Dietary changes that may be helpful

The following dietary changes have been studied in connection with breast cancer.

Avoidance of alcohol
An analysis of studies using the best available methodology found that women who drink alcohol have a higher risk of breast cancer compared with teetotalers.¹ Alcohol consumption during early adulthood may be more of a risk factor than alcohol consumption at a later age.²

Some,^{3 4} though not all,⁵ studies have reported that alcohol increases estrogen levels. Increased estrogen levels might explain the increase in risk.

In a preliminary report, drinkers with low intake of folic acid had a 32% increased risk of breast cancer compared with nondrinkers; however, the excess risk was only 5% in those drinkers who consumed adequate levels of folic acid.⁶ In the same report, women taking multivitamins containing folic acid and having at least 1.5 drinks per day had a 26% lower risk of being diagnosed with breast cancer compared with women drinking the same amount of alcohol but not taking folic acid-containing vitamins.⁷

Fiber
Insoluble fiber from grains delays the onset of mammary (breast) cancer in animals.⁸ In an analysis of the data from many studies, people who eat relatively high amounts of whole grains were reported to be at low risk for breast cancer.⁹

In some studies, the protective effect of fiber against the risk of breast cancer has been stronger in young women than in older women.¹⁰ This finding might occur because fiber has been reported to lower estrogen levels in premenopausal women but not in postmenopausal women.^{11 12} Other researchers, however, report that fiber appears to equally reduce the risk of breast cancer in women of all ages.¹³ One leading researcher has suggested the active components in fiber may be phytate and isoflavones, substances that may provide protection even in the absence of a decrease in estrogen levels.¹⁴ If these substances do protect against breast cancer, they might be as helpful in older women as in younger women.

Consuming a diet high in insoluble fiber is best achieved by switching from white rice to brown rice and from bakery goods made with white flour or mixed flours to 100% whole wheat bread, whole rye crackers, and whole grain pancake mixes. Refined white flour is generally listed on food packaging labels as “flour,”“enriched flour,”“unbleached flour,”“durum wheat,”“semolina,” or “white flour.” Breads containing only whole wheat are usually labeled “100% whole wheat.”

Vegetarianism
Compared with meat eaters, most,¹⁵ but not all,¹⁶ studies have found that vegetarians are less likely to be diagnosed with cancer. Vegetarians have also been shown to have stronger immune functioning, possibly explaining why vegetarians may be partially protected against cancer.¹⁷ Female vegetarians have been reported to have lower estrogen levels compared with meat-eating women, possibly explaining a lower incidence of breast cancer that has been reported in vegetarian women.¹⁸

Fruits and vegetables
An analysis of 17 studies on breast cancer risk and diet found that high consumption of vegetables was associated with a 25% decreased risk of breast cancer compared with low consumption.¹⁹ The same report analyzed 12 studies that found high consumption of fruit was associated with a 6% reduction of breast cancer incidence compared with low consumption. However, when data from only the eight largest and best studies were combined, high intake of fruits and/or vegetables did not correlate with protection from breast cancer.²⁰ Therefore, the protective effect of fruit and vegetable consumption against breast cancer remains unproven.²¹

Tomatoes
Tomatoes contain lycopene—an antioxidant similar in structure to beta-carotene. Most lycopene in our diet comes from tomatoes, though traces of lycopene exist in other foods. Lycopene has been reported to inhibit the proliferation of cancer cells in test tube research.²²

A review of published research found that higher intake of tomatoes or higher blood levels of lycopene correlated with a reduced risk of a variety of cancers in 57 of 72 studies. Findings in 35 of these studies were statistically significant.²³ Evidence of a protective effect for tomato consumption was strongest for cancers other than breast cancer (prostate, lung, and stomach cancer), but some evidence of a protective effect also appeared for breast cancer.

Meat and how it is cooked
Most,^{24 25} but not all,²⁶ studies show that consumption of meat is associated with an increased risk of breast cancer. This association probably depends in part on how well the meat is cooked. Well-done meat contains more carcinogenic material than does lightly cooked meat.²⁷ Evidence from preliminary studies shows that women who eat well-done meat have a high risk of breast cancer.²⁸ Genetic factors may determine which women increase their risk of breast cancer by eating well-done meat.²⁹

Fish
Fish eaters have been reported to have a low risk of breast cancer.³⁰ The omega-3 fatty acids found in fish are thought by some researchers to be the components of fish responsible for protection against cancer.³¹

Coffee, unrelated to risk
Coffee drinking has been reported to increase breast pain associated with noncancerous lumps in the breast—a group of conditions commonly called fibrocystic breast disease. The presence of some forms of fibrocystic breast disease have been reported by some researchers to increase the risk of breast cancer.³² As a result of these separate findings, some women may be concerned coffee drinking might increase the risk of breast cancer. However, most research has shown that coffee drinkers are at no higher risk of breast cancer than are women who do not drink coffee.^{33 34 35}

Olive oil
Olive oil consumption has been associated with a reduced risk of breast cancer in several preliminary reports.^{36 37 38} Oleic acid, the main fatty acid found in olive oil, does not appear to be the cause of this protective effect,³⁹ and scientists now guess that some as-yet undiscovered substance in olive oil might be responsible for the apparent protective effect of olive oil consumption.⁴⁰

The dilemma over dietary fat
Olive oil and fish are two sources of dietary fat considered potentially helpful in protecting against breast cancer.^{41 42 43 44} Each has been discussed separately above. The information below discusses fat sources that some researchers are concerned might increase the risk of cancer.

