Epidemiological data from the 1960s and 1970s showed a fairly strong correlation between dietary fat intake and the risk of breast cancer.1 Figure 1 shows that the death rate from breast cancer was several fold higher in countries where dietary fat intake was very high compared to countries where the fat intake was much lower.

Calorie Control ImportantThis epidemiological evidence was supported by earlier research in animals, which also found that animals fed higher fat diets were also more likely to develop breast cancer.2 However, the breast cancer promoting effect of high-fat diets in animals largely disappears when calorie intake is tightly controlled. This suggests that high-fat diets primarily promote breast cancer by increasing calorie intake. Over a lifetime, increased calorie intake leads to earlier menarche, increased body fat stores and higher estrogen levels. Indeed, moderate calorie restriction in animals has been proven to slow the aging process and so dramatically reduce the age-adjusted risk of developing most types of cancer.In Asian countries, where fat intake and the risk of breast cancer is quite low, the blood levels of estradiol have been shown to be 30-70% lower than in the U.S. which has a high fat diet and high prevalence of breast cancer.3, 4, 5 There is now overwhelming evidence that higher levels of estrogens, like estradiol, increase the risk of developing breast cancer.6, 7 Conflicting Studies ExplainedIn marked contrast to this earlier evidence, a large prospective study which followed women aged 30-55 years for 14 years found no evidence that a lower fat intake or the type of fat consumed was associated with a reduced risk of developing breast cancer.8 However, the data from this study was corrected for age of menarche, height, BMI and weight change after 18 years. Increased height, earlier menarche, increased BMI, and greater weight gain after age 18 years, are all well-established risk factors for breast cancer. However, it seems likely that high-fat diets may be promoting breast cancer primarily by increasing calorie intake. This means that correcting for age of menarche, height, BMI, and adult weight gain would largely eliminate the effect of a diet high in fat. This would be like finding a diet high in saturated fat and cholesterol does not promote heart disease after correcting for differences in blood lipids. Substantial weight gain in adulthood was recently shown to significantly increase the risk of breast cancer in postmenopausal women.9 Weight gain may increase the risk of breast cancer in postmenopausal women because it elevates estradiol levels. MigrationMigration studies have shown that when adult women migrated from a low-risk country to a high-risk country, their risk of breast cancer rose somewhat but never approached that of women born in the “high-risk” country. However, the daughters and grand-daughters of these women, who grew up in the high-risk country developed breast cancer at almost the same rate as other women in that country.10 This suggests that most of the protective effects of a non-Westernized diet occurs by age 30-35.It seems likely then that the adoption of a moderate fat diet (25-30%) by middle-aged women is unlikely to markedly reduce the risk of breast cancer, particularly if it does not result in weight loss. A study of women who followed a very-low-fat, high-fiber, near vegetarian diet for several weeks found a 40% drop in estradiol levels.11 Such a drop in estradiol levels, if maintained in the long-run would likely reduce the risk of developing breast cancer in post-menopausal women. Increased alcohol consumption has been repeatedly shown to be associated with an increased risk of developing breast cancer.12 Alcohol has been shown to rapidly increase estrogen levels. It is already well established that a diet higher in fruits and vegetables and lower in fatty meats is likely to reduce many types of cancer including breast cancer.13 The Bottom LineA low-fat (<20%), high-fiber (>35 g/day), more vegetarian diet with plenty of fruits and vegetables, limited alcohol and regular exercise will likely help prevent breast cancer by promoting weight loss and reducing excessive levels of estradiol, especially if it is adopted early in life. Even better still, this diet will also greatly reduce the risk of cardiovascular disease, which kills more than 10 times as many women each year as breast cancer. By James J. Kenney, PhD, RD, FACN References1. Carroll KK. J Am Oil Chem Soc 1984;61:18892. Carroll KK, et al. Prog Biochem Pharmacol 1975;10: 308-533. Shimizu H, et al. Br J Cancer 1990;62: 451-34. Bernstein L. Cancer Causes Control 1990;1:51-85. Key TJ, et al. Br J Cancer 1990;62:631-66. Pike MC, et al. Epidemiol Rev 1993;15:17-357. Bernstein L, et al. Epidemiol Rev 1993:15:48-658. Holmes MD, et al. JAMA 1999;281:914-309. Huang Z, et al. JAMA 1997;278:1407-1110. Ziegler RG, et al. J Natl Cancer Inst 1993;85:1819-2711. Heber D, et al. Nutrition 1991;7:137-4112. Hunter DJ, Willett WC. Cancer Causes Control 1996;7:56-6213. Willett W. Am J Clin Nutr 1994;59:116S-70S