and fruits and vegetables) and breast cancer with mixed results. A recent meta-analysis concluded there was no association between breast cancer and dietary fat consumption.133 It has been suggested that soy consumption may reduce breast cancer risk, in part because of historically low breast cancer rates among Asian women, who have a diet high in soy. A metaanalysis showed that soy intake was inversely associated with breast cancer risk in Asian but not Western populations, perhaps because Asian women generally consume more soy products beginning at an earlier age than Western women.134 There is limited but growing evidence that high levels of fruit and/or vegetable consumption may reduce the risk of HR- breast cancer.135-137 These findings are supported by studies linking lower breast cancer risk to higher 18 Breast Cancer Facts & Figures 2019-2020 blood levels of carotenoids (micronutrients found in fruits and vegetables).138-140 Studies also suggest that calcium-rich diets may be linked to lower risk of breast cancer.136 The effect of diet on breast cancer risk remains an active area of research, with studies particularly focused on the timing of exposure, specific dietary components, and risk differences by tumor hormone receptor status. Alcohol Numerous studies have confirmed that alcohol consumption increases the risk of breast cancer in women by about 7%-10% for each 10 grams (roughly one drink) of alcohol consumed per day on average.141 Women who have 2-3 alcoholic drinks per day have a 20% higher risk of breast cancer compared to non-drinkers. There is also some evidence that alcohol consumption before first pregnancy may particularly affect risk.141, 142 Although mechanisms are not well understood, alcohol may increase risk indirectly by increasing estrogen and other hormone levels.143 Alcohol use appears more strongly associated with risk for HR+ than HR- breast cancers.50, 144 Tobacco Accumulating research indicates that smoking may slightly increase breast cancer risk, particularly longterm, heavy smoking and among women who start smoking before their first pregnancy.145, 146 A review by American Cancer Society researchers found that women who initiated smoking more than 10 years before the birth of their first child had a 18% higher risk of breast cancer than women who never smoked.146 Some studies suggest secondhand smoke may increase risk, particularly when exposure happens in childhood and for premenopausal breast cancer.147-148 Environmental and other risk factors Radiation Radiation exposure has been shown to increase breast cancer risk in studies of atomic bomb survivors and females treated with high-dose radiation therapy to the chest between 10 and 30 years of age, such as for Hodgkin lymphoma. This may be because breast tissue is most susceptible to carcinogens before it is fully differentiated, which occurs with first childbirth. Breast cancer risk starts to rise about 8 years after radiation treatment and continues to be elevated for more than 35 years.149 Although radiation treatments have evolved to include lower doses given over smaller areas, recent studies suggest that the elevated breast cancer risk persists.150 Diethylstilbestrol (DES) exposure From the 1940s through 1971, some pregnant women were given the drug DES because it was thought to lower the risk of miscarriage. These women have an increased risk (about 30%) of developing breast cancer compared to women who have not taken DES.151 It remains unclear whether women born to mothers who took DES also have a higher risk.151-153 Environmental chemicals and pollutants Many occupational, environmental, and chemical exposures have been proposed as causes of breast cancer. In general, epidemiological studies have not found clear relationships between environmental pollutants, such as organochlorine pesticides, and breast cancer. Studies to date have found no association between increased concentrations of organochlorines (e.g., dichlorodiphenyl trichloroethane, or DDT) in blood and fat tissue of adults and breast cancer risk,154 although a recent study found in utero exposure to DDT was linked to elevated breast cancer risk later in life.155 Animal studies have demonstrated that prolonged, high-dose exposure to many chemicals can increase mammary tumor development, but it is unknown whether the much lower dose exposures that occur in the general environment increase human breast cancer risk. Furthermore, many relevant chemicals have not been adequately studied in humans and this is an active area of research.156-158 Night shift work Most studies of nurses who work night shifts and flight attendants who experience circadian rhythm disruption caused by crossing multiple time zones have found increased risks of breast cancer associated with longterm employment.159, 160 Elevated risk appears to be most strongly associated with shift working during early Breast Cancer Facts & Figures 2019-2020 19 adulthood.161, 162 Exposure to light at night disrupts the production of melatonin, a hormone that regulates sleep. Experimental evidence suggests that melatonin may also inhibit the growth of small, established tumors and prevent new tumors from developing.163 Based on the results of studies in humans and animals, the International