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Fruit and vegetable consumption and breast cancer incidence: Repeated measures over 30 years of follow-up.

Farvid et al,

2019

Int J Cancer

A large prospective study of 182,145 women in the Nurses’ Health Study found that higher fruit and vegetable consumption was associated with an 11% lower risk of invasive breast cancer, with the strongest benefits from cruciferous vegetables (like broccoli and cabbage) and yellow/orange vegetables (like carrots and squash). Women consuming more than 5.5 servings per day had significantly lower breast cancer risk compared to those eating 2.5 servings or less, with particularly strong protective effects against aggressive tumor types including estrogen receptor-negative (16% risk reduction per 2 additional servings/day), HER2-enriched (21% reduction), and basal-like (16% reduction) breast cancers. The protective associations were most pronounced for tumor subtypes that tend to be more aggressive and harder to treat. These findings suggest that a diet rich in fruits and vegetables, especially cruciferous and yellow/orange varieties, may help prevent breast cancer and particularly reduce the risk of developing more aggressive forms of the disease.

Postmenopausal breast cancer and occupational exposure to chemicals.

Videnros et al,

2019

J Work Environ Health

A prospective study of 16,084 Swedish women found that occupational exposure to chemicals was associated with a 26% increased risk of postmenopausal breast cancer, with risk increasing based on duration of exposure. Women exposed to diesel exhaust for more than 10 years showed a 69% increased risk, while exposure to organic solvents showed a non-significant elevated risk. The researchers estimated that occupational chemical exposures accounted for 2% of all breast cancer cases in this population, highlighting workplace chemical exposure as a potentially important but often overlooked risk factor for breast cancer.

Use of dietary supplements containing soy isoflavones and breast cancer risk among women aged >50 y: a prospective study.

Touillaud et al,

2019

Am J Clin Nutr

A large French cohort study of 76,442 women over age 50 followed for 11 years found that current soy supplement use was associated with a 22% reduced risk of estrogen receptor-positive (ER+) breast cancer but a 101% increased risk of ER-negative breast cancer compared to never users. The risk profile varied significantly by personal characteristics: women with a family history of breast cancer showed a 36% increased risk with soy supplement use, while those without family history showed an 18% reduced risk; premenopausal or recently postmenopausal women showed a 50% risk reduction, while women more than 5 years past menopause showed a 6% increased risk. These findings suggest that soy supplements—often marketed as natural alternatives to hormone therapy—may have complex and opposing effects on breast cancer risk depending on tumor biology and individual characteristics, cautioning against their use particularly in women with breast cancer family history.

Night shift work and breast cancer: a pooled analysis of population-based case-control studies with complete work history.

Cordina-Duverger et al,

2018

Eur J Epidemiol

This large international study pooling data from over 13,000 women across five countries found that night shift work increased breast cancer risk by 26% in pre-menopausal women, with the risk rising substantially for those working longer shifts (≥10 hours), more frequent nights (≥3 nights per week), or longer durations (≥10 years). Pre-menopausal women working both long durations and high frequency had a 2.5 times higher breast cancer risk, with current or recent night workers at higher risk than those who had stopped more than two years ago. Notably, no increased risk was found in post-menopausal women, and the elevated risk was primarily for estrogen receptor-positive tumors, particularly those that were also HER2-positive, suggesting that disruption of hormones and circadian rhythms during reproductive years may be key factors in how night work affects breast cancer risk.

Occupational exposure to organic solvents and risk of male breast cancer: a European multicenter case-control study.

Laouali et al,

2018

Scand J Work Environ Health

A European multicenter case-control study of 104 male breast cancer cases and 1,901 controls found that high cumulative lifetime exposure to trichloroethylene (>23.9 ppm-years) was associated with a 110% increased male breast cancer risk compared to non-exposure, with the association persisting when only exposures occurring 10 or more years before diagnosis were considered, indicating a true latency effect. The study also suggested possible roles for benzene and ethylene glycol in male breast cancer risk, though no clear dose-response relationships were observed for these chemicals. These findings are particularly important given that male breast cancer is rare, often diagnosed at later stages, and has worse prognosis than female breast cancer, and they add to evidence that occupational solvent exposures—particularly trichloroethylene used in metal degreasing, dry cleaning, and industrial cleaning—may be underrecognized risk factors for breast cancer in both men and women, warranting stricter workplace exposure limits and enhanced surveillance of workers in high-exposure industries.

Breast Cancer Incidence and Exposure to Metalworking Fluid in a Cohort of Female Autoworkers.

Garcia et al,

2018

Am J Epidemiol

Job| Human| Epidemiological Studies| Human Studies| Occupation

A cohort study of 4,503 female autoworkers in Michigan exposed to metalworking fluids (MWFs)—oil and chemical mixtures used in metal manufacturing—found that increased exposure to straight mineral oil MWFs was associated with a 13% increased breast cancer risk per interquartile range increase in cumulative exposure. Among younger women who developed premenopausal breast cancer, exposure to synthetic MWFs (chemical lubricants without oil) showed elevated risk, suggesting potentially different carcinogenic mechanisms in younger versus older women. This occupational study addresses a critical gap in breast cancer research by providing quantitative exposure-response data for a specific chemical mixture affecting a large workforce, offering one of the few leads on modifiable environmental risk factors for breast cancer.

Adipose tissue levels of polybrominated diphenyl ethers and breast cancer risk in Chinese women: A case-control study.

He et al,

2018

Environ Res

Furniture & Electronics| Human| Epidemiological Studies| Flame Retardants| Human Studies

A Chinese case-control study of 209 breast cancer patients and 165 controls found that women with the highest levels of PBDE flame retardants in their adipose tissue had dramatically elevated breast cancer risk, with some congeners showing 447% to 545% increased risk in the highest exposure groups compared to the lowest. Most individual PBDE congeners—including BDE-47, BDE-99, BDE-100, BDE-209, and total PBDEs—were significantly higher in breast cancer cases than controls (median 95.0 vs 73.7 ng/g lipid) and showed strong dose-response relationships with increasing exposure levels. The associations remained robust for estrogen receptor-positive breast cancers and in multivariate models adjusting for age and reproductive factors, providing compelling evidence that PBDE exposure may play an important role in breast cancer development, particularly in populations with high environmental exposures to these persistent flame retardant chemicals.

