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Lifestyle factors

Lifestyle factors on this page have been the subject of research to investigate whether they are associated with an increased or decreased risk of breast cancer. Study results have shown these lifestyle factors, including fruit, soy products, caffeine, passive smoking and bras, to be unproven or unlikely to influence risk of breast cancer.

Fruit

Inconclusive

The evidence is too limited to determine the likelihood of an association with increased or decreased risk of breast cancer.

There is no conclusive evidence that eating fruit is associated with a decreased risk of breast cancer.

Some studies have shown a link between fruit intake and decreased risk of postmenopausal breast cancer however other studies have shown no effect. A recent good quality study provides supportive evidence for a reduced risk of breast cancer associated with eating a high amount of fruit.

Fruits are the edible parts of a plant that consist of seeds and surrounding tissues. Types of fruit include apples, bananas, berries, figs, grapes, mangos, melons, citrus fruits such as oranges and lemons, and dried fruits. Several of the components of fruit could affect the risk of breast cancer.

Evidence classification: Inconclusive

The evidence for any association between fruit intake and breast cancer risk is inconclusive.

Studies have shown inconsistent results.  A recent large cohort study reported a positive association between high versus low amounts of fruit intake after long-term follow-up. 

Proposed mechanisms

‘Fruit’ refers to the edible part of a plant that consists of seeds and surrounding tissues. Types of fruit include apples, bananas, berries, figs, grapes, mangos, melons, citrus fruits such as oranges and lemons, and dried fruits.

Potential mechanisms for any association between fruit intake and breast cancer risk may be via components of fruits, such as vitamins C and E, minerals, fibre and other bioactive compounds (for example, antioxidants and polyphenols in berries). These compounds may reduce breast cancer risk by reducing oxidative damage to DNA, increasing programmed cell death (apoptosis) or increasing the activity of enzymes able to detoxify carcinogens.

Evidence 

The World Cancer Research Fund International/ American Institute for Cancer Research (WCRF/ AICR) classified the evidence for an association between fruit intake and risk of breast cancer as ‘limited – no conclusion’ for both premenopausal and postmenopausal breast cancer.1

Despite grading the evidence as ‘Limited – no conclusion’, meta-analyses in the WCRF/AICR review indicated that fruit intake was associated with reduced risk of breast cancer overall and of postmenopausal breast cancer, with a dose-response relationship (relative risk [RR] per 200 g/day 0.94, 95% confidence interval [CI] 0.90‒0.98; 0.92, 95 % CI 0.87–0.98; respectively).2

Two large prospective cohort studies published since the WCRF/AICR review have produced inconsistent results. One prospective cohort study of 335,054 women with a median follow-up of 11.5 years found no association between fruit intake and breast cancer risk.3 Another prospective cohort study of 182,145 women with a median follow-up of 23.7 years found that high versus low fruit consumption was associated with a reduced risk of breast cancer (HR >2.5 servings/day versus ≤4 servings/week 0.91, 95% CI 0.84–0.99).4

Read the full Review of the Evidence

References
  1. World Cancer Research Fund/American Institute for Cancer Research (2018). Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity and breast cancer. London, UK.
  2. World Cancer Research Fund (2017). Continuous Update Project Systematic Literature Review: The associations between food, nutrition and physical activity and the risk of breast cancer. London, UK.
  3. Emaus MJ, Peeters PH, Bakker MF, et al. (2016). Vegetable and fruit consumption and the risk of hormone receptor-defined breast cancer in the EPIC cohort. American Journal of Clinical Nutrition 2103(1):168–177.
  4. Farvid MS, Chen WY, Rosner BA, et al. (2018) Fruit and vegetable consumption and breast cancer incidence: Repeated measures over 30 years of follow-up. International Journal of Cancer Jul 6. DOI: 10.1002/ijc.31653
Phytoestrogens and soy products

Inconclusive

The evidence is too limited to determine the likelihood of an association with increased or decreased risk of breast cancer.

There is no conclusive evidence that phytoestrogens (plant oestrogens) and soy products in the diet are associated with an increased risk of breast cancer. Only a small number of studies have been done and these have produced inconsistent results.

