Breast cancer is the most common cancer in women, and its incidence has increased by more than 20 percent worldwide since 2008. A study from Spain shows that women on a high-vegetable Mediterranean-type diet had close to one third the rate of breast cancer when compared to a control group that was only given advice to reduce fat intake (AMA Oncol, January 21, 2016, and JAMA Intern Med, Nov 1, 2015;175(11):1752-60). Mediterranean-type diets are high in plants and contain fish but are low in other animal products.
From 2003 to 2009, 4282 women aged 60 to 80 years who were at high risk for heart attacks were randomly assigned to follow:
• a Mediterranean-type diet supplemented with extra–virgin olive oil,
• a Mediterranean-type diet supplemented with mixed nuts, or
• no special diet, with instructions only to reduce dietary fat (control group)
After up to 4.8 years, 35 of the women had developed breast cancer. The rates per 1000 person–years for each group were:
1.1 for the Mediterranean diet with extra–virgin olive oil group,
1.8 for the Mediterranean diet with mixed nuts group, and
2.9 for the control group
Why This Study is Significant
The authors of this study recognize that their total number of cancer cases is small, so their findings "need to be confirmed in longer-term and larger studies." However, the study is significant because it is one of the first prospective studies linking breast cancer risk to dietary factors. Virtually all of the previous studies are retrospective, in which women who had breast cancer were asked to report what diets they had followed in the past. In this study, a large number of women who did not have breast cancer were placed on different diets, so the women who developed breast cancer during the study period could be accurately tabulated based on their diet group. Prospective studies are much harder to do, and they eliminate the bias of subjects who report what they think they should say rather than what they actually did, or who cannot recall what they ate.
Other Studies on Breast Cancer and Diet
A new report from The Nurses Study at Harvard shows that women who ate a lot of fiber in their teens (fruits, vegetables and seeds) had significantly reduced rates of breast cancer when they grew older (Pediatrics, February 1, 2016;137(3):). For nearly 44,000 women aged 33 to 52, each 10 g per day increase in fiber in adolescence and early adulthood was associated with a 13 percent reduction in breast cancer risk. Breast cancer in pre-menopausal women usually starts in adolescence. The authors of this report propose that a high-fiber diet lowers high blood estrogen levels that increase risk for pre-menopausal breast cancer.
Previous studies linking breast cancer risk to various dietary factors include:
• Post-menopausal women who followed a traditional Mediterranean diet in Greece had lower breast cancer rates (Am J Clin Nutr, Sept 2010;92(3):620-5).
• Women who ate the most calories, exercised the least and had the most body fat were at increased risk for breast cancer (Breast Cancer Res Treat, May 2006;97(1):97-106).
• Eating large amounts of foods that cause high rises in blood sugar increased breast cancer risk among postmenopausal women, regardless of whether they were overweight, exercised vigorously or took female hormones (Int J Cancer, May 1, 2006;118(9):2372).
There is no guarantee that anything you do or fail to do will prevent breast cancer, or keep it from recurring if you have been successfully treated in the past. However, many studies such as those reported here suggest that you can reduce your risk by following a healthful lifestyle.
• eat a high-plant diet, such as a Mediterranean-type diet with fruits, vegetables, seafood and olive oil
• restrict red meat and processed meats
• restrict foods that cause a high rise in blood sugar (sugar-added foods and drinks and other refined carbohydrates such as foods made from flour)
• avoid being overweight
I also continue to recommend not taking estrogen and progesterone, the female hormones. Most other doctors agree with this recommendation because the hormones have been linked to increased breast cancer risk.