Daughters of women who have been diagnosed with breast cancer or who have a BRCA1 or BRCA2 mutation are at higher risk for breast cancer than the general population. While there are some breast cancer risk factors such as height and age at first period over which parents have limited control, there are other sources of risk that they can influence. These generally fall under diet and lifestyle factors, although minimizing exposure to certain pollutants and ionizing radiation are also important. We have divided this topic into three time periods: prenatal period and infancy; childhood and puberty; and teenage years and young adulthood.
Diet of mother and other factors during pregnancy
Birth weight and breast cancer risk
Low birth weight has been found to be associated with decreased risk of premenopausal breast cancer and high birth weight is associated with increased risk of breast cancer in adulthood. High birth weight is associated with dense breasts, a known risk factor for breast cancer. Older mothers (over age 39) are also more likely to have daughters with dense breasts. Women who weighed more than 8.8 lbs at birth have been found to have a significantly higher risk of breast cancer than women who weighed less than 5.5 lbs. However, since large babies tend to be healthier and more intelligent than small babies, it does not make sense to undernourish babies in the womb by greatly restricting the mother's calorie intake.
In fact, severe calorie restriction during pregnancy has been shown to dramatically increase the risk of breast cancer in adulthood. One study examined the effects of the famine that took place in the Netherlands at the end of World War II from November 1944 until liberation in May 1945. People were surviving on an estimated 400 to 800 calories per day and the diet lacked variety. Daughters of women who endured famine conditions during the first trimester of their pregnancies were found to have 8.3 times the risk of dying of breast cancer during adulthood than daughters not exposed to famine in the womb. While there is some limited evidence that excessive weight gain during pregnancy could increase breast cancer risk in daughters, greatly restricting calories in an attempt to limit birth weight does not appear to be a safe or effective risk reduction strategy.
Effect of high BMI before pregnancy
Are overweight women more likely to have daughters who eventually develop breast cancer? There is no clear answer, but the effect of high body mass index (BMI), if any, appears to be small. One study found that sex hormone-binding globulin (which is associated with reduced breast cancer risk) was inversely related to pre-pregnancy BMI and weight gain during pregnancy, and positively related to vegetable and legume intake. The authors concluded that avoiding excess calorie intake and emphasizing plant foods might contribute to reducing the risk of breast cancer in daughters. However, another study that included 814 mothers of nurses with breast cancer reported that the mothers' pre-pregnancy BMI was not associated with the daughters' breast cancer risk during adulthood. Weight gain during pregnancy also was not found to be associated with daughters' breast cancer risk. The authors of this study concluded that the known association between birth weight and breast cancer risk is likely due to factors independent of mothers' pre-pregnancy BMI or weight gain during pregnancy.
Folic acid supplementation during pregnancy and daughter's breast cancer
Currently, it is recommended that all women of childbearing age take 0.4 mg folic acid per day or to obtain this amount from fortified foods. This is because folic acid reduces the risk of neural tube defects such as spina bifida and anencephaly when taken before conception and during early pregnancy. However, a 2010 study found that offspring of rats fed a diet supplemented with folic acid during pregnancy and for a time after birth were more likely to develop mammary tumors. Therefore, we suggest that women who plan to have children get their folate levels checked well before becoming pregnant. This will give them the opportunity to increase their circulating folate levels through their diets (see the list of foods below) and by eliminating alcohol consumption, if necessary. Women with folate levels in the high end of the normal range do not need folic acid supplementation during pregnancy. Those that are not able to achieve this level may need to take folic acid supplements.
The following foods are good sources of folate:
Note that both green and black tea reduce intestinal absorption of folate and should not be consumed with high-folate foods by those wishing to increase their circulating folate levels.
Harmful plastics
Some plastics contain endocrine disruptors such as bisphenol A (BPA), styrene, and phthalates that can leach into food when the plastics are heated, microwaved, put under pressure, or simply scuffed and worn. Prenatal exposure to BPA has been shown to increase the risk of mammary tumors in mice. Plastics that may leach these substances include (1) polyvinyl chloride (PVC), which may be found in cling wrap, some plastic squeeze bottles, and cooking oil bottles; (2) polystyrene, which may be found in styrofoam food containers and disposable cups and bowls; and (3) polycarbonates, which may be found in soup and soda can linings, plastic baby bottles, water bottles, and clear plastic sippy cups. Pregnant women should avoid all but temporary, low temperature uses of these products. Plastic containers may be marked with a number in a triangle-like icon. Plastics marked 1, 2, 4 or 5 use less toxic additives in their manufacture. Products that use polyvinyl chloride should be marked with 3, polystyrene with a 6, and polycarbonate with a 7 — these are the ones to avoid.