Long-term exposure to road traffic Noise and incidence of breast cancer: a cohort study.

Andersen et al,

2018

Breast Cancer Res

A nationwide study of 22,466 Danish female nurses found that long-term exposure to road traffic Noise was associated with increased breast cancer risk, particularly for hormone receptor-positive tumors. For every 10-decibel increase in 24-year average Noise levels at their residence, women showed a 23% increased risk of estrogen receptor-positive (ER+) breast cancer and a 21% increased risk of progesterone receptor-positive (PR+) breast cancer, with no significant association for hormone receptor-negative tumors. The association was especially strong among nurses who worked night shifts, showing a 236% increased risk of ER+ breast cancer compared to a 21% increase in those not working nights. These findings suggest that chronic traffic Noise exposure may contribute to breast cancer development, potentially through stress-related hormonal pathways, with night shift work potentially amplifying this risk.

Possible role of stress, coping strategies, and life style in the development of breast cancer.

Yıldırım et al,

2018

Int J Psychiatry Med

A hospital-based study of 250 breast cancer patients and 250 controls in Turkey found that recent major stressors—particularly those occurring within the last five years—were associated with a 372% increased risk of breast cancer (the strongest factor identified). Other significant risk factors included inadequate social support (83% increased risk), loss of a father during childhood (168% increased risk), family history of cancer (55% increased risk), and history of psychiatric disorders (95% increased risk), suggesting that psychological trauma and social factors may play substantial roles in breast cancer development alongside genetic predisposition.

Elevated risk for male breast cancer after occupational exposure to gasoline and vehicular combustion products.

Hansen et al,

2000

Am J Indust Med

This Danish nationwide study of 230 male breast cancer cases found that men occupationally exposed to gasoline and its combustion products had a 2.5 times higher risk of breast cancer, with the risk rising to 5.4 times higher among men first exposed before age 40. Gasoline contains several known carcinogens including benzene and produces cancer-causing combustion products, which may explain this elevated risk. Since male breast cancer is rare but shares similar biology with female breast cancer, these findings suggest that gasoline exposure may also increase breast cancer risk in women and warrant further investigation in female workers.

Breast cancer incidence after hormonal treatment for infertility: A meta-analysis of population-based studies.

Zaman et al,

2025

Saudi Med J

A systematic review and meta-analysis of 15 studies including 92,555 women found no evidence that hormonal fertility medications increase breast cancer risk, with pooled risk ratio analysis showing no association (RR=1.00, 95% CI: 0.97-1.02) and hazard ratio analysis initially suggesting a protective effect that became non-significant after accounting for study heterogeneity. The analysis revealed low heterogeneity in risk ratio studies but substantial heterogeneity in hazard ratio studies, which the authors attributed to methodological differences between studies rather than true variation in effects. These findings provide reassuring evidence for women considering or undergoing fertility treatment, though the authors caution that results should be interpreted carefully given the study heterogeneity and note that longer-term follow-up studies with standardized methodologies are needed to definitively establish the safety profile of hormonal fertility medications with respect to breast cancer risk, particularly for specific medication types, dosages, and treatment durations.

Consumption of fast foods and ultra-processed foods and breast cancer risk: a systematic review and meta-analysis.

Karimi et al,

2025

Global Health Res Pol

A meta-analysis of 17 observational studies found that the highest consumption of fast foods and ultra-processed foods (FFs/UPFs) was associated with a 25% increased breast cancer risk compared to the lowest consumption levels. The association was particularly strong in case-control studies and in Latin American populations, and remained significant regardless of sample size or BMI adjustment, though no association was found in cohort studies or when stratified by menopausal status. These findings suggest that diets high in ultra-processed foods—which often contain carcinogenic compounds formed during processing, additives, packaging contaminants, and unhealthy nutrient profiles—may contribute to breast cancer development, highlighting the urgent need for public health strategies and regulatory policies targeting food processing standards, marketing restrictions, improved labeling, and reduced accessibility of ultra-processed foods to reduce cancer risk, particularly as global consumption of these products continues to rise.

Association between nitrogen dioxide and incident breast cancer in Alberta’s tomorrow project.

Ahmadi et al,

2025

Sci Rep

A secondary analysis of 15,536 post-menopausal women from the Alberta Tomorrow Project cohort examined whether exposure to traffic-related air pollution (TRAP), measured using nitrogen dioxide (NO₂) levels, was associated with breast cancer risk over 12.6 years of follow-up during which 523 breast cancer cases were diagnosed. After adjusting for confounders including age, education, births, diet, income, physical activity, BMI, and smoking, the study found no significant association between NO₂ exposure and post-menopausal breast cancer risk (HR = 1.10 per 10-ppb increase in NO₂; 95% CI = 0.90, 1.34). The authors suggest that while the magnitude of risk observed was similar to other studies, the lack of statistical significance may be due to the study’s sample size limitations and the fact that NO₂ levels in Alberta are lower than in many other regions globally where associations have been found. These findings indicate that traffic-related air pollution may not significantly impact post-menopausal breast cancer risk in regions with relatively low pollution levels, though larger studies in more polluted areas may be needed to fully assess this relationship.

Effects of endocrine disrupting chemicals, blood metabolome, and epigenetics on breast cancer risk: A multi-dimensional mendelian randomization study.

Song et al,

2025

Ecotoxicol Env Saf

A Mendelian randomization study using genetic data from European populations found that specific endocrine-disrupting chemicals (EDCs) were causally associated with different breast cancer subtypes: n-butyl paraben (n-BuP) increased Luminal A risk, mono-methyl phthalate decreased Luminal B risk, and mono-iso-butyl phthalate (MiBP) increased triple-negative breast cancer risk. Mediation analysis revealed that blood metabolites—including caffeic acid sulfate and caffeine metabolism ratios—partially explained the n-BuP effect on Luminal A, while methylsuccinate mediated the MiBP effect on triple-negative cancer, and epigenetic analysis identified specific DNA methylation sites associated with EDC exposure and breast cancer risk. These findings provide the first genetic evidence suggesting causal relationships between specific EDC exposures and breast cancer subtypes through distinct metabolic and epigenetic pathways, identifying potential biomarkers for early detection and highlighting the heterogeneous effects of different EDCs on breast cancer biology—underscoring the need for chemical-specific and subtype-specific prevention strategies rather than treating all EDCs or breast cancers as uniform entities.

