Breast Cancer: Nutritional Considerations
Researchers have long noted the low incidence of breast cancer in countries where traditional diets based on plant foods prevail.28-30 A striking increase in breast cancer incidence has been noted in migrants who have abandoned traditional diets (eg, rice, vegetables, soy foods) and adopted Western diets high in meat, dairy products, and fat.31 These observations have led scientists to hypothesize that diet-related factors, particularly obesity, play a key role in breast cancer risk. Part of this risk may be related to an increase in estrogen production from adipose tissue, and to eating fatty foods that increase circulating estrogen concentrations.32 In contrast, high-fiber foods tend to interrupt the enterohepatic circulation of estrogens and encourage their excretion.33
Maintenance of healthy body weight. Large studies including the Women's Health Initiative (WHI) indicate that breast cancer risk increases significantly with overweight and obesity, although risk seems to be limited to women who have never used hormone replacement therapy.34
Exercise, of course, assists in weight management. However, evidence shows that regular exercise may influence risk through other mechanisms as well, perhaps through reduction of circulating estrogens and androgens and higher levels of sex-hormone binding globulin.35
Reductions in saturated fat and meat. The association of diets high in saturated fat and meat with increased risk may be part of the reason that breast cancer is much more common in areas where Western diets and lifestyle prevail.36, 37 And it may help explain why incidence increases successively in first- and second-generation immigrants to North America.13,38 In the Nurses' Health Study II, premenopausal women in the highest quintile of fat intake had a slightly increased risk for breast cancer. Risk was attributed mainly to the intake of red meat and high-fat dairy products.37 Several mechanisms have been proposed to explain this association. Fats in general (not just saturated fat) tend to promote weight gain, which, in turn, increases serum estrogen levels. Dietary factors may also influence the age of menarche.
Aside from saturated fat, there may be benefit in reducing consumption of omega-6 fats (arachidonic acid, linoleic acid). One presumptive mechanism, aside from the effect of fat on estrogen production, is a reduction in formation of prostaglandin E2 through the cyclooxygenase (COX) enzyme. A role for this mechanism is supported by the fact that regular use of COX-blocking drugs (ASA and other NSAIDs) was associated with a reduced breast cancer risk.39,40
The Women's Health Initiative (WHI) Dietary Modification Trial, which included 48,835 women, tested a diet that aimed to reduce fat intake to 20% of energy and to increase vegetable and fruit consumption. The actual fat intake achieved by study participants averaged 24% of energy at one year and drifted upward toward baseline values by the study's end. After 8.1 years of follow-up, breast cancer risk was 9% lower in the intervention group, compared to a control group, although this result did not reach statistical significance. One exception was for progesterone-receptor-negative tumors, for which the risk decreased by 24% (P = .001).41 While the study results fueled pessimism about the ability of dietary changes to significantly alter breast cancer risk, it should be noted that the intervention diet included much more fat, meat, and dairy products than the Asian diets associated with lower cancer risk.
High-fiber diet. A large study of postmenopausal women found that those eating the most fiber had the lowest risk for breast cancer. In this group risk was even lower among women eating the least fat.42 With the addition of exercise, this type of diet is known to help keep blood glucose levels within normal limits and to lower the risk for adult-onset diabetes, both of which have been related to increased breast cancer risk.25,26 A low-fat diet high in fruits and vegetables reduced mammographic density (a breast cancer risk marker) by 6.1%, compared with 2.1% for the control diet.43 High-fiber, low-fat diets also reduce serum estradiol, which is known to be associated with breast cancer risk.44
Similarly, consumption of legumes (including soy products) that are high in isoflavones and lignans is associated with lower risk for breast cancer, an effect that is greater if intake of these foods begins before or during adolescence.45 A greater intake of isoflavones was also found to protect against breast cancer in a study of a large number of native Japanese women.46
Folate. Eating foods high in folate may help lower the risk for breast cancer, especially in women who consume alcohol. Studies show that as little as one serving of alcohol per day may increase the risk for breast cancer, and the protective effect of folate on breast cancer risk appears to be greater in women who consume alcohol.45 Foods that contain folic acid (green leafy vegetables, legumes, oranges) are likely to be preferable to folate supplements, due to their fiber, beta carotene, vitamin C, and phytochemical content, all of which may have important roles in reducing breast cancer and cancer risk per se.
Reducing or eliminating alcohol. Alcohol intake is associated with a linear increase in incidence of breast cancer up to five drinks a day. People who had two (approximately 30 g ethanol) or more drinks per day had a relative risk of 1.41 compared with nondrinkers.47 Even one drink daily increases risk 9% to 10%.47,48 The risk is additive with HRT.49 Increased risk of breast cancer due to alcohol use may be mediated by the effect on sex hormone levels.
Survival after Diagnosis
Lower body weight. Breast cancer patients with greater-than-average body weight experience a shortened survival time,50 and Japanese women who have less body fat are more likely to survive breast cancer.51 In the Nurses' Health Study, weight before diagnosis was positively associated with breast cancer recurrence and death.52
Lower-fat diets. In Japan and certain other countries, age-adjusted death rates for breast cancer increased when high-fat foods (particularly butter and margarine, cheese, ham and sausage, and dairy products) became available.53 Prospective studies suggest that those consuming less fat at the time of diagnosis, and perhaps those who later alter their diets to reduce fat intake, have a better prognosis.54,56
Results from a large randomized clinical trial, the Women's Intervention Nutrition Study (WINS), showed that postmenopausal women who ate a low-fat diet were less likely to develop a recurrence of breast cancer than those who ate a more typical diet.57
Fruits and vegetables. Limited evidence suggests that a diet high in fruits and vegetables may enhance survival from breast cancer. The Women's Healthy Eating and Living (WHEL) study, currently in progress, combines a low-fat diet with an additional emphasis on vegetables and fruits for postmenopausal women previously treated for breast cancer. Evidence to date shows that women who eat less fat and more vegetables and fruits have lower blood concentrations of estradiol and bioavailable estradiol58 and longer disease-free survival.59,60 Part of the apparent benefit may be attributable to the combinations of carotenoids, folate, and phytochemicals present in these foods.45 In the National Breast Screening Study, a significantly lower risk of dying from breast cancer was observed with higher intakes of vitamin C and beta carotene, two nutrients found in fruits and vegetables.61
During active treatment, dietary orders should be written in consultation between the treating physician and a consulting dietitian.
After treatment, a low-fat, vegetable-rich, plant-based diet appears to be prudent.
Exercise prescription (30 minutes per day of walking or equivalent energy expenditure).
Alcohol cessation or minimization.
What to Tell the Family
The families of breast cancer patients play two key roles. The first is to assist the patient who is undergoing treatment, which can be arduous at times. Particularly important is helping the patient make diet and lifestyle changes that can support good health. Second, breast cancer sometimes runs in families. It is important not only for family members to have regular screening for the disease, but also to reduce their risk to the extent possible through the diet and lifestyle changes noted above.
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