Radiation in high doses is a strong cancer initiator of breast cells. Women who were exposed to high doses of radiation, as occurred in Hiroshima and Nagasaki in Japan in World War I, had very high rates of breast cancer compared with unexposed women. Women who have had chest area radiation as a child or young woman as treatment for another cancer (such as lymphoma) have a significantly increased risk for breast cancer. Women who in past decades had high exposure to medical X-ray procedures such as fluoroscopy also appear to be at high risk. The increased risk seems to occur only in women who were exposed to radiation before age forty.

The very small doses of radiation present in mammograms do not appear to increase breast cancer risk, and for virtually all women any tiny risk from that radiation is far outweighed by the life-saving effect mammography in detecting early cancers. In addition, most women do not have mammograms before age forty, so that the radiation from mammograms should not pose a problem.

Some suspected breast carcinogens (cancer-causing substances) include chlorinated hydrocarbons such as the pesticide DDT and PCBs. The pesticides may act as powerful estrogens once in the body, stimulating breast cells to proliferate, that is, to replicate themselves. There are several ongoing studies looking more closely into these environmental contaminants. So far, however, none have been shown to increase the risk for breast cancer. More definitive research results should be available in a few years.

Some studies have indicated that women exposed to large amounts of electromagnetic fields have an increased risk for breast cancer. This increased risk has been observed only in electrical workers. There is no good evidence from scientific studies that exposure typically encountered in the home from appliances increases the risk of breast cancer. If electromagnetic fields do increase risk, it may occur through effects on the pineal gland in the brain, which controls the production and release of melatonin, a hormone that can affect estrogen levels.

Nutrition

High-fat, low-fiber diets, with few fruits and vegetables, have been implicated in increased breast cancer risk, but little has been conclusively proved. Much of our information about diet and breast cancer comes from comparisons across countries. These studies show that women who live in countries with low-fat, high-fruit/vegetable, and high-fiber diets have much lower risk of developing breast cancer, compared with women who live in countries with high-fat, low-fruit/vegetable, and low-fiber diets. Women from the low-risk countries such as Japan and China who immigrate to high-risk countries like the United States tend to take on the high-risk profile of women in their adopted countries. Other clues to a relationship between diet and breast cancer have come from animal studies. Laboratory animals fed diets high in fats of any kind tend to develop more breast cancers than animals fed low-fat diets. Studies of women in the United States have been less informative, probably because very few American women have diets with a low enough percent of calories from fat to make a difference in their risk.

Many nutrition-cancer experts believe that dietary fat must be kept at less than 20 percent of daily calories in order to reduce risk of breast cancer. Another problem with studying women’s diets directly is that women who have lower-fat diets tend to also have higher intakes of fruits, vegetables, and fiber. So it is unclear which dietary factors are the most important in controlling risk. It may be that the whole package of a diet that is low-fat, high-fruit/vegetable, and high-fiber will be important for women to adopt.

Alcohol

If you drink two or more alcoholic drinks per day you may have a doubled risk of getting breast cancer, compared with nondrinkers. It doesn’t appear to matter what kind of alcohol you drink—all alcohol appears to be associated with increased risk for breast cancer. Alcohol increases the levels of estrogen in the blood, possibly through the toxic effects of alcohol on the liver. Since we know that some aspects of estrogen exposure are related to increased risk for breast cancer, it makes sense that drinking alcohol would increase risk. Alcohol can also depress immune function, which may be important in breast cancer development. A recent analysis from the Nurses’ Health Study, a follow-up study of more than 120,000 United States nurses, indicates that folic acid supplementation may help to alleviate the increased risk from alcohol.

Tobacco

Although other types of cancer are more strongly related to tobacco use, recent studies suggest that smoking can also increase the risk of developing breast cancer in some women. The increased risk associated with smoking may be genetically determined.

Body Size and Shape

Researchers have known for many years that women with bigger body size are at higher risk for getting breast cancer than women with smaller body size. Taller women have higher risk than shorter women. On average, women who are 5’8″ or taller have one and a half to two times greater risk compared with women who are 5’2″ or shorter. Tall women may have higher levels of growth hormones, which might stimulate and promote cancer cell growth. Alternatively, tallness may just be a marker of good nutrition during childhood—perhaps girls with access to large quantities of high-calorie and high-fat foods both grow taller and have higher risk of breast cancer from the types and amounts of foods they ate as children and teens.

Researchers have also long noted an association between increasing weight and breast cancer risk. Most researchers now look at weight corrected for height. The most common measure is called the body mass index, or BMI. The higher your BMI, the higher your risk of getting breast cancer after menopause, the time when most breast cancers occur. Paradoxically, a higher BMI offers some protection against developing breast cancer before you reach menopause. We think that these different associations between BMI and breast cancer risk in younger versus older women are related to hormones. Women who are overweight in their twenties, thirties, and forties are more likely to have irregular menstrual periods and to be ovulating only occasionally, which reflects lower estrogen levels compared with other women of similar ages. On the other hand, women with large amounts of fat stores make estrogen in their body fat, which makes their estrogen levels higher than those of women who are thin. After menopause, this amount of estrogen made in fatter women’s body fat is enough to push overweight and obese women into a higher-risk category. The placement of body fat could also be important, as we describe here.

The Nurses’ Health Study reported on more than 95,000 nurses who had been asked about their weight and lifetime weight patterns, and were followed for sixteen years for risk for breast cancer. In that study, women who had a BMI of 31 or greater (who were obese) had a 60 percent higher risk for breast cancer than women with a BMI of 20 or less. Women who had gained fifty pounds or more after age eighteen had double the risk for breast cancer compared with women whose weight stayed the same after age eighteen. They also found an interaction between weight and use of hormones.