rheumatoid arthritis (some evidence of lower risk).238

section reviewed 01/05/14
section updated 18/02/15

  • Cosmetic breast implants (though may be associated with advanced stage at diagnosis).152
  • Statins.183
  • Calcium channel blockers.217
  • Diuretics (some evidence of higher risk with long-term use in obese women).136

section reviewed 01/05/14
section updated 21/01/15

  • Red meat.174,175
  • Processed meat.174
  • Green or black tea.160,232
  • Folate (dietary and blood levels; some evidence of varying effect with dose).201,215
  • Vitamin supplements.161,219
  • Vitamin D levels (from diet, supplements, or circulating in the blood).176,210,220,226
  • Eggs (though some evidence of higher risk).180
  • Fish.159

section reviewed 01/05/14
section updated 17/02/15

section reviewed 01/05/14
section updated 05/01/14

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Updated: 21 May 2014