Phytoestrogens are naturally occurring chemicals found in plants. Different types of phytoestrogens are found in soy products, some types of fruits and vegetables, grains and seeds, and red wine.

Phytoestrogens have a similar chemical structure to the hormone oestrogen, which occurs naturally in women’s bodies. It has been suggested that phytoestrogens might act like oestrogen and affect a woman’s risk of breast cancer; however the evidence does not support this.

Some studies have actually linked phytoestrogens and soy products with a decreased risk of breast cancer; however most studies have not shown an association.

Evidence classification: Inconclusive

The evidence for any association between phytoestrogens and risk of breast cancer is inconclusive.

The small number of studies examining various different types of phytoestrogens, such as lignans, isoflavones and soya and soya products, have provided inconsistent results. However, meta-analyses of the higher-quality prospective cohort studies show no association with breast cancer risk for intake of any specific groups of dietary phytoestrogens including ‘isoflavones’ and ‘soy and soy products’.

Evidence 

Phytoestrogens are plant-derived compounds that have a similar chemical structure to oestrogens. They include isoflavones (from soy beans, soya products and vegetables), flavonoids (from red and yellow fruits and vegetables), lignans (from flaxseed, whole grains, fruits and vegetables), coumestans (from peas, beans, alfalfa and sunflower seeds) and stilbenes (from red wine).

The World Cancer Research Fund International/American Institute for Cancer Research (WCRF/AICR) has classified the evidence for any association between breast cancer risk and ‘phytoestrogens’, ‘isoflavones’, and ‘soya and soya products’ as ‘limited – no conclusion’.1 Small numbers of studies were available for the different types of phytoestrogens, with high heterogeneity and inconsistent findings. Most studies showed no association with breast cancer risk.

Two recent meta-analyses have also shown no association between highest versus lowest intakes of total dietary flavonoids2 or soy food3 and breast cancer risk. No associations with breast cancer risk were found for any of the individual dietary flavonoids studied.2

Longer-term follow-up of the Shanghai Women’s Study cohort (for which earlier results were included in the analysis by the WCRF/AICR) showed an association between adult soy intake and decreased risk of breast cancer, as well as decreased risk of premenopausal breast cancer.4

Read the full Review of the Evidence

References
  1. World Cancer Research Fund/American Institute for Cancer Research (2018). Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity and breast cancer. London, UK.
  2. Grosso G, Godos J, Lamuela‐Raventos R, et al. (2017). A comprehensive meta‐analysis on dietary flavonoid and lignan intake and cancer risk: level of evidence and limitations. Molecular Nutrition & Food Research 61(4).
  3. Wu J, Zeng R, Huang J, et al. (2016). Dietary protein sources and incidence of breast cancer: a dose–response meta-analysis of prospective studies. Nutrients 8(11):730.
  4. Baglia ML, Zheng W, Li H, et al. (2016). The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. International Journal of Cancer 139(4):742–748.
Coffee, tea and caffeine

Inconclusive

The evidence is too limited to determine the likelihood of an association with increased or decreased risk of breast cancer.

There is no conclusive evidence that drinking coffee or tea is associated with decreased risk of breast cancer. While a large number of studies have been done, some have shown a decreased risk however others have shown no association.

Coffee and tea contain caffeine, a naturally occurring plant chemical, as well as other compounds. It has been suggested that these could decrease the risk of breast cancer by affecting levels of some hormones in the body.

Evidence classification: Inconclusive

The evidence for any association between coffee and/or tea intake and risk of breast cancer is inconclusive.

The majority of evidence is for coffee and caffeine. For coffee, findings are inconsistent across meta-analyses of large numbers of studies, although there is some evidence that higher versus lower levels of coffee may be associated with a decreased risk of postmenopausal breast cancer. Meta-analyses of black or green tea consumption, or caffeine intake, have not shown any associations with risk of breast cancer.

Proposed mechanisms

Coffee and tea contain caffeine, a naturally occurring plant alkaloid. It has been suggested that a high intake of caffeine may decrease levels of circulating oestrogen and testosterone via an increase in sex hormone-binding globulin, thereby decreasing risk of breast cancer. Coffee and tea also contain other compounds, such as polyphenols, that might have anti-carcinogenic effects.