High-fat diets increase the risk of mammary cancer in animals.⁴⁵ From country to country, breast cancer risk in women is proportionate to the level of total fat consumed in the diet.⁴⁶ Estrogen levels, body weight, and breast density have all been reported to decrease when women are put on low-fat diets—all changes that are thought to reduce the risk of breast cancer.^{47 48 49 50} Moreover, breast cancer patients have been reported to reduce their chances of survival by eating a diet high in saturated fat.⁵¹ (Saturated fat is found mostly in meat and dairy fat.) Similarly, breast cancer patients have been reported to be at increased risk of suffering a recurrence if they eat higher levels of fatty foods, such as butter, margarine, red meat, and bacon.⁵²

Analysis of human trials, using a research design dependent on the memories of subjects, also has shown women consuming high-fat diets to be at high risk of breast cancer.⁵³ In some cases, the correlation has been quite strong.⁵⁴ However, most,^{55 56 57} but not all,⁵⁸ “prospective” studies—which avoid problems caused by faulty memories—have not found any association between fat intake and the risk of breast cancer.

Why do some research findings suggest that fat increases the risk of cancer and other studies find no association? Some studies finding dietary fat unrelated to cancer risks have not factored out the effects of olive oil or fish fat; both may protect against cancer.^{59 60 61 62} Adding them to the total dietary fat intake and then studying whether “more fat causes more cancer” is therefore misleading. Some studies finding no association between fat intake and breast cancer have made one or both of these errors.^{63 64}

Scientists know cancers caused by diet most likely occur many years after the causative foods are regularly consumed. When one group of researchers compared dietary intakes to cancer rates occurring ten years after the consumption of food, and also eliminated from consideration the effect of fat from fish consumption, they found a high degree of correlation between consumption of animal fat (other than from fish) and the risk of breast cancer death rates for women at least 50 years of age.⁶⁵

In the debate over whether dietary fat increases breast cancer risks, only one fact is indisputable: women in countries that consume high amounts of meat and dairy fat have a high risk of breast cancer, while women in countries that mostly consume rice, soy, vegetables, and fish (instead of dairy fat and meat) have a low risk of breast cancer.⁶⁶

The complex relationship between soy consumption and risk
Asian countries in which soy consumption is high generally have a low incidence of breast cancer. However, the dietary habits in these countries are so different from diets in high-risk countries that attributing protection from breast cancer specifically to soy foods on the basis of this evidence alone is premature.⁶⁷ Similarly, within a society, women who frequently consume tofu have been reported to be at low risk of breast cancer.⁶⁸ Consumption of tofu might only be a marker for other dietary or lifestyle factors that are responsible for protection against breast cancer.

Genistein, one of the isoflavones found in many soy foods, inhibits proliferation of breast cancer cells in test tube studies. Most animal studies report that soybeans and soy isoflavones protect against mammary cancer.⁶⁹ However, the protective effect in animals have occurred primarily when soy has been administered before puberty.⁷⁰ If the same holds true in humans, consuming soy products in adulthood might provide little, if any, protection against breast cancer.

The findings of several recent studies suggest that consuming soy might, under some circumstances, increase the risk of breast cancer.^{71 72 73 74 75} When ovaries were removed from animals—a situation related to the condition of women who have had a total hysterectomy—dietary genistein was reported to increase the proliferation of breast cancer cells.⁷⁶ When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.⁷⁷ Although premenopausal women have shown decreases in estrogen levels in response to soy consumption,^{78 79}  proestrogenic effects have also been reported.⁸⁰ When premenopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.⁸¹ In yet another trial, healthy breast cells from women previously given soy supplements containing isoflavones showed an increase in proliferation rates—an effect that might also increase the risk of breast cancer.⁸²

The commonly held belief that consuming soybeans or isoflavones such as genistein will protect against breast cancer is, therefore, far from proven.^{83 84 85 86 87} Possibly, consuming soybeans in childhood may ultimately be proven to have a protective effect.⁸⁸ Doing the same in adulthood, however, may have very different effects.^{89 90 91 92 93}

Some scientists, at least under some circumstances, remain hopeful about the potential for soy to protect against breast cancer. These scientists recommend consumption of foods made from soy (such as tofu), as opposed to taking isoflavone supplements. Several substances in soybeans other than isoflavones have shown anticancer activity in preliminary research.⁹⁴

Reduction in sugar
Preliminary studies have reported associations between an increased intake of sugar or sugar-containing foods and an increased risk of breast cancer,⁹⁵ though this link does not appear consistently in published research.⁹⁶ Whether these associations exist because sugar directly promotes cancer or because sugar consumption is only a marker for some other dietary or lifestyle factor remains unknown.