Industrial Air Emissions and Breast Cancer Incidence in a United States-wide Prospective Cohort.

Ish et al,

2025

Epidemiol

A prospective study of 46,150 Sister Study participants followed for a median 13.4 years (4,155 breast cancer cases) used EPA Toxics Release Inventory data to quantify residential air emissions of 28 industrial compounds—many carcinogenic or endocrine-disrupting—during the 10 years before enrollment (2003-2006). Emissions within 3 km of residences showed non-significant associations with breast cancer for nickel compounds (HR = 1.3; 95% CI: 1.0-1.6 for highest vs. no exposure) and trichloroethylene (HR = 1.3; 95% CI: 1.0-1.6), while exposure continuum mapping identified 25 mixture profiles explaining 72% of emissions variance, with the joint-exposure response function suggesting higher breast cancer incidence among individuals with rare, high-emission profiles, though the overall mixture trend was not statistically significant (p = 0.09). These findings indicate that residential proximity to industrial air emissions of certain carcinogens—particularly nickel compounds and trichloroethylene—may be associated with increased breast cancer risk, though the lack of overall mixture association may reflect that individual compounds or specific emission sources are more important than cumulative exposure profiles, or that most participants experienced relatively low emissions with elevated risks concentrated among small subgroups with high exposure to specific pollutants.

Associations between phenol and paraben exposure and the risk of developing breast cancer in adult women: a cross-sectional study.

Xiong et al,

2025

Sci Rep

A large U.S. study of 4,455 people found that exposure to triclosan (TRS)—an antibacterial chemical commonly found in soaps, toothpaste, and personal care products—was associated with more than double the risk of breast cancer at moderate exposure levels, with the relationship following an inverted U-shape pattern. The association was strongest among overweight individuals, people under 60 years old, and white participants, while other tested chemicals (including bisphenol A, benzophenone-3, and parabens) showed no significant link to breast cancer risk. These findings suggest that triclosan exposure may be an important modifiable risk factor for breast cancer, particularly for certain population groups, though more research is needed to understand the underlying mechanisms and confirm causality.

Multilevel Stressors and Systemic and Tumor Immunity in Black and White Women With Breast Cancer.

Harris et al,

2025

JAMA Netw Open

A cross-sectional study of 121 Black and White women with breast cancer found that chronic stressors—including perceived stress, inadequate social support, discrimination, and neighborhood deprivation—were associated with harmful immune and tumor changes, with particularly pronounced effects in Black women who also lived in significantly more socioeconomically deprived neighborhoods. Higher stress, discrimination, and neighborhood deprivation were linked to increased systemic inflammation, immune-suppressive tumor environments (including tumor-promoting M2 macrophages), and elevated tumor mutational burden, while greater social support was associated with beneficial immune-stimulatory changes including increased natural killer cells in breast tissue. Black women showed distinct stress-related immunologic signatures including enhanced chemotaxis, immune suppression at the systemic level, and increased tumor-associated myeloid cells, suggesting that chronic psychosocial and environmental stressors may biologically contribute to breast cancer disparities by creating a pro-tumorigenic immune environment—findings that underscore the urgent need for interventions addressing social determinants of health as cancer prevention strategies.

The Association of Sleep Trouble and Physical Inactivity with Breast Cancer Risk in Nova Scotia: Evidence from the Atlantic PATH Cohort.

Feng et al,

2025

Int J Environ Res Public Health

A prospective study of 10,305 women in Nova Scotia found that frequent sleep trouble (“all of the time”) was associated with a 141% increased breast cancer risk, while high physical activity was associated with a 42% reduced risk. Interestingly, sleep duration itself showed no association with breast cancer risk, suggesting that sleep quality—rather than quantity—may be the more important factor. These findings add sleep disturbances to the list of modifiable lifestyle risk factors for breast cancer and reinforce the protective benefits of physical activity, though the authors note that further research is needed to understand the biological mechanisms linking poor sleep quality to increased cancer risk.

The mediating roles of anthropo-metabolic biomarkers on the association between beverage consumption and breast cancer risk.

Lin et al,

2025

Nutr J

A prospective cohort study of 13,567 Chinese women followed for nearly 15 years found that consuming one or more servings of sugar-sweetened beverages (SSBs) per week was associated with a 58% increased breast cancer risk compared to consuming less than one serving weekly. The association was partly mediated by body mass index (4.2%) and uric acid (18.8%), with genetic analyses identifying additional metabolic mediators including cholesterol and fatty acid ratios accounting for small portions of the effect. Interestingly, higher soy milk consumption (3-6 portions weekly) was associated with a 69% reduced breast cancer risk, while dairy milk showed a non-significant trend toward increased risk, and no associations were found for juice, coffee, tea, or alcohol, suggesting that reducing SSB consumption and addressing the associated metabolic disruptions could be effective breast cancer prevention strategies.

Ambient Air Pollution Exposure and Breast Cancer Risk Worldwide: A Systematic Review of Longitudinal Studies.

Tippila et al,

2024

Int J Environ Res Public Health

A systematic review of 25 epidemiological studies (2013-2022) found that seven out of eight investigated outdoor air pollutants showed significant associations with increased breast cancer risk, with benzo[a]pyrene showing the strongest relationship. The review found that 100% of studies examining nitrogen oxides (NOₓ), 83% of PM₂.₅ studies, 69% of nitrogen dioxide (NO₂) studies, and 43% of PM₁₀ studies demonstrated positive associations with breast cancer risk, with hazard ratios ranging from 1.05-1.56 and odds ratios from 1.03-1.86, while ozone and cadmium showed negative or no associations. These findings strengthen the evidence that outdoor air pollution—particularly traffic-related pollutants and fine particulate matter—contributes to breast cancer development, though the authors note that further research is needed to establish causal mechanisms, particularly through epigenetic pathways, and acknowledge that the review’s focus on English-language articles from developed countries may limit generalizability.

Indoor and outdoor artificial light-at-night (ALAN) and cancer risk: A systematic review and meta-analysis of multiple cancer sites and with a critical appraisal of exposure assessment.

Palomar-Cros et al,

2024

Sci Tot Environ

A comprehensive systematic review and meta-analysis of 28 studies including over 2.5 million individuals found that higher levels of outdoor artificial light at night (ALAN) were associated with a 12% increased breast cancer risk, though indoor ALAN showed a non-significant 7% increased risk, with no differences by menopausal status. For prostate cancer, the analysis suggested a 43% increased risk with outdoor ALAN exposure, though this was not statistically significant, and qualitative synthesis revealed positive associations with non-Hodgkin lymphoma, colorectal, pancreatic, and thyroid cancers. The authors note a critical limitation: most studies relied on low-resolution satellite imagery (1-5 km resolution from the Defense Meteorological Program) without information on light color or spectral composition, which may have led to exposure misclassification and underestimation of true effects, highlighting the urgent need for studies using higher-resolution exposure assessment methods and investigation of light pollution effects on cancers beyond breast cancer.

Assessing the impact of contraceptive use on reproductive cancer risk among women of reproductive age-a systematic review.

Jahanfar et al,

2024

Front Glob Women’s Health

A systematic review of 51 studies (2 RCTs and 49 observational studies) examined the association between modern contraceptive use and the risk of breast and reproductive cancers in women of reproductive age. The review found that hormonal contraceptive use significantly reduced the risk of ovarian cancer by 36% and endometrial cancer, while cervical cancer rates were lower among contraceptive users compared to non-users; notably, no increased breast cancer risk was found among healthy women (RR 1.00), but BRCA1/2 mutation carriers using oral contraceptives showed a 39% increased risk of breast cancer. These findings highlight the complex cancer-related effects of hormonal contraceptives: while they offer protective benefits against ovarian and endometrial cancers for most women, BRCA mutation carriers face elevated breast cancer risk, emphasizing the need for personalized contraceptive counseling that accounts for individual genetic risk factors. The study underscores the importance of healthcare providers considering family history and genetic profiles when discussing contraceptive options with women, particularly those with hereditary cancer susceptibility.

International Pooled Analysis of Leisure-Time Physical Activity and Premenopausal Breast Cancer in Women From 19 Cohorts.

Timmins et al,

2024

J Clin Oncol

The aim of this pooled analysis (taking data from 19 cohort studies from around the world) was to determine the effect (if any) of leisure-time physical activity on the premenopausal risk of breast cancer. Leisure time physical activity is defind as physical activity occuring during free time (so, not related to work). This analysis took data from a little over half a million women, where the incidence of breast cancer was about 10,000 across the studies. Hazard ratios (how often breast cancer occured in groups w/ high activity versus low activity) was used to asses relative risk. High levels of leisure time activity were associated with an approximately 10% reduction in breast cancer risk after adjusting for BMI. This association was found to be particuarily strong for human epidermal growth factor receptor 2–enriched breast cancer, with a 16-61% reduction in risk.

Association between brominated flame retardants and risk of endocrine-related cancer: A systematic review and meta-analysis.

Shen et al,

2024

Toxicol Lett

A meta-analysis of 15 studies including 3,468 cancer cases and 4,187 controls found that brominated flame retardant (BFR) exposure in adipose tissue was significantly associated with increased breast cancer risk, though no association was observed for thyroid cancer. The analysis revealed that BFR exposure generally elevates the risk of endocrine-related cancers, with BDE-28—a lower-brominated congener—showing particularly strong associations with increased cancer risk. These findings identify BFRs as a significant environmental risk factor for breast cancer and suggest that certain BFR congeners may be more carcinogenic than others, though the authors emphasize the need for further research to establish causal mechanisms and clarify how these ubiquitous flame retardant chemicals disrupt endocrine function to promote cancer development.

Examining the relationship between per-and polyfluoroalkyl substances and breast, colorectal, prostate, and ovarian cancers: a meta-analysis.

Sezavar et al,

2024

Crit Rev Toxicol

A meta-analysis of 13 observational studies through May 2022 examined the dose-response relationship between per- and polyfluoroalkyl substances (PFAS) exposure—including PFOS, PFOA, PFNA, PFDA, and PFHxS—and risk of breast, prostate, colorectal, and ovarian cancers. The highest versus lowest exposure analysis found no significant associations between any PFAS compound and breast cancer (ORs ranging from 0.88-1.29), ovarian cancer (OR = 1.43), prostate cancer (OR = 1.05), or colorectal cancer (OR = 0.77). However, linear dose-response analysis revealed unexpected inverse associations, with each 1 ng/ml increase in PFNA and 2 ng/ml increase in PFOA associated with significant decreases in breast cancer risk (RR = 0.67 and 0.94, respectively), though non-linear analysis found no significant changes. The findings provide no evidence that PFAS exposure increases cancer risk for these sites, and the unexpected inverse associations observed in linear dose-response analyses—suggesting potential protective effects—are unexplained and may reflect study artifacts, reverse causation, or confounding factors rather than true biological protection, warranting further investigation to clarify these paradoxical relationships and determine whether they reflect real phenomena or methodological limitations.

Exposure to air pollutants and breast cancer risk: mediating effects of metabolic health biomarkers in a nested case-control study within the E3N-Generations cohort.

Mercoeur et al,

2024

Breast Cancer Res

A nested case-control study within the French E3N-Generations cohort examined 523 breast cancer cases and 523 matched controls to investigate whether thirteen metabolic health biomarkers mediate the relationship between exposure to three air pollutants (nitrogen dioxide, PCB153, and benzo[a]pyrene) and breast cancer risk. The study found that benzo[a]pyrene exposure was associated with a significant 2.32-fold increased breast cancer risk (highest vs. lowest quartile), PCB153 showed inconsistent positive associations, and nitrogen dioxide showed no association; among biomarkers, estradiol was associated with increased breast cancer risk (OR = 1.22 per SD). Four-way decomposition mediation analysis revealed suggestive evidence that albumin, HDL and LDL cholesterol, parathormone, and estradiol may partially mediate the associations between all three pollutants and breast cancer risk, though findings were limited by statistical power. These results provide preliminary mechanistic insights suggesting that air pollutants may influence breast cancer risk through alterations in metabolic biomarkers—particularly lipid metabolism and hormone regulation—though larger studies are needed to confirm these pathways and establish the clinical significance of these mediating effects in the relationship between environmental exposures and breast cancer development.

Randomized trials of estrogen-alone and breast cancer incidence: a meta-analysis.

Chlebowski et al,

2024

Breast Cancer Res Treat

A meta-analysis of 10 randomized controlled trials including 14,282 participants and 591 breast cancers found that estrogen-alone hormone therapy was associated with a significant 23% reduction in breast cancer incidence (3.6% vs 4.7% in estrogen vs placebo groups, RR 0.77, 95% CI 0.65-0.91, P=0.002). This finding was driven primarily by the large Women’s Health Initiative trial but was supported by nine smaller trials showing similar directional effects, with estradiol formulations showing a 37% non-significant risk reduction. These results from randomized trials contradict findings from observational cohort studies and challenge conventional wisdom about hormone therapy and breast cancer, suggesting that estrogen-alone therapy (in women without a uterus) may actually protect against breast cancer rather than increase risk—a finding with important implications for counseling postmenopausal women about menopausal hormone therapy decisions.

Recreational physical activity and breast cancer risk by menopausal status and tumor hormone receptor status: results from the Nurses’ Health Studies.

Fortner et al,

2024

Breast Cancer Res Treat

A large prospective study of 187,278 nurses followed for up to 31 years found that high levels of recreational physical activity (≥27 vs <3 MET-hours/week, roughly equivalent to ≥9 hours/week of brisk walking) were associated with 17% reduced breast cancer risk in both premenopausal and postmenopausal women. The protective effect was strongest for hormone receptor-positive (ER+/PR+) tumors in both groups, with no significant associations observed for hormone receptor-negative cancers, and while the benefit in postmenopausal women was partly explained by lower body weight, the association in premenopausal women remained independent of BMI. These findings provide strong evidence that regular recreational physical activity is a modifiable risk factor that reduces breast cancer risk across the lifespan, regardless of menopausal status, supporting public health recommendations for regular exercise as an accessible breast cancer prevention strategy for all women.

Alcohol cessation and breast cancer risk stratified by hormone receptor status.

Terry et al,

2024

Breast Cancer Res

A meta-analysis combining data from three cohort studies and one case-control study (3,793 estrogen receptor positive [ER+] and 627 ER- breast cancer cases) examined whether stopping alcohol consumption affects breast cancer risk by hormone receptor subtype. The study found that women who stopped drinking alcohol had a 12% lower risk of developing ER+ breast cancer compared to those who continued drinking (RR = 0.88), but cessation was not associated with reduced risk of ER- breast cancer (RR = 1.23). These findings suggest that quitting alcohol may reduce the risk of ER+ breast cancer specifically, which is noteworthy given that alcohol is an established risk factor for breast cancer, particularly hormone-receptor positive subtypes. The authors note that further research examining how long women have abstained from alcohol is needed to better understand the relationship between cessation duration and breast cancer risk.

BMI and breast cancer risk around age at menopause.

Von Holle et al,

2024

Cancer Epidemiol

A large collaborative study of 609,880 women from 16 prospective studies, including 9,956 breast cancer cases before age 55, examined when the relationship between body mass index (BMI) and breast cancer risk transitions from protective (before menopause) to harmful (after menopause). During the critical age window of 45-55 years, the study found that a five-unit BMI increment remained associated with reduced or neutral breast cancer risk across all menopausal status groups: HR=0.87 for premenopausal women, HR=1.00 for women after natural menopause, HR=0.99 after interventional loss of ovarian function, and HR=0.88 after hysterectomy without bilateral oophorectomy. The findings indicate that the well-documented reversal from BMI being protective to being a risk factor for breast cancer occurs after age 55, later than previously thought, suggesting that the transition to increased risk with higher BMI is not directly tied to the menopausal transition itself but occurs in the years following menopause. This timing provides important insight into how adiposity influences breast cancer risk across the lifespan and suggests that the hormonal and metabolic changes associated with higher BMI have different effects on breast tissue depending on age and years since menopause.

Association between sleep duration, depression and breast cancer in the United States: a national health and nutrition examination survey analysis 2009-2018.

Cai et al,

2024

Ann Med

Sleeping Patterns| Human| Epidemiological Studies| Human Studies| Sleeping Patterns

A study of 1,789 participants (including 263 breast cancer patients) from the National Health and Nutrition Examination Survey (NHANES) examined the associations between sleep duration, depression, and breast cancer risk, and developed machine learning algorithms to predict breast cancer. The study found that participants with depression had nearly double the odds of breast cancer (OR = 1.99; 95% CI: 1.55-3.51), while no significant association was observed between sleep duration (<7 hours, >9 hours compared to 7-9 hours) and breast cancer risk. Among six machine learning algorithms tested, the AdaBoost model performed best in predicting breast cancer with an area under the curve (AUC) of 0.84 (95% CI: 0.81-0.87), demonstrating good predictive capability. These findings suggest that depression, but not sleep duration, is significantly associated with increased breast cancer risk, providing new insights into the relationship between mental health and breast cancer while highlighting the need for further research into the underlying pathological mechanisms connecting depression and breast cancer development.

Association between processed and ultra-processed food intake and the risk of breast cancer: a case-control study.

Nouri et al,

2024

BMC Cancer

A case-control study of 133 breast cancer cases and 266 controls in Tehran, Iran used a food frequency questionnaire and the NOVA classification system to examine the association between processed foods (PFs) and ultra-processed foods (UPFs) consumption and breast cancer risk. In the initial model, women in the highest tertile of UPF consumption had nearly double the odds of breast cancer compared to the lowest tertile (OR = 1.93; 95% CI: 1.08-3.45), though this association was no longer significant after additional adjustments in a second model. However, when stratified by menopausal status, premenopausal women in the highest tertile of UPF consumption had more than 3.5 times higher odds of breast cancer (OR = 3.66; 95% CI: 1.33-10.08) in the fully adjusted model. These findings suggest that high consumption of ultra-processed foods may be particularly associated with increased breast cancer risk among premenopausal women, adding to the limited and sometimes contradictory evidence on the relationship between ultra-processed food intake and breast cancer.

Impact of Alcohol Consumption on Breast Cancer Incidence and Mortality: The Women’s Health Study.

Mostofsky et al,

2024

J Women's Health

A prospective cohort study of 39,811 women followed for a median 25 years documented 2,830 breast cancer cases (including 237 deaths) and found that each additional alcoholic drink per day was associated with a 10% higher breast cancer rate (HR = 1.10; 95% CI: 1.04-1.16) in a linear dose-response manner, with the association significant for ER+ tumors (HR = 1.12; 95% CI: 1.06-1.18) but not ER- tumors (HR = 0.95; 95% CI: 0.82-1.10). Modeling 100,000 women over 10 years revealed that compared to rarely/never drinking, at-least-monthly consumption would result in 64 additional cases (NNH = 1,567) and >1 drink/day would result in 279 additional cases (NNH = 358), with approximately 4.1% of breast cancer cases attributable to alcohol consumption exceeding one drink per month. The study demonstrates that alcohol consumption increases breast cancer risk in a linear fashion even within recommended limits of up to one drink per day, particularly for ER+ tumors, and quantifies the substantial population-level burden of alcohol-attributable breast cancer among women.

High adherence to Western dietary pattern increases breast cancer risk (an EPIC-Spain study).

Castelló et al,

2024

Maturitas

A prospective study of 24,892 Spanish women (639 breast cancer cases) from the EPIC cohort examined associations between three dietary patterns (Western, Prudent, Mediterranean) and breast cancer risk over time. Women with moderate-to-high adherence to the Western dietary pattern showed a non-linear 37% increased breast cancer risk compared to those with lowest adherence (HR = 1.37; 95% CI: 1.07-1.77 for third quartile and HR = 1.37; 95% CI: 1.03-1.83 for fourth quartile), with particularly strong associations in postmenopausal women (HR = 1.42; 95% CI: 1.04-1.94 in highest quartile) and for ER+/PR+/HER2- tumors (HR = 1.71; 95% CI: 1.11-2.63 in highest quartile), while Prudent and Mediterranean patterns showed no clear associations. The findings suggest that Western dietary patterns characterized by high-fat dairy, red and processed meats, refined grains, sweets, caloric drinks, and convenience foods may increase breast cancer risk, especially for hormone receptor-positive tumors in postmenopausal women.

Hair Dye and Relaxer Use among Cisgender Women in Embu and Nakuru Counties, Kenya: Associations with Perceived Risk of Breast Cancer and Other Health Effects.

Llanos et al,

2024

Int J Env Res Pub Health

This study surveyed 746 Kenyan women about their hair product use and found that nearly 60% had used chemical relaxers and one-third had used hair dyes, despite many expressing concerns about health risks including breast cancer. Older women and those working in sales and service industries were more likely to use these products, which may contain endocrine-disrupting chemicals linked to breast cancer. Surprisingly, women’s awareness of potential health risks did not consistently lead to reduced use of these products, suggesting that concern alone may not be enough to change behavior. These findings are particularly important for Kenya, where breast cancer rates are rising, and highlight the need for better education about chemical exposures in hair products and strategies to reduce potentially harmful exposures.

Heavy-metal associated breast cancer and colorectal cancer hot spots and their demographic and socioeconomic characteristics.

Tomlinson et al,

2024

Cancer Causes Control

A population-based study using Kentucky cancer registry data (77,637 breast cancer and 56,598 colorectal cancer cases) found that higher ambient air concentrations of carcinogenic metals—cadmium, arsenic, nickel, and chromium(VI)—were associated with increased odds of residing in breast and colorectal cancer hotspots, independent of individual risk factors including age, race, smoking, and neighborhood socioeconomic characteristics. Cancer hotspot populations were disproportionately Black and exhibited markers of lower socioeconomic status, and importantly, the metal-cancer associations persisted even after adjusting for these factors, suggesting environmental metal exposure is an independent contributor to geographic cancer clustering. These findings provide evidence that historically marginalized communities face disproportionate exposure to carcinogenic metals through environmental pollution, likely contributing to cancer disparities, and underscore the urgent need for environmental justice interventions including stricter air quality regulations, cleanup of contaminated sites, and individual-level exposure assessments to fully understand how metal exposures drive cancer inequities in vulnerable populations.

Exposure to di-2-ethylhexyl phthalate (DEHP) increases the risk of cancer.

Yang et al,

2024

BMC Pub Health

This study used NHANES 2011–2018 data (n = 6,147) to evaluate the association between DEHP exposure and cancer risk. After adjusting for confounders, DEHP and its metabolites were significantly associated with increased risk of overall cancer prevalence, especially female reproductive system cancers (OR > 1.0, p < 0.05).

Exposure to Pesticides and Breast Cancer in an Agricultural Region in Brazil.

Panis et al,

2024

Ecotoxicol Public Health

A case-control study of 758 women in southwestern Paraná, Brazil—a region with intensive pesticide use and 41% higher breast cancer rates than the national average—found that even women not working in fields but handling contaminated equipment and laundry tested positive for Glyphosate, atrazine, and 2,4-D. While the overall breast cancer risk association with pesticide exposure was non-significant after adjustment (OR = 1.30), exposed women had significantly higher risk of lymph node metastasis (OR = 2.19; 95% CI: 1.31-3.72), indicating more aggressive disease. These findings suggest pesticide exposure in agricultural communities may be associated with development of more aggressive breast cancer, highlighting the need for monitoring both occupational and household pesticide exposure in rural populations.

Increasing rates of early-onset Luminal A breast cancers correlate with binge drinking patterns.

Chen et al,

2024

Breast Cancer Res

An ecological study using SEER Cancer Registry data examined correlations between alcohol consumption patterns and breast cancer diagnosed in women under age 40, accounting for a 10-year latency period and conducting cumulative 10-year aggregate exposure analyses. Both moderate (≥1 drink/day) and heavy (≥2 drinks/day) alcohol consumption were associated with 5% increased rates of early-onset Luminal A breast cancer (IRR = 1.05 for both), while binge drinking was associated with 6% increased rates of Luminal A BC in the lag model (IRR = 1.06; 95% CI: 1.02-1.11) and 4-5% increases in both Luminal A and Luminal B BC in cumulative models, with no associations found for ERBB2-enriched or triple-negative subtypes. These findings support the hypothesis that increasing alcohol consumption and binge drinking trends may be contributing to the observed rise in early-onset breast cancer among young U.S. women, particularly hormone receptor-positive Luminal A and B subtypes that have been increasing fastest in this population. The study highlights alcohol—especially binge drinking—as a modifiable risk factor that may be driving concerning upward trends in breast cancer incidence among young women, with effects appearing specifically for hormone-responsive cancer subtypes consistent with alcohol’s known estrogenic mechanisms.

Cadmium exposure and risk of breast cancer: A meta-analysis.

Florez-Garcia et al,

2023

Environ Res

A meta-analysis of 17 epidemiological studies examining cadmium exposure and breast cancer risk found no statistically significant associations overall or when examining specific exposure routes separately (dietary cadmium or biomarker-based studies), with substantial heterogeneity between studies and no clear patterns by menopausal status. The inconclusive findings leave critical questions about whether specific exposure routes (occupational, air pollution, smoking) pose different risks than dietary intake or whether residual confounding by tobacco smoke constituents may influence observed associations. These results highlight the need for future research with better exposure assessment methods that can distinguish between different cadmium sources and routes of exposure, particularly occupational and environmental air pollution exposures that may be more relevant than diet for populations living near industrial areas where cadmium contamination is prevalent.

Light at night exposure and risk of breast cancer: a meta-analysis of observational studies.

Luo et al,

2023

Frontiers

A meta-analysis of 21 studies including 734,372 participants worldwide found that light at night (LAN) exposure was associated with a 12% increased breast cancer risk overall, with case-control studies showing 16% increased risk and cohort studies showing 8% increased risk. The association was particularly pronounced in Asian populations (24% increased risk) and for ER-positive breast cancers (10% increased risk), while outdoor LAN specifically showed a 7% increased risk. These findings support the hypothesis that artificial light exposure at night disrupts circadian rhythms and suppresses melatonin production—a hormone with anti-cancer properties—though the authors caution against taking melatonin supplements for prevention without medical advice until mechanisms are better understood, and emphasize the need for high-quality research accounting for environmental confounding factors to clarify the role of light pollution in breast cancer development.

Linking Physical Activity to Breast Cancer via Inflammation, Part 2: The Effect of Inflammation on Breast Cancer Risk.

Lou et al,

2023

Cancer Epidemiol Biomark Prev

A systematic review and meta-analysis of 34 observational studies and 3 Mendelian randomization studies found that women with the highest levels of C-reactive protein (CRP)—a marker of systemic inflammation—had a 13% increased breast cancer risk compared to those with the lowest levels, though the quality of evidence was rated as very low to moderate. While adiponectin showed a protective association (24% reduced risk), this finding was not supported by genetic evidence from Mendelian randomization studies, and there was little evidence that other inflammatory markers like TNFα and IL-6 affected breast cancer risk. These findings suggest that while chronic low-grade inflammation measured by CRP may modestly increase breast cancer risk, the overall role of inflammation in breast cancer development remains unclear, with limited support beyond CRP—highlighting the need for higher-quality prospective studies and mechanistic research to clarify whether inflammation is truly causal or merely a marker of other underlying processes that drive breast carcinogenesis.

Dose-response Relationships Between Cigarette Smoking and Breast Cancer Risk: A Systematic Review and Meta-analysis.

Scala et al,

2023

J Epidemiol

A comprehensive systematic review and meta-analysis of 169 observational studies published through January 2020 examined the association between cigarette smoking and breast cancer risk using random-effects models and dose-response analyses. The pooled analysis found that current smokers had 7% increased breast cancer risk, former smokers 8% increased risk, and ever smokers 9% increased risk compared to never smokers, with results consistent across both case-control and cohort studies and across most covariates including genetic mutations (BRCA) and relevant polymorphisms. Importantly, breast cancer risk increased in a linear dose-response manner with both smoking intensity (12% increased risk for 20 cigarettes/day, 26% for 40 cigarettes/day) and duration (5% increased risk for 20 years, 11% for 40 years), demonstrating clear dose-response relationships. This large and comprehensive meta-analysis, which employed innovative search methods, provides strong evidence supporting a causal role of tobacco smoking in breast cancer development, resolving previous controversy on this association and establishing smoking as a modifiable risk factor for breast cancer with risks increasing proportionally to the amount and duration of smoking exposure.

mTOR pathway candidate genes and physical activity interaction on breast cancer risk in black women from the women’s circle of health study.

Ilozumba et al,

2023

Breast Cancer Res Treat

A study of 1,398 Black women (567 breast cancer cases, 831 controls) found preliminary evidence of gene-environment interactions between genetic variants in the mTOR signaling pathway and vigorous physical activity affecting breast cancer risk, though results did not survive correction for multiple testing. Specific variants in AKT1 were associated with 49-85% reduced ER-positive breast cancer risk only among physically active women, while an MTOR variant showed a 124% increased ER-positive cancer risk and an EIF4E variant showed dramatically elevated ER-negative cancer risk (over 20-fold), but only in the context of vigorous physical activity. These exploratory findings suggest that the relationship between physical activity and breast cancer may vary by genetic background in Black women, potentially explaining some of the heterogeneity in physical activity-breast cancer associations, though larger studies with multiple testing correction are needed to confirm whether these gene-exercise interactions truly modify breast cancer risk.

Dietary Inflammatory Index and risk of breast cancer: evidence from a prospective cohort of 67,879 women followed for 20 years in France.

Hajji-Louati et al,

2023

Eur J Nutr

A large prospective cohort study of 67,879 French women followed for 21 years found that higher dietary inflammatory potential was associated with a 4% increased breast cancer risk per standard deviation increase in DII score, with women in the highest versus lowest quintile showing a 13% increased risk in a linear dose-response relationship. The association was slightly stronger among non-smokers (6% increased risk per standard deviation) and low alcohol consumers (5% increased risk per standard deviation), suggesting that inflammatory diet effects may be most pronounced in women without other pro-inflammatory exposures. These findings from one of the largest and longest prospective studies provide strong evidence that promoting anti-inflammatory dietary patterns—rich in fruits, vegetables, whole grains, legumes, nuts, and fish while limiting processed foods, red meat, refined carbohydrates, and saturated fats—could contribute meaningfully to breast cancer prevention as part of comprehensive public health strategies.

Ethylene oxide emissions and incident breast cancer and non-Hodgkin lymphoma in a US cohort.

Jones et al,

2023

J Natl Cancer Inst

A large cohort study of 451,945 National Institutes of Health-AARP Diet and Health Study participants used EPA Toxics Release Inventory data to estimate historical environmental ethylene oxide (EtO) exposures based on proximity to EtO-emitting facilities, wind patterns, and emission levels from enrollment in 1995-1996. Among 173,670 postmenopausal women, living within 10 km of EtO facilities was associated with statistically significant breast cancer risk for invasive disease (HR = 1.03; 95% CI: 0.97-1.09); women in the highest quartile of the airborne emissions index showed elevated risk of in situ breast cancer at 10 km (HR = 1.25; 95% CI: 1.02-1.53), with no clear patterns for non-Hodgkin lymphoma risk. These findings reveal a novel potential association between environmental EtO exposure and in situ breast cancer—but not invasive breast cancer or lymphohematopoietic cancers—contrasting with occupational studies that found associations with invasive disease. The differential association with in situ versus invasive disease suggests EtO may influence early-stage breast carcinogenesis, though the mechanism remains unclear and warrants further investigation to understand why environmental exposures show different patterns than occupational exposures and why the effect appears limited to pre-invasive lesions.

Case-Cohort Study of the Association between PFAS and Selected Cancers among Participants in the American Cancer Society’s Cancer Prevention Study II LifeLink Cohort.

Winquist et al,

2023

Environ Health Perspect

A case-cohort study within the prospective Cancer Prevention Study II (CPS-II) LifeLink cohort examined associations between serum per- and polyfluoroalkyl substances (PFAS) concentrations and cancer risk by analyzing blood samples collected 1998-2001 from 999 randomly selected participants and 3,762 cancer cases (breast, bladder, kidney, pancreas, prostate, and hematologic cancers), with particular attention to histologic subtypes. The study found that higher serum PFOA concentrations were positively associated with renal cell carcinoma in women (HR per PFOA doubling: 1.54; 95% CI: 1.05-2.26) but not men, while higher PFHxS concentrations were associated with chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) in men (HR per PFHxS doubling: 1.34; 95% CI: 1.02-1.75), with some variation in associations observed across histologic subtypes within cancer sites. These findings in a general population cohort support previous observations linking PFOA to kidney cancer in women and identify a new association between PFHxS and CLL/SLL in men, highlighting the importance of considering both sex differences and specific histologic cancer subtypes when evaluating PFAS-cancer relationships. The study demonstrates that PFAS exposure at levels found in the general U.S. population may be associated with increased risk of certain cancers, extending concerns beyond highly exposed occupational or community populations.

Endocrine-disrupting chemicals and breast cancer: a meta-analysis.

Liu et al,

2023

Frontiers

This meta-analysis aimed to evaluate the association between exposure to endocrine-disrupting chemicals (EDCs), including phthalates and other common environmental pollutants, and breast cancer risk. The study found that certain EDCs—such as p,p′-DDT, chlordane, HCH, and specific PCBs—were positively associated with increased breast cancer risk, while a few compounds like BBP and PFDoDA showed a negative association.

Air pollution with NO(2), PM(2.5), and elemental carbon in relation to risk of breast cancer- a nationwide case-control study from Denmark.

Poulsen et al,

2023

Environ Res

A nationwide Danish study of 55,745 breast cancer cases matched with controls found that each 10 μg/m³ increase in fine particulate matter (PM₂.₅) air pollution exposure over 20 years was associated with a 21% increased breast cancer risk, with the association particularly strong (32% increased risk) among women diagnosed before age 55. While elemental carbon and nitrogen dioxide also showed modest associations initially, these disappeared in multi-pollutant models, suggesting PM₂.₅ is the primary air pollutant linked to breast cancer risk. These findings add breast cancer to the growing list of cancers associated with air pollution exposure and suggest that younger women may be especially vulnerable, though the authors caution that unmeasured confounding factors may influence the results.

Association of Social Determinants and Tumor Biology With Racial Disparity in Survival From Early-Stage, Hormone-Dependent Breast Cancer.

Hoskins et al,

2023

JAMA Oncol

A retrospective analysis of 60,137 women with early-stage, ER-positive, node-negative breast cancer found that Black women had an 82% increased risk of breast cancer death compared to White women, with social determinants of health (neighborhood disadvantage and insurance status) mediating 19% of this disparity and tumor biological characteristics (including genomic recurrence scores) mediating 20%. When all factors were combined in a fully adjusted model, 44% of the racial survival disparity was explained, suggesting that social determinants and aggressive tumor biology contribute roughly equally to worse outcomes in Black women, though over half of the disparity remains unexplained. Notably, neighborhood disadvantage itself mediated 8% of racial differences in high-risk recurrence scores, indicating that social factors may influence tumor biology, and highlighting that addressing breast cancer disparities requires dual approaches targeting both structural barriers to healthcare access and quality while investigating the biological mechanisms—including ancestry-related genetic variants and molecular pathways—that may drive more aggressive disease in Black women.

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