Researchers have long speculated that the diet may help explain why nations in the Mediterranean region have historically had lower rates of heart disease and some cancers, including breast cancer, compared with other European countries and the U.S.
Until now, only two other studies have looked at the relationship between Mediterranean-style eating and the risk of breast cancer, both done in the U.S. Each found a connection between the diet and lower breast cancer risk, although in one the link was limited to breast cancers that lack receptors for the hormone estrogen — which account for about one-quarter of breast tumors.
The current study focused on women in Greece, as it is the “cradle” of the Mediterranean diet, and a large segment of the population still adheres to it, Dr. Dimitrios Trichopoulos, the senior researcher on the work, told Reuters Health by email.
At the outset, the study participants completed detailed dietary questionnaires and gave information on their lifestyle habits and demographics. Each woman was given a Mediterranean diet score, ranging from 0 to 9, based on how often they consumed vegetables, legumes, fruit and nuts, whole grains, fish and olive oil or other sources of monounsaturated fats; they also won points by limiting meat and dairy.
Of the 14,800 women included, 240 were diagnosed with breast cancer over an average follow-up of 10 years.
Overall, postmenopausal women whose Mediterranean diet scores were in the 6-to-9 range were 22 percent less likely to develop breast cancer than their counterparts with scores between 0 and 3. That was with factors such as age, education, smoking history, weight and exercise habits taken into account.
The findings show an association between Mediterranean eating and lower breast cancer risk, but do not prove cause-and-effect, according to Trichopoulos, who is with the Harvard School of Public Health in Boston and the Bureau of Epidemiologic Research at the Academy of Athens in Greece.
Further studies are needed to confirm the results, he said.
However, other evidence suggests ways the Mediterranean diet might curb cancer risk.
Research has found, for instance, that women who closely follow the diet tend to have lower levels of estrogen, which fuels the growth of the majority of breast cancers, than other women do. Other studies in the lab suggest that the fats found in the Mediterranean diet — both olive oil and the omega-3 fats in oily fish — may slow the growth of cancer cells.
The diet is also typically rich in antioxidants, which protect body cells from damage that can eventually lead to disease, including cancer. Trichopoulos said that if the Mediterranean diet does have a protective effect against cancer, it is “likely” to involve that antioxidant component.
It also makes sense, said the researcher, that the diet could affect the risk of postmenopausal, but not premenopausal, breast cancer.
Younger women who develop breast cancer, he explained, often have a genetic vulnerability to the disease, whereas in older women, lifestyle and environmental exposures may be relatively more important contributors to risk.
Based on their findings, Trichopoulos and his colleagues write, the association between the Mediterranean diet and breast cancer is of “modest, but not negligible, strength.”
In the U.S., a woman's chance of being diagnosed with breast cancer rises from about a half a percent, or one in 233, during her 30s, to about four percent, or one in 27, during her 60s.
Established risk factors for breast cancer include older age and having had a first-degree relative diagnosed with the disease. Research has also linked obesity, sedentary lifestyle, use of hormone replacement therapy and high alcohol intake to an increased risk.
SOURCE: Journal of Clinical NutritionCopyright 2017. Diet Advice Pty Ltd. All Rights Reserved.