Exposure to pesticides
Girls whose mothers worked in greenhouses in the first trimester of pregnancy have been found to have earlier breast development than girls without such prenatal exposure. This appears to be because some currently approved pesticides possess endocrine disrupting properties.
Smoking
Prenatal exposure to maternal smoking has been found to be associated with early first period, which in turn is linked to increased risk of breast cancer.
Omega-3 to omega-6 fatty acid ratio during pregnancy
There is also evidence that a relatively high omega-3 to omega-6 fatty acid ratio in the maternal diet may reduce the risk of subsequent breast cancer in daughters. This means emphasizing foods such as walnuts, canola oil and sources of marine fatty acids, and de-emphasizing corn oil, soybean oil and other high omega-6 vegetable oils. Flaxseed is another dietary source of omega-3 fatty acids, but flaxseed can accumulate unacceptable levels of cadmium (a known breast cancer promoter). We recommend that pregnant women consume only flaxseed and flaxseed oil that are stated to have low levels of cadmium. In addition, while fatty fish are the best sources of marine omega-3 fatty acids, it may be best for pregnant women to consume high quality fish or krill oil supplements that have low levels of contaminants like mercury, PCBs and dioxins, instead of eating fatty fish.
Whole grains
Maternal consumption of whole grains during pregnancy have been found to be associated with lower risk of subsequent breast cancer in offspring, possibly by enhancing DNA damage repair mechanisms. There is also some evidence that adequate choline consumption during pregnancy is important.
Foods to include during pregnancy and nursing
The following foods may reduce the subsequent risk of breast cancer in daughters when included in the mother's diet during pregnancy and while nursing:
- Canola oil
- Cauliflower
- Dry beans
- Eggs
- Fish oil supplements, high quality
- Flaxseed, cadmium free
- Macadamia nuts and oil
- Spinach
- Walnuts
- Whole wheat, oats or rye
Foods and supplements to limit or avoid during pregnancy and nursing
The following foods and supplements may increase the subsequent risk of breast cancer in daughters when consumed frequently during pregnancy or while nursing:
The following foods and other products may increase the risk of other cancers or birth defects in daughters or cause problems for the pregnancy when consumed regularly during pregnancy:
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Infancy and subsequent breast cancer
Parents should avoid using baby care products containing lavender or tea tree oil, which have been shown to produce estrogenic effects in children, and parabens, which are suspected to increase female breast cancer incidence. In addition, herbal treatments for colic containing fennel (Foeniculum vulgare) should not be used since they may cause premature breast development. Also to be avoided are clear plastic baby bottles, sippy cups, and toys containing polycarbonates, which have been shown to be carcinogenic. Plastic products containing polycarbonates may be marked on the base with a triangle containing the number 7.
Having been breastfed as an infant does not appear to influence the risk of breast cancer in adulthood. However, breastfeeding has other advantages for babies and it appears to reduce the risk of breast cancer in the mother, even if undertaken only for a short period of time. The more difficult question is whether infant formula made from cow's milk or soy is safer for our high risk daughters (little evidence is available for other types of infant formula). This is a topic that can lead to highly emotional reactions. We summarize the available scientific evidence in the following two sections.
Cow's milk formula
Although milk consumption during adulthood has been found to be associated with higher risk of breast cancer, milk has been found to protect against breast cancer when consumed in infancy and childhood. A partial explanation for this finding is the fact that milk is a very good source of calcium, which is known to protect against breast cancer.
Soy formula
Soy infant formula contains phytoestrogens, which have been shown to accelerate puberty in female rats and delay puberty in male rats. However, female rats fed soy protein isolate (the principal ingredient of soy formula) have been found to have reduced incidence of carcinogen-induced mammary tumors. Soy formula could adversely affect thyroid function and, based on animal studies, might increase body weight and reduce eventual fertility. Nevertheless, the few studies that have compared the health of milk formula-fed children with soy formula fed children have not found any adverse effects for soy. Based on the available evidence, boys fed soy formula could suffer from subtle demasculinization effects. In addition, while isoflavone-rich diets in mice during pregnancy and lactation reduce breast cancer in offspring, soy formula may have unforeseen and unintended long-term effects.
Additional comments
Pregnant women who themselves have an elevated risk of breast cancer should eat a wide variety of the foods on our recommended food list and limit or avoid those on our avoid list, in addition to paying particular attention to the foods and other products on the lists above.