Here’s a bunch of studies about the protective effects of sun exposure on breast cancer:
Abstract C046: Dietary Calcium and Vitamin D and Sun Exposure with the Risk of Breast Cancer Among African American Women (2020)
Conclusion: Due to the vitamin D synthesis-blocking effects of skin melanin, dark-skinned people tend to suffer both higher rates of vitamin D deficiency and vitamin D deficiency diseases, including cancer. This study evaluated and compared the dietary and supplementary intake of vitamin D and sun exposure habits of 1033 black women with breast cancer and 391 healthy black women without breast cancer – all in New Jersey. Researchers found that the women who took vitamin D supplements enjoyed a “significant decreased overall” breast cancer risk. Researchers also found that “more daylight hours spent outdoors in a year predicted a lower risk of pre- and postmenopausal BrCa [breast cancer]”
Conclusion: This is a meta-analysis of 13 different studies investigating associations between daily sun exposure (in adolescence, adulthood, and lifetime) and risk of breast cancer. Researchers found that adults who spend 1+ hours in the sun daily during summer have a lower risk of developing breast cancer compared to those who spend less than 1 hour in the sun daily. Regular sun exposure during adolescence was found to also possibly provide a chemoprotective benefit into adulthood, even if sun exposure declines with age. Indirect sunlight (ambient UV) was found to offer no benefits.
Intake of Vitamin D and Calcium, Sun exposure, and Risk of Breast Cancer Subtypes Among Black Women (2019)
Conclusion: To investigate associations between vitamin D, sun exposure, and risk of breast cancer in black women, researchers evaluated 1724 black female breast cancer patients and 1233 healthy matched controls for their reported total amount of daylight hours spent outside (in the sun). What they found is that those who spent the most time outside had the lowest risk of breast cancer, for all types of breast cancer, compared to the women who reported the least time spent outside. It’s also important to mention that blacks, due to the vitamin D production-blocking effects of melanin, tend to suffer much higher rates of vitamin D deficiency and black women are nearly twice as likely to develop the more aggressive types of breast cancer.
Conclusion: Data from the Nurses’ Health Study II was evaluated for associations between sun exposure history and breast cancer incidence. The Nurses’ Health Study II is a giant study following 112,447 registered American nurses since 1989. This study used data from 1989 to 2013. Over the course of this period 3,959 of the nurses would develop breast cancer. Included in this study was self-reporting on the nurses’ sun exposure habits in childhood, at age 15, and age 30, along with the latitudes they lived at. What this study found was a slight association between higher self-reported UV exposure early in life and a lower risk for estrogen receptor-negative breast cancer.
Environmental and Lifestyle Risk Factors of Breast Cancer in Malta—A Retrospective Case-Control Study (2016)
Conclusion: 200 breast cancer patients and 403 matched controls in Malta were evaluated for a variety of lifestyle factors, including sun exposure, and risk of breast cancer. Researchers found that breast cancer rates were lower in those who reported receiving more summer sun exposure. “Though prolonged sunlight exposure damages the skin, it has been found that it reduces the risk of breast cancer—possibly due to vitamin D production and its role in breast cell growth.”
Role of Vitamin D Deficiency and Lack of Sun Exposure in the Incidence of Premenopausal Breast Cancer: A Case Control Study in Sabzevar, Iran (2014)
Conclusion: Sun exposure habits and vitamin D levels were evaluated and compared in 60 premenopausal breast cancer patients and 116 matched, healthy controls in northern Iran. Researchers found that 95% of women in both groups were vitamin D-deficient, but women who reported ever consuming any vitamin D supplements were less likely to develop breast cancer. 98% of the women reported receiving ZERO sun exposure due to how Iranian women dress, which has them completely covered up all the time. Iranian women are also known to start developing breast cancer a decade earlier compared to women in other countries. Could their sun-starved way of dressing be killing them?
Conclusion: This study of breast cancer cases by latitude in Australia found that the south half of the country (farther from the equator) has more than double the rate of breast cancer cases compared to the north half of the country.
Sun Exposure, Vitamin D Receptor Genetic Variants, and Risk of Breast Cancer in the Agricultural Health Study (2014)
Conclusion: Sun exposure habits and breast cancer rates were evaluated in 31,021 ladies for a decade. Out of this data set, researchers compared 293 breast cancer cases with 586 matched controls and found that women who get 1+ hours of sun exposure daily have a significantly lower risk of breast cancer compared to women who get less sun exposure. “Our results suggest that sun exposure may be associated with reduced risk of breast cancer.”