Evidence 

The World Cancer Research Fund International/American Institute for Cancer Research (WCRF/AICR) has classified the evidence for any association between consumption of coffee or tea and risk of breast cancer as ‘limited – no conclusion’ for both premenopausal and postmenopausal breast cancer.1

Dose–response meta-analyses undertaken by the WCRF/AICR found a borderline dose-response association for 1 cup/day increments in coffee consumption and decreased risk of breast cancer overall and postmenopausal breast cancer.2 There was no association for premenopausal breast cancer. The WCRF/AICR also found no associations between consumption of black or green tea and risk of breast cancer. 2

A recent meta-analysis of 13 prospective cohort studies reported a significant dose-response association between coffee consumption and decreased risk of postmenopausal, but not premenopausal breast cancer or breast cancer overall.3 A previous meta-analysis of 16 cohort and 10 case-control studies had found a borderline significant inverse association between coffee intake and risk of breast cancer.4 However, another meta-analysis of 17 prospective and 20 case-control studies did not find a significant association between coffee consumption and breast cancer risk.5

A recent meta-analysis of 9 prospective studies reported no association between highest versus lowest intakes of caffeine and risk of breast cancer.6

Read the full Review of the Evidence

References
  1. World Cancer Research Fund/American Institute for Cancer Research (2018). Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity and breast cancer. London, UK.
  2. World Cancer Research Fund (2017). Continuous Update Project Systematic Literature Review: The associations between food, nutrition and physical activity and the risk of breast cancer. London, UK.
  3. Lafranconi A, Micek A, De Paoli P, et al. (2018). Coffee intake decreases risk of postmenopausal breast cancer: a dose-response meta-analysis on prospective cohort studies. Nutrients 10(2).
  4. Li KJ, Ren ZJ, Qin JW, et al. (2013). Coffee consumption and risk of breast cancer: an up-to-date meta-analysis. PLoS One 8(1):e52681.
  5. Jiang W, Wu Y& Jiang X (2013). Coffee and caffeine intake and breast cancer risk: an updated dose-response meta-analysis of 37 published studies. Gynecologic Oncology 129(3):620–629.
  6. Grosso G, Godos J, Galvano F et al. (2017). Coffee, caffeine, and health outcomes: an umbrella review. Annual Review of Nutrition 37:131–156.
Passive smoking

Inconclusive

The evidence is too limited to determine the likelihood of an association with increased or decreased risk of breast cancer.

There is no conclusive evidence that passive smoking, or second-hand smoking is associated with an increased risk of breast cancer. Study findings have been inconsistent, with the best quality studies not finding any association.

Passive smoking means breathing in the smoke from other people’s cigarettes, cigars and pipes. This smoke is made up of the smoke that the smoker breathes out and the smoke that comes from the burning cigarette. It contains the same cancer-causing chemicals that are inhaled by smokers.

Evidence classification: Inconclusive

The evidence for any association between exposure to environmental tobacco smoke (passive smoking; second-hand smoke) and risk of breast cancer is inconclusive. 

The evidence is inconsistent. Some case-control studies have reported a positive association between passive smoking and risk of breast cancer however the more robust cohort studies do not support an association.

Proposed mechanisms

Environmental tobacco smoke (ETS) is the combination of the smoke that a smoker breathes out and the smoke that comes directly from their burning cigarette or other tobacco product. This smoke contains the same carcinogens that are inhaled by smokers. Environmental tobacco smoke is classified by the International Agency for Research on Cancer (IARC) as a Group 1 carcinogen and IARC concluded that ETS is a cause of lung cancer in never smokers.1

Evidence 

IARC considered the evidence for an association between ETS and breast cancer to be inconsistent for breast cancer overall and for premenopausal breast cancer.1 IARC noted a number of limitations of the studies on which this conclusion was based – for example, the information on exposure to ETS relies heavily on recall of past exposures outside the home, which might be underestimated.

A number of recent meta-analyses2,3,4,5 have found an increased risk of breast cancer associated with some exposures to ETS when all studies are combined but when stratified by study type, the association is mainly observed only in case–control studies, with one meta-analysis finding an increased risk also in cohort studies3 and another showing an increased risk of premenopausal breast cancer but not postmenopausal breast cancer among cohort studies.2

This latter meta-analysis, which included 47 studies (15 cohort studies, 30 case-control studies and 2 nested case–control studies), found an association between ‘any adult exposure’, ‘total exposure’, ‘spouse ever smoked’ and ‘at home exposure’ to ETS and increased breast cancer risk. However, associations were not significant for workplace exposure and ‘at-home exposure’.2 Definitions of environmental tobacco smoke exposure varied markedly across studies and there was significant heterogeneity between studies.

Dose-response analyses have provided inconsistent evidence of an association.2,5

Read the full Review of the Evidence

References
  1. International Agency for Research on Cancer (2012). Personal habits and indoor combustions, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, volume 100E, IARC Working Group on the Evaluation of Carcinogenic Risk to Humans, Lyon, http://monographs.iarc.fr/ENG/Monographs/vol100E/mono100E.pdf.
  2. Lee PN & Hamling JS (2016). Environmental tobacco smoke exposure and risk of breast cancer in nonsmoking women: an updated review and meta-analysis. Inhalation Toxicology 28(10):431–454.
  3. Macacu A, Autier P, Boniol M, et al. (2015) Active and passive smoking and risk of breast cancer: a meta-analysis. Breast Cancer Research and Treatment 154:213–224.
  4. Chen Z, Shao J, Gao X, et al. (2015). Effect of passive smoking on female breast cancer in China: a meta-analysis. Asia Pacific Journal of Public Health 27(2): NP58–NP64.
  5. Chen C, Huang Y-B, Liu X-O, et al. (2014). Active and passive smoking with breast cancer risk for Chinese females: a systematic review and meta-analysis. Chinese Journal of Cancer 33(6):306.
Bras

Inconclusive

The evidence is too limited to determine the likelihood of an association with increased or decreased risk of breast cancer.

There is no conclusive evidence that wearing a bra is associated with an increased risk of breast cancer.

Only a small number of poor quality studies have been done, and the best evidence does not show an association.

Some media stories have suggested that wearing a bra and particularly an underwire bra increases the risk of breast cancer. This is based on the idea that bras might block the breast’s lymphatic system causing the accumulation of lymphatic fluid and ‘toxins’ in the breast. There is no evidence to support this or any other suggested mechanism. 

Evidence classification: Inconclusive

The evidence of any association between aspects of bra wearing and risk of breast cancer is inconclusive.

There is a very limited amount of poor quality evidence examining aspects of bra-wearing and breast cancer risk. The best available evidence shows no association between any aspect of wearing a bra and breast cancer risk.

Proposed mechanisms

The proposed mechanisms for a link between bra use and breast cancer include an effect of bras on increasing the surface temperature of the breast and on impedance of lymphatic drainage from the breast allowing accumulation of ‘toxins’. There is no evidence to support these proposed mechanisms.1,2

Evidence 

There are no published cohort studies. Only one population-based case-control study has examined an association between bra wearing and risk of breast cancer.1 This study of approximately 1500 postmenopausal women found no association between the number of hours per day that bras were worn, or the age when regular bra wearing began, or wearing an underwire bra, and risk of breast cancer.

Additional evidence is too low in quality to reliably contribute to the evidence base.3

Read the full Review of the Evidence

References
  1. Chen L, Malone KE, Li CI (2014). Bra wearing not associated with breast cancer risk: a population-based case–control studyCancer Epidemiology, Biomarkers & Prevention 17:43–50.
  2. Gansler T, Jemal A (2009). Axillary lymphatic disruption does not increase risk of breast carcinoma. The Breast Journal 15(4):438-439.
  3. So WKW, Chan DNS, Lou Y, et al. (2015). Brassiere wearing and breast cancer risk: a systematic review and meta-analysisWorld Journal of Meta-Analysis 3:193–205.



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