Mediterranean Diet Cuts Some Breast Cancer Risk by 40%

March 08, 2017

Closely following a Mediterranean diet in everyday life may significantly reduce the risk for types of breast cancer that are associated with poorer prognoses in postmenopausal women, new research indicates.

The traditional Mediterranean diet is characterized by a high intake of plant proteins, whole grains, fish, and monounsaturated fat, as well as moderate alcohol intake and low intake of refined grains, red meat, and sweets, say the study authors, led by Piet A. van den Brandt, PhD, an epidemiologist at the Maastricht University Medical Center in the Netherlands.

This diet has been repeatedly shown to be associated with decreased risk for cardiovascular diseases, but the evidence in cancer, including breast cancer, is less established, say the authors.

The new findings come from 62,573 Dutch women aged 55 to 69 years who provided information on dietary and lifestyle habits in 1986 and have since been followed for more than 20 years.

The study was published online March 5 in the International Journal of Cancer.

It consisted of multivariate case–cohort analyses of 2321 incident breast cancer patients and 1665 women who were without breast cancer and for whom complete diet data were available.

The investigators found that women who most closely adhered to a Mediterranean diet had a 40% reduced risk for estrogen receptor–negative (ER-) breast cancer compared to women who adhered to the diet the least (hazard ratio [HR], 0.60; ptrend = .032).

They found a 39% reduced risk for progesterone receptor–negative (PR-)/ER- disease when comparing these same high- and low-adherence groups (HR, 0.61; ptrend = .047).

Notably, in these results, the definition of the diet excluded alcohol intake, because the consumption of alcohol is a known risk factor for breast cancer. Alcohol was, however, one of many variables controlled for, along with other factors, such as age, body mass index, family history of breast cancer, use of hormone replacement therapy, and smoking status.

The authors also report that there were no significant associations with the diet and the risk of ER+ disease or total breast cancer.

It makes sense that the protective effect of the diet was not seen in women with ER+ disease, say the authors.

"Any potential influence of dietary factors may be difficult to detect in ER+ tumors given the strong influence of hormonal factors. In ER- tumors, other risk factