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2025
Ecotoxicol Environ Saf
This study examined the link between cadmium (Cd) exposure and breast cancer risk using data from nearly 6,000 participants and a meta-analysis of 20 studies. The researchers found that higher cadmium levels significantly increased breast cancer risk, with the strongest association in estrogen receptor-positive (ER+) breast cancer subtypes, and identified glycated hemoglobin (HbA1c, a marker of blood sugar control) as a potential mechanism linking cadmium exposure to cancer development. The findings suggest that cadmium, an endocrine-disrupting metal found in the environment, may contribute to breast cancer through metabolic disruption. These results highlight the importance of reducing cadmium exposure and understanding how environmental toxins interact with metabolic health to influence cancer risk.
2025
Environ Health
This case-control study of 942 Brazilian women (471 with breast cancer and 471 matched controls) found that higher blood levels of certain per- and polyfluoroalkyl substances (PFAS)—persistent environmental chemicals found in products like non-stick cookware and food packaging—were significantly associated with increased breast cancer risk. Specifically, elevated concentrations of n-perfluoroheptane sulfonate (n-PFHpS) doubled the breast cancer risk, and this association was particularly strong for hormone receptor-positive breast cancers and varied by ethnicity. The study emphasizes the importance of analyzing specific PFAS chemical variants (isomers) rather than treating all PFAS as identical, as different forms showed different associations with breast cancer risk across ethnic groups. These findings add to growing evidence that PFAS exposure may be a modifiable environmental risk factor for breast cancer, though the researchers note that previous studies have shown mixed results.
2024
Environ Res
This large Danish study of over 5,500 breast cancer cases found that daughters whose mothers worked in horticulture (greenhouse and nursery work) before conception or during pregnancy had significantly increased breast cancer risk—79% higher for estrogen receptor-positive tumors and 148% higher for estrogen receptor-negative tumors. The associations were particularly strong for maternal horticulture work compared to paternal work, suggesting that pesticide and chemical exposures during critical windows of fetal development may affect daughters’ breast cancer risk decades later. These findings are concerning because horticultural workers have high exposure to pesticides and other agricultural chemicals, and the study suggests that even preconceptional maternal exposure—before pregnancy begins—may influence a daughter’s future cancer risk, highlighting the potential for intergenerational effects of occupational chemical exposures.
2023
Int J Cancer
This study of over 1,200 postmenopausal women found that higher blood levels of PFOS (a common “forever chemical” found in items like stain-resistant fabrics and food packaging) were associated with 59-134% increased risk of hormone receptor-positive breast cancers, while PFOA (another widespread PFAS chemical) showed modest associations with hormone receptor-negative tumors. PFAS are called “forever chemicals” because they persist indefinitely in the environment and human body, and these findings suggest they may contribute to breast cancer through hormone disruption. These results add to growing concerns about PFAS exposure from contaminated water, food packaging, and consumer products, though the different associations for PFOS and PFOA by tumor subtype require further investigation to fully understand how these chemicals affect breast cancer development.
2023
Environ Res
This case-control study of 5587 women with breast cancer and age-matched controls for which there was data on maternal employment. Maternal occupational exposure to diesel exhaust (OR=1.13, 95%CI,1.01-1.27) and bitumen fumes (OR=1.51., 95%CI,1.00-2.26) was associated with breast cancer. Further, exposures to diesel exhaust (OR=1.23, 95%CI, 1.01-1.50) were more strongly associated with ER- tumors than with ER+ tumors.
2022
Ann Surg
A genomic study of 308 women with stage I-IV breast cancer (non-Hispanic White, Hispanic White, Hispanic Black, and non-Hispanic Black) using peripheral blood analysis found that increasing West African (WA) ancestry was associated with 6% higher odds of triple-negative breast cancer per percentage increase (OR = 1.06; 95% CI: 1.001-1.126) and higher neighborhood socioeconomic status (nSES) showed a protective effect (OR = 0.343; 95% CI: 0.151-0.781), though WA ancestry’s association with TNBC was attenuated when adjusting for nSES. Local ancestry analysis revealed nSES-independent enriched WA ancestral segments at specific chromosomal loci (χ²=42004914, p=3.70×10⁻⁵) associated with TNBC, while multinomial logistic regression demonstrated that women from low nSES neighborhoods remained more likely to have TNBC independent of genetic ancestry. These findings reveal the complex interplay between genetic ancestry and socioeconomic environment in TNBC etiology: while specific genetic variants linked to West African ancestry contribute to TNBC risk through biological mechanisms, neighborhood socioeconomic disadvantage exerts independent effects, highlighting that both genetic susceptibility and environmental/social determinants contribute to the disproportionate TNBC burden in Black women and underscoring the critical importance of studying gene-environment interactions as drivers of aggressive breast cancer disparities.
2021
Environ Int
Long-term exposure to benzo[a]pyrene (BaP), a toxic air pollutant from gas and diesel vehicle exhaust, was associated with a 15% increased risk of breast cancer in a large French study of over 10,000 women. The risk was particularly elevated in women transitioning through menopause and for hormone receptor-positive breast cancers, with each increase in BaP exposure levels raising breast cancer odds by 15-20%. This study provides important real-world evidence that BaP air pollution may contribute to breast cancer development, especially affecting hormone-sensitive tumors.
2021
Scand J Work Environ Health
This large Danish study of over 38,000 women with breast cancer found that occupational exposure to diesel exhaust was associated with a modestly increased risk of estrogen receptor-negative breast tumors in women under age 50, with the risk increasing with longer duration and higher levels of exposure. No significant associations were found for overall breast cancer risk or for exposure to polycyclic aromatic hydrocarbons (PAHs). These findings suggest that workplace diesel exhaust exposure may be particularly relevant for early-onset, harder-to-treat forms of breast cancer, highlighting the importance of protecting workers from diesel fumes.
2021
Sci Total Environ
This Japanese case-control study of 405 matched pairs of women found that higher serum concentrations of PFAS chemicals were generally associated with reduced breast cancer risk rather than increased risk, with women in the highest exposure quartile showing 79-85% lower odds of breast cancer for linear isomers of PFOS and PFOA. However, the study revealed important differences between chemical structures: among postmenopausal women, while the linear form of one PFAS (PFTrDA) was protective, the branched form showed a marginally increased risk (74% higher odds approaching significance at medium exposure levels). These unexpected inverse associations contrast with several other studies showing increased breast cancer risk from PFAS exposure, highlighting the complexity of PFAS research and the importance of distinguishing between different chemical structures (branched vs. linear isomers) that may have opposite health effects, though the reasons for these protective associations remain unclear and warrant further investigation.
2020
Breast Cancer Res
A study of 118,760 postmenopausal women in the NIH-AARP cohort found that combined estrogen-progestogen therapy (EPT) was associated with a 54% increased breast cancer risk overall, with risk doubling for women using EPT for 10 or more years and reaching an 80% increased risk for those who continued use through 2004. Importantly, women who discontinued EPT showed no increased risk (14% non-significant increase), suggesting the elevated risk may be reversible after stopping treatment. Estrogen-only therapy (ET) showed no increased risk at baseline, though a 35% increased risk emerged in women who continued use through 2004, and EPT was associated with elevated risk across all breast cancer subtypes including in situ disease, invasive cancers, and tumors of varying hormone receptor status.
2020
Environ Res
A 24-year study of over 112,000 U.S. nurses found that women living within 10 kilometers (about 6 miles) of municipal solid waste incinerators had a 15% increased breast cancer risk compared to those not living near these facilities, with risk rising to 25% for women within 5 kilometers (3 miles). The association strengthened with longer duration of residence near incinerators and higher dioxin emission levels, and was consistent across hormone receptor subtypes and menopausal status. These findings provide evidence that residential exposure to dioxins—toxic chemicals released from waste incineration that persist in the environment and accumulate in the food chain—may increase breast cancer risk, highlighting the need for stricter emissions controls on incinerators and consideration of proximity to such facilities in community planning and cancer prevention strategies.
2019
Cancer Epidemiol
A 22-year Italian study following nearly 9,000 women found that those with the highest dietary cadmium intake (a toxic metal found in foods like grains, vegetables, and shellfish) had a 54% increased breast cancer risk compared to those with the lowest intake, with even stronger associations in premenopausal women (73% increased risk). Cadmium—classified as a proven human carcinogen—enters the food supply primarily through contaminated soil and water, with diet being the main exposure source for non-smokers at an average intake of about 8 micrograms per day in this population. The increased risk was consistent across all breast cancer subtypes regardless of hormone receptor status, supporting cadmium as a dietary risk factor for breast cancer and highlighting the need for strategies to reduce cadmium levels in the food supply through soil remediation and crop selection.
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.
2019
Int J Mol Sci
This case-control study of 509 breast cancer patients and 1,170 controls in examined whether cadmium exposure—a metalloestrogen that mimics estrogen—is associated with specific breast cancer subtypes based on urinary cadmium levels. Women with the highest cadmium levels (>0.33 μg/g creatinine) had 53% increased odds of ductal breast cancer (OR=1.53, 95% CI: 1.15-2.04) compared to those with the lowest levels, with the strongest associations observed for hormone receptor-positive tumors: 34% increased odds for ER+ cancers (OR=1.34, 95% CI: 1.14-1.59), 33% for PR+ cancers (OR=1.33, 95% CI: 1.09-1.61), and 35% for ER+/PR+ cancers (OR=1.35, 95% CI: 1.11-1.65). The study found significant associations with HER2-negative cancers, with the strongest link for ER+/PR+/HER2- breast cancers, supporting the hypothesis that cadmium acts as an endocrine disruptor. These findings provide evidence that cadmium exposure is specifically associated with hormone receptor-positive and HER2-negative breast cancer risk, independent of tumor type, suggesting cadmium’s estrogenic properties may contribute to hormone-driven breast cancers.
2019
Environ Int
This population-based case-control study investigated whether occupational exposure to alkylphenolic compounds—endocrine-disrupting chemicals widely used in industry—is associated with breast and prostate cancer. The study included over 5,600 participants and found that occupational exposure to alkylphenolic compounds was modestly associated with increased breast cancer risk (23% increase), particularly from exposure to domestic cleaning products and nonylphenol ethoxylates. No significant associations were found between alkylphenolic compound exposure and prostate cancer overall, except among men exposed through cosmetics and personal hygiene products. The authors conclude these findings suggest a modest link between occupational alkylphenolic compound exposure and breast cancer that requires further confirmation in additional studies.
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.
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.
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.
2024
Int J Mol Sci
This comprehensive review covers breast cancer biology from classification and risk factors through diagnosis and treatment, highlighting how the disease varies by subtype and between racial groups. The review examines both traditional factors like staging systems and molecular subtypes (Luminal A/B, Triple Negative, HER2-enriched) as well as emerging research on genetic mutations, epigenetic changes, and microbiome imbalances that may contribute to breast cancer development and progression. Recent evidence suggests that disruptions in the body’s microbial communities may play a role in breast cancer, with patterns potentially differing across populations, adding a new dimension to understanding racial disparities in breast cancer outcomes.
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.
2024
Br J Cancer
A large prospective study of 1,275,783 women aged 45+ years followed for a median 12.7 years from 2004 examined associations between menopausal hormone therapy (HT) types and breast cancer risk, finding that oral estrogen combined with daily progestin carried the highest risk (HR=2.42; 95% CI: 2.31-2.54), with drug-specific hazard ratios ranging from Cliovelle® (HR=1.63) to Kliogest® (HR=2.67), while vaginal estradiol showed no association with breast cancer. The associations varied substantially by molecular subtype (stronger for luminal A: HR=1.97 vs. other subtypes), detection mode (stronger for interval-detected: HR=2.00 vs. screen-detected cancers: HR=1.33 in women 50-71), and BMI (decreasing hazard ratios with increasing BMI). These contemporary findings confirm that oral and transdermal HT use increases breast cancer risk with important variation by specific formulations, administration routes, tumor characteristics, and patient factors, providing critical updated evidence for clinical decision-making about menopausal hormone therapy use and emphasizing the need for individualized risk-benefit assessment accounting for HT type, body weight, and cancer screening participation.
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.
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.
2023
Eur J Clinc Nutr
A prospective study of 65,574 postmenopausal French women followed for 20 years found that high adherence to a Paleolithic diet—characterized by high intake of fruits, vegetables, nuts, fish, and lean meat while limiting dairy, grains, legumes, refined sugar, and alcohol—was associated with a 17% reduced breast cancer risk compared to low adherence. Each standard deviation increase in Paleolithic diet score was associated with an 8% lower breast cancer risk, and notably, the protective association was consistent across all breast cancer subtypes, suggesting involvement of non-hormonal mechanisms. These findings provide the first long-term evidence linking Paleolithic dietary patterns to breast cancer prevention and suggest that this eating pattern—which emphasizes whole, unprocessed foods while restricting potentially inflammatory foods—may offer protective benefits beyond what has been observed with other dietary patterns, though further research is needed to identify the specific biological mechanisms involved.
2023
Adv Nutr
A systematic review and meta-analysis of 31 studies (29 included in quantitative analysis) following Cochrane methodology examined associations between healthy lifestyle indices (incorporating factors like healthy body weight, diet, physical activity) and breast cancer risk. Comparing the highest versus lowest adherence categories, the analysis found a 20% breast cancer risk reduction in prospective studies (HR=0.80; 95% CI: 0.78-0.83) and 26% reduction in retrospective studies (OR=0.74; 95% CI: 0.63-0.86), with inverse associations remaining significant when stratified by menopausal status (except for premenopausal BC in prospective studies) and across molecular subtypes: ER+/PR+ (HR=0.68; 95% CI: 0.63-0.73), ER+/PR- (HR=0.78; 95% CI: 0.67-0.90), and ER-/PR- (HR=0.77; 95% CI: 0.64-0.92). These findings—with most studies scoring low risk of bias and moderate certainty of evidence—demonstrate that adherence to healthy lifestyles reduces breast cancer risk by approximately 20-30% regardless of molecular subtype or menopausal status, supporting the prioritization of lifestyle-based recommendations for population-level breast cancer prevention as a key modifiable approach to reducing the substantial disease burden.
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.
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.
2022
Cancers
A meta-analysis of 19 case-control studies found that oral contraceptive (OC) use had markedly different effects on breast cancer risk depending on tumor receptor status: OC use was associated with a 37% increased risk of triple-negative breast cancer and a 20% increased risk of ER-negative breast cancer, while showing an 8% reduced risk of ER-positive breast cancer and a non-significant 5% reduced risk of HER2-positive breast cancer. These contrasting associations suggest that oral contraceptives may influence breast cancer development through different biological mechanisms depending on tumor subtype, with particularly concerning implications for triple-negative breast cancer—the most aggressive subtype with limited treatment options. The findings highlight the importance of considering breast cancer heterogeneity when evaluating hormonal contraceptive risks and suggest that women at high risk for triple-negative or ER-negative breast cancer may need alternative contraceptive counseling.
2022
Int J Cancer
A comprehensive two-sample Mendelian randomization study using Breast Cancer Association Consortium data examined potential causal associations of 23 risk factors and biomarkers with breast cancer risk overall and by molecular subtypes, identifying significant associations for 15 traits including reproductive factors (age at menarche/menopause), anthropometric measures (BMI, waist-to-hip ratio, height), lifestyle factors (physical activity, smoking, sleep duration, chronotype), and six blood biomarkers (estrogens, IGF-1, SHBG, telomere length, HDL-cholesterol, fasting insulin). Notably, the study revealed heterogeneous subtype-specific associations: increased SHBG reduced ER+ breast cancer risk (OR=0.83; 95% CI: 0.73-0.94) but elevated risk for ER- (OR=1.12) and triple-negative cancers (OR=1.19, p-heterogeneity=0.01), while higher fasting insulin increased HER2-negative cancer risk (OR=1.94; 95% CI: 1.18-3.20) but reduced HER2-enriched cancer risk (OR=0.46; 95% CI: 0.26-0.81, p-heterogeneity=0.006). These findings—consistent across sensitivity analyses—provide strong genetic evidence for causal relationships between multiple modifiable and biological risk factors and breast cancer, while revealing that SHBG and insulin have paradoxical opposite effects on different molecular subtypes, suggesting distinct etiologic pathways for ER+/ER- and HER2+/HER2- breast cancers that may require subtype-specific prevention strategies.
Risk of breast cancer in Danish women occupationally exposed to organic solvents, including ethanol.
2022
Am J Indust Med
A Danish nested case-control study of 845 breast cancer patients and 1,500 matched controls found that occupational exposure to organic solvents was associated with a 220% increased breast cancer risk among ever-exposed women compared to never-exposed women. The elevated risk was observed across all breast cancer subtypes—both estrogen receptor-positive and negative tumors—and in both pre- and postmenopausal women, though surprisingly, no dose-response relationship was detected with duration or cumulative exposure levels. Occupational ethanol exposure showed no association with breast cancer, suggesting that specific properties of other organic solvents may drive the carcinogenic effect, and highlighting the need for workplace protections and further research to identify which solvent types pose the greatest risk.
2022
Front Endocrinol
A meta-analysis of 8 studies (5 cohort and 3 case-control studies) examined whether fertility treatments increase breast cancer risk in genetically susceptible women, including those with a family history of breast cancer or BRCA mutations. The analysis found no significant increase in breast cancer risk associated with fertility treatments in genetically susceptible women overall (OR 1.18), women with a family history of breast cancer (OR 1.35), or BRCA mutation carriers (OR 1.02), with similarly reassuring results across subgroups including BRCA1 carriers, BRCA2 carriers, and women treated with specific fertility medications like in vitro fertilization, clomiphene citrate, or gonadotropins. This first meta-analysis on this topic provides reassuring evidence that fertility treatments do not significantly increase breast cancer risk even in women with hereditary susceptibility, though the authors note that larger prospective studies with more detailed information are needed to fully understand potential risks. Future research should examine whether risks vary by breast cancer subtype, explore the genetic mechanisms underlying hormone-related breast cancer, and investigate the relationship between BRCA mutations and hormone receptor-positive breast cancer specifically.
2021
Ocup Environ Med
A large Danish registry-based case-control study of 38,375 women under age 70 with primary breast cancer and matched controls used objective lifetime employment histories from the Danish Supplementary Pension Fund Register and a job exposure matrix to assess occupational ultraviolet radiation (UVR) exposure as a reliable measure of long-term solar UVR exposure. While no overall association was observed between occupational UVR exposure and breast cancer risk, among women aged ≥50 years, longer duration of UVR exposure (≥20 years: OR=0.83; 95% CI: 0.75-0.92) and highest cumulative exposure (OR=0.89; 95% CI: 0.83-0.95) were inversely associated with breast cancer risk, with no notable differences by estrogen receptor status. These findings suggest that long-term occupational UVR exposure may protect against late-onset breast cancer, supporting the hypothesis that solar UVR (likely through vitamin D production) may have protective effects on breast cancer development, particularly in postmenopausal women, though the mechanism requires further investigation in future occupational studies with objective exposure assessment.
2021
Sci Rep
A nested case-control study of 4,401 breast cancer cases and 4,401 matched controls from the French E3N cohort found no overall association between long-term airborne cadmium exposure and breast cancer stage or tumor grade, but identified a striking 240% increased risk of invasive tubular carcinoma (ITC)—a specific breast cancer subtype—among women in the highest versus lowest quintile of cadmium exposure. The dose-response analysis suggested a linear relationship between cadmium exposure and ITC risk specifically, though no associations were found for other histological subtypes or more advanced disease. These findings suggest that cadmium’s estrogenic properties may selectively promote certain breast cancer subtypes rather than broadly increasing all breast cancer risk, highlighting the importance of examining cancer heterogeneity in environmental exposure studies and raising concerns about air pollution from industrial sources, waste incineration, and fossil fuel combustion that release cadmium into the environment.
2021
Eur J Epidemiol
A large prospective study of 318,686 European women followed for 14 years found that consuming a pro-inflammatory diet was associated with a 4% increased breast cancer risk per standard deviation increase in inflammatory diet score, with women in the most pro-inflammatory diet group showing a 12% increased risk compared to those consuming the least inflammatory diets. The association was particularly strong in premenopausal women (8% increased risk per standard deviation), and notably, the pro-inflammatory diet effect was independent of body mass index, physical activity, and alcohol consumption, suggesting that dietary inflammation contributes to breast cancer risk through pathways distinct from these other established risk factors. The consistent associations across all hormone receptor-defined breast cancer subtypes suggest that dietary inflammation may promote breast cancer through non-hormonal mechanisms, reinforcing the importance of anti-inflammatory dietary patterns—rich in fruits, vegetables, whole grains, and omega-3 fatty acids while limiting processed foods, red meat, and refined carbohydrates—as a modifiable strategy for breast cancer prevention across all women, particularly those still premenopausal.
2021
Aging
A meta-analysis of 14 studies including 312,885 women found that those consuming the most pro-inflammatory diets had a 37% increased breast cancer risk compared to women with the most anti-inflammatory diets. The association was significant in both premenopausal women (87% increased risk) and postmenopausal women (23% increased risk), with notably stronger effects observed in younger women. These findings suggest that dietary patterns promoting chronic inflammation are an independent risk factor for breast cancer across all ages, and that dietary interventions focused on anti-inflammatory foods—such as fruits, vegetables, whole grains, and omega-3 fatty acids—could be an important prevention strategy, particularly for premenopausal women who showed the strongest association.
2019
Medicine
A meta-analysis of 10 studies including 8,585 breast cancer cases among 686,305 participants examined the relationship between age at first oral contraceptive (OC) use and breast cancer risk through June 2018. The pooled analysis found a 24% increased breast cancer risk associated with earlier age at first OC use (RR = 1.24; 95% CI: 1.10-1.41), with a significant linear dose-response relationship indicating that younger age at first use was associated with higher breast cancer risk. However, subgroup analyses showed inconsistent results with no statistical significance when restricted to studies from Western countries, lower quality studies, smaller sample sizes, shorter follow-up periods, or when stratified by breast cancer subtypes defined by estrogen receptor (ER), progesterone receptor (PR), or HER2 status. The findings suggest that starting oral contraceptive use at a younger age may increase breast cancer risk in a dose-dependent manner, though this association appears to be influenced by study characteristics and may not differ consistently across hormone receptor-defined breast cancer subtypes, highlighting the need for further research to clarify these relationships and inform contraceptive counseling for young women.
2019
BMJ
A Mendelian randomization study using UK Biobank data (156,848 women including 7,784 breast cancer cases) and Breast Cancer Association Consortium data (122,977 cases, 105,974 controls) examined causal effects of sleep traits on breast cancer risk using genetic variants associated with chronotype, sleep duration, and insomnia symptoms. Two-sample MR analysis confirmed that morning preference reduced breast cancer risk by 12% per category increase (OR = 0.88; 95% CI: 0.82-0.93) and provided suggestive evidence that each additional hour of sleep duration increased risk by 19% (OR = 1.19; 95% CI: 1.02-1.39) for both ER+ and ER- breast cancer subtypes, with inconsistent evidence for insomnia symptoms. These findings—robust to sensitivity analyses accounting for horizontal pleiotropy—provide genetic evidence that being a “morning person” may protect against breast cancer while longer sleep duration may increase risk, suggesting that circadian rhythm patterns and sleep duration represent modifiable risk factors, though the counterintuitive finding regarding sleep duration requires further investigation given that adequate sleep is generally considered health-protective.
2019
Eur J Clinic Nutr
A meta-analysis of 23 prospective studies including 41,516 breast cancer cases and 1,171,048 individuals found that higher folate intake was associated with reduced risk of ER-/PR- breast cancer (RR = 0.82; 95% CI: 0.68-0.97), with each 100 μg/day increment decreasing risk by 6% for ER- and 10% for ER-/PR- subtypes. Additionally, high folate intake showed protective effects in premenopausal women (RR = 0.94) and individuals with moderate-to-high alcohol consumption (RR = 0.82), suggesting folate may be particularly beneficial for hormone receptor-negative breast cancers and specific high-risk populations.
2018
Breast Cancer Res
A pooled analysis of three population-based case-control studies including 6,320 women (3,934 cases, 2,386 controls) aged 35-64 years examined whether the associations between body mass index (BMI) and breast cancer risk differ by tumor subtype defined by estrogen receptor (ER), progesterone receptor (PR), and HER2 status. Higher BMI at age 18 was inversely associated with premenopausal breast cancer risk across all ER/PR/HER2 subtypes (≥25 vs. <20 kg/m²: OR=0.72), with the strongest protection seen in premenopausal women who had high BMI both at age 18 and currently (46% reduced risk; OR=0.54; 95% CI: 0.38-0.78), while no significant associations were found for postmenopausal breast cancer. The findings indicate that high BMI during late adolescence provides similar protective effects against all molecular subtypes of premenopausal breast cancer regardless of hormone receptor or HER2 status, and this protection appears to be maximized when women maintain higher BMI consistently throughout their premenopausal years. These results suggest that the mechanisms by which adiposity protects against premenopausal breast cancer operate broadly across different tumor subtypes rather than being specific to hormone receptor-positive disease.
2018
PLOS One
A case-control study of 399 breast cancer patients with receptor status information and 324 controls found that current menopausal hormone therapy (MHT) use was associated with approximately double the risk of hormone receptor-positive breast cancers, including ER+ (104% increased risk), ER+/PR+ (129% increased risk), and notably ER+/PR+/HER2- subtypes (130% increased risk). Past MHT use showed no elevated risk for any subtype, and current use was not significantly associated with hormone receptor-negative cancers, indicating the effect is specific to hormonally-driven tumors. These findings provide mounting evidence that MHT specifically increases risk of the ER+/PR+/HER2- subtype—the most common breast cancer type—adding to concerns about current hormone therapy use and supporting the need for women and clinicians to carefully weigh the duration of MHT treatment against cardiovascular and quality-of-life benefits versus breast cancer risks.
2018
BMC Cancer
A prospective analysis of 657 incident breast cancer cases from the EPIC-Germany Study (n=27,012) with immunohistochemical characterization of tumors by six markers (ER, PR, HER2, Ki67, Bcl-2, p53) found that among postmenopausal women not using hormone therapy (HT), higher BMI was significantly associated with increased risk of less aggressive tumors characterized by ER+, PR+, HER2-, low Ki67, Bcl-2+, and p53- status (HR per 5 kg/m²: 1.44; 95% CI: 1.10-1.90) but not more aggressive subtypes, while among postmenopausal HT users, BMI showed a significant inverse association with less aggressive tumors (HR per 5 kg/m²: 0.68; 95% CI: 0.50-0.94). Among pre- and perimenopausal women, no significant linear associations emerged, though the highest BMI tertile showed significantly lower risk of less aggressive tumors (HR: 0.55; 95% CI: 0.33-0.93). These findings suggest that obesity’s relationship with breast cancer is highly nuanced, being associated with increased risk of less aggressive, hormone-responsive tumors in postmenopausal women not using HT, paradoxically protective in HT users and younger women, highlighting the complex interplay between adiposity, endogenous and exogenous hormones, menopausal status, and tumor biology that requires replication in larger pooled prospective studies.
2018
Breast Cancer Res Treat
A study of 27,153 postmenopausal women in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial created an estrogen-related lifestyle score (ERLS) combining four factors: estrogenic diet, alcohol intake, body mass index (BMI), and physical activity, with scores ranging from 0-6 and higher scores representing lower estrogenic lifestyle patterns. Women with ERLS scores of 4 or ≥5 had 23% and 34% lower breast cancer risk, respectively, compared to those with scores ≤2, with similar protective effects observed for invasive cases and estrogen receptor-positive subtypes. The findings indicate that adopting multiple low-estrogen lifestyle behaviors together—including a low estrogenic diet, minimal alcohol consumption, healthy body weight, and high physical activity—has a combined protective effect against postmenopausal breast cancer that is stronger than any single factor alone. The researchers suggest this combined effect likely works by influencing estrogen metabolism, highlighting the importance of addressing multiple lifestyle factors simultaneously for breast cancer prevention in postmenopausal women.
2018
Curr Oncol Rep
Obesity is now recognized as a leading preventable cause of cancer, particularly postmenopausal estrogen receptor-positive breast cancer, and is associated with worse outcomes across all breast cancer subtypes. Multiple interconnected mechanisms link obesity to breast cancer, including elevated estrogen levels, altered hormone-like molecules from fat tissue (leptin and adiponectin), disrupted insulin signaling, changes in gut bacteria, and chronic inflammation throughout the body. Understanding these complex pathways could lead to new prevention and treatment strategies to reduce the growing burden of obesity-related breast cancers worldwide.
2018
Food Chem Tox
A systematic review of 25 studies examining phthalates (plastic chemicals) and breast cancer found that while laboratory studies show certain phthalates can activate estrogen receptors and promote cancer cell growth, epidemiological studies in humans have produced mixed and inconclusive results. The main source of phthalate exposure is through diet—particularly from food and beverages in plastic packaging—but current human studies have significant limitations in how they measure exposure and account for other risk factors. The review calls for better-designed future studies that use hair samples instead of urine for more accurate long-term exposure assessment, include dietary factors and genetic markers as confounders, and investigate phthalates’ effects beyond just estrogen-driven cancers to include all breast cancer subtypes.
2018
Maturitas
A Spanish population-based case-control study of 1,006 breast cancer cases and 1,370 controls found that high total meat intake was associated with a 39% increased breast cancer risk in postmenopausal women, with processed/cured meat showing a 47% increased risk overall and a striking 152% increased risk specifically for triple-negative breast cancers. Cooking methods and doneness preferences significantly modified risk: well-done red meat was associated with 62% increased risk and stewed red meat with 49% increased risk (particularly for hormone receptor-positive tumors), while pan-fried or breaded white meat showed 38% increased overall risk and 78% increased risk in premenopausal women. These findings suggest that breast cancer risk could be reduced not only by limiting meat consumption—especially processed meats—but also by modifying cooking practices to avoid well-done or high-temperature cooking methods that generate carcinogenic compounds like heterocyclic amines and polycyclic aromatic hydrocarbons.
2018
Breast Cancer Res Treat
A meta-analysis of 25 epidemiological studies (23 cohort studies and 2 randomized trials) found that hormone replacement therapy (HRT) was associated with a 33% increased breast cancer risk overall, with combined estrogen-progestin therapy (EPT) showing stronger associations than estrogen-only therapy (ET). EPT was associated with both ductal (51% increased risk) and lobular breast cancer (38% increased risk), and all HRT types were linked to ER-positive but not ER-negative breast cancers, consistent with hormone-driven carcinogenesis. Notably, Asian women using HRT showed higher breast cancer risk than Western women, possibly due to differences in baseline hormone levels, genetic susceptibility, body composition, or HRT formulations used—a finding highlighting the importance of considering racial and ethnic differences when counseling women about menopausal hormone therapy risks and benefits.
2020
Sci Rep
A study comparing breast tissue from Black and White women with different breast cancer subtypes found distinct patterns of bacteria living in the tissue that varied by both race and cancer type. In Black women with triple-negative breast cancer (an aggressive subtype), tumors had lower bacterial diversity compared to surrounding normal tissue, while White women showed the opposite pattern with higher diversity in tumors. These findings suggest that the bacterial communities in breast tissue differ between racial groups and may play a role in the known disparities in breast cancer subtypes and outcomes, though more research is needed to understand how these bacteria influence cancer development.