Joint Effects of Dietary Vitamin D and Sun Exposure on Breast Cancer Risk from the French E3N Cohort (2011)
Conclusion: The French E3N Cohort study has been following nearly 100,000 French women since 1991 and recording their health data. For this study, researchers evaluated 10 years of data on 67,721 women to look for associations between sun exposure and risk of breast cancer. In this 10-year period, 2,871 of the women developed breast cancer. What researchers found was that post-menopausal women living in the areas of France with the highest UV had a significantly lower incidence rate of breast cancer.
Ultraviolet Sunlight Exposure During Adolescence and Adulthood and Breast Cancer Risk: A Population-based Case-Control Study Among Ontario Women (2011)
Conclusion: 3,101 breast cancer patients and 3,471 healthy, age-matched controls in Canada were evaluated for lifetime sun exposure habits and time spent outdoors. Researchers found that women who reported more lifetime sun exposure (21+ hours outside per week versus less than 6 hours per week) enjoyed a lower risk of breast cancer and that even increased sun exposure in adolescence alone carried a protective effect into adulthood, even if sun exposure decreased with age.
Incident Invasive Breast Cancer, Geographic Location of Residence, and Reported Average Time Spent Outside (2009)
Conclusion: The Women’s Health Initiative Observational Study followed 71,662 participants over a decade. Over that period 2,535 women would develop breast cancer. What researchers found was that women who reported spending 2+ hours outside daily had a 20% lower risk of breast cancer compared to women who reported spending less than 30 minutes outside daily. “The observed association between time spent outside and breast cancer risk support the hypothesis that vitamin D may protect against breast cancer.”
Relationship Between Low Ultraviolet B Irradiance and Higher Breast Cancer Risk in 107 Countries (2008)
Conclusion: This study looked at breast cancer rates in 107 different countries and found that countries closer to the equator, getting more UVB sunlight, enjoy significantly lower incidence rates of breast cancer compared to countries father away from the equator.
Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Breast Cancer Risk in a Multiethnic Population (2007)
Conclusion: This study evaluated 1,788 newly diagnosed breast cancer patients and 2,129 healthy controls for lifetime sun exposure. Researchers found that light-skinned women who enjoyed the most sun exposure had a 47% lower risk of advanced breast cancer.
Vitamin D From Dietary Intake and Sunlight Exposure and the Risk of Hormone-Receptor-Defined Breast Cancer (2007)
Conclusion: Canada study where researchers compared vitamin D intake and lifetime sun exposure habits between 759 breast cancer patients and 1,135 healthy, matched controls. The study found that women with a higher vitamin D intake and increased lifetime sun exposure enjoy a significantly lower risk of estrogen-receptor positive and progesterone-receptor positive breast cancers, and a slightly lower risk for estrogen and progesterone negative breast cancers. For sun exposure specifically, they found that increased frequency of outdoor activities and ever having an outdoor job between the ages of 10 and 29 was associated with lower breast cancer risk, suggesting that breast cancer could possibly be due to developmental defects caused by early life vitamin D deficiency. The study also found that women who reported exposing more naked skin to the sun enjoyed a lower breast cancer risk. Early life sunburns, while not good for skin cancer risk, are actually associated with lower breast cancer risk.
Note: Article exploring emerging evidence that people who live night-shift / night owl lifestyles are at a higher risk for breast cancer. This is partly due to this lifestyle providing far less sun exposure, which has been strongly documented to have a protective effect against breast cancer, but also the melatonin our brains produce from sun exposure to your eyes has been found to also provide some protection against cancer.
Sunlight and Mortality from Breast, Ovarian, Colon, Prostate, and Non-Melanoma Skin Cancer: A Composite Death Certificate Based Case-Control Study (2002)
Conclusion: American cancer deaths and matched non-cancer control deaths in 24 American states were evaluated for lifetime sun exposure by evaluating the subjects based on the solar region they lived their lifetime in (state) and their occupation (how much time their job had them in the sun versus indoors) – all this info was collected from their death certificates. Researchers found that the people who received the most lifetime sun exposure enjoyed significantly lower rates of internal cancers, and these associations were stronger in the states with more intense sunlight. These people also had the highest rates of non-melanoma skin cancer, but that’s a good trade. These associations were the strongest for female breast and colon cancer, but also significant in ovarian and prostate cancer.
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