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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, a meta-analysis showed.164

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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, a meta-analysis showed; among  BRCA2 carriers, breast cancer risk is not associated with age at first birth.164

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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

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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, and ethnicity.18,20

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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

Breast cancer risk is around doubled in women with one first-degree relative with breast cancer, compared with women with no first-degree relatives, meta- and pooled-analyses have shown.34,35 The risk is further increased with a larger number of affected first-degree relatives, or relatives affected aged under 50.34,35 The risk increase is similar for first-degree relatives with ER-positive or ER-negative breast cancer.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 Environmental and lifestyle factors explain around three-quarters of breast cancer risk, with hereditary factors explaining only around a quarter.37 The reasons for breast cancer clustering in families remain largely unclear, but a small proportion of families share breast cancer predisposition genes, some of which are discussed below.

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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

Li Fraumeni syndrome (caused by TP53 mutation) and Cowden syndrome (caused by PTEN mutation) are high-penetrance breast cancer predisposition genes, but they are both rare and so account for a very low proportion of breast cancer cases overall and among cases with first-degree family history.42,46 NHS screening for TP53 and PTEN mutation is available.

Mutations in CHEK2, ATM, BRIP1, and PALB2 confer an intermediate risk of breast cancer in carriers, but again are rare. Mutations in a number of other genes are more common but confer a lower risk of breast cancer.42,46 Breast cancer risk in some other rare genetic mutation syndromes, such as Peutz-Jeghers syndrome (caused by STK11 mutation), and hereditary diffuse gastric cancer syndrome (caused by CDH1 mutations), remains unclear.42,46

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Overweight and obesity

Greater body fatness (measured by BMI) is classified by WCRF/AICR as a cause of post-menopausal breast cancer; and abdominal fatness, and weight gain during adulthood, as probable causes (Table 4.1).5 Greater body fatness is classified by WCRF/AICR as possibly protective against pre-menopausal breast cancer.5 Body fatness is the largest modifiable risk factor for post-menopausal breast cancer in the UK, and because breast cancer is much more common in post-menopausal than pre-menopausal women, body fatness is overall linked with excess cases in the population.1 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

Greater birth weight is classified by WCRF/AICR as a probable cause of pre-menopausal breast cancer (Table 4.1).5

Among post-menopausal women, breast cancer risk is 15% higher in those who are overweight (BMI 25-29.9) or obese (BMI 30+), compared with 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 is 50% higher in post-menopausal women with the highest waist-to-hip ratio (WHR, a measure of abdominal obesity) compared with 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 risk is 50% higher in non HRT-using post-menopausal women with the highest levels of adulthood weight gain, compared with 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

Greater birth weight is classified by WCRF/AICR as a probable cause of pre-menopausal breast cancer (Table 4.1).5

Among pre-menopausal women, breast cancer risk is slightly lower in those who are overweight or obese, compared with healthy weight, but meta-analyses indicate this may be limited to Caucasians and Africans (not Asians), and to ER- and PR-positive tumours.51,55,56,170 The association between BMI and pre-menopausal breast cancer risk was not significant in one meta-analysis.50

Abdominal fatness may be more closely associated with pre-menopausal breast cancer risk.53 Breast cancer risk is 79% higher in pre-menopausal women with the highest WHR compared with those with the lowest WHR, a meta-analysis showed.52 Risk is 8% higher for every 0.1 unit WHR increase.56

Pre-menopausal breast cancer risk overall is not associated with weight gain during adulthood, a pooled-analysis showed.54 This may vary by hormone receptor status and parity, a case control study indicated.181

Women with a higher birth weight are at increased risk of breast cancer, 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 is 30% higher in men with the highest BMI versus those with the lowest.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 more than four times higher in women with the most dense breasts (at least 75% 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

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

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'. Have 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

Increased risk of breast cancer following some previous cancers may be associated with radiotherapy treatment for the previous cancer.

Breast cancer

Breast cancer survivors have between two and five times increased risk of developing a second primary breast cancer, cohort studies show.78-80 Excluding 'second cancers' found within two years of the primary, which may actually be spread from the primary tumour, risk of second primary breast cancer remains significantly elevated for 20 years from the primary diagnosis.79 Assessing second primary breast cancer risk in breast cancer survivors is difficult, because international cancer coding rules preclude the recording of two identical cancers in the same body site (even if the second cancer is in the opposite breast).81

Among breast cancer survivors, the risk of second primary breast cancer increased by 14% per 5-unit BMI increase, a meta-analysis showed.82 Among survivors of ER-positive breast cancer, second primary breast cancer risk is reduced by around 40% by taking tamoxifen for five years, a meta-analysis found.83

Second primary breast cancer risk in BRCA1 or BRCA2 mutation carriers is around doubled in those with a previous breast cancer, versus those without a previous breast cancer, a cohort study showed.208

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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 cancer in women aged 75 and under are caused by exposure to diagnostic x-rays.93 X-ray-associated breast cancer risk is further elevated in women with BRCA1 or BRCA2 mutation.94 Mammograms are associated with a very small number of breast cancer: of 10,000 women who are screened every three years between the ages of 47 and 73, between three and six will develop cancer during their lifetime because of mammogram radiation.95 Exposure to computed tomography (CT) scans in childhood or adolescence does not appear to be linked with increased breast cancer risk.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

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

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

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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

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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

Exposure to light at night is associated with higher levels of sex hormones, because it disturbs the circadian system, which suppresses melatonin production, and melatonin is thought to reduce circulating oestrogen.120,121 This 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

Breast cancer risk is 19-48% higher in women who have ever worked overnight shifts, meta-analyses have shown.122-126 Risk estimates appear to be higher in older and better-quality studies. Nurses and flight attendants are the night workers most commonly studied.125-127

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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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Anti-hypertensive medicine

Breast cancer risk is around 20% higher in pre-/peri-menopausal women taking anti-hypertensive medication for five years or longer, a large cohort study found.136

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Diabetes

Breast cancer risk is 10-23% higher in diabetic women, compared with non-diabetic women, meta-analyses show.137-139 The association may be limited to post-menopausal women and those with Type II diabetes.138 Breast cancer risk may vary by treatment type among diabetic women, with lower risk in users of pioglitazone or metformin.140-143 Male breast cancer risk is 19% higher in diabetics versus non-diabetics, a meta-analysis showed.187

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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, compared with those with the lowest, a meta-analysis showed.165 Accordingly, breast cancer risk is 15% lower in users of bisphosphonates (used to treat bone thinning) compared with non-users, a meta-analysis showed.166

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

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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 Canadian case-control study indicated.100

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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 decreased in people with coeliac disease, a large cohort study found, however mechanisms are unclear.145

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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

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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.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

section reviewed 01/05/14
section updated 11/08/14

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
  • Cosmetic breast implants (though may be associated with advanced stage at diagnosis).152
  • Pregnancy-induced hypertension in mother.57,146
  • Statins.183
  • Red meat.174,175
  • Green or black tea.160
  • Dietary folate.201
  • Vitamin supplements.161
  • Vitamin D levels (from diet or circulating in the blood).176,210
  • Eggs (though some evidence of risk increase).180
  • Undergoing in vitro fertilisation treatment involving ovarian stimulation203,204 (though some evidence of increased risk with multiple clomiphene citrate cycles205,206).
  • Polycystic ovary syndrome.211

section reviewed 01/05/14
section updated 12/08/14

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Updated: 21 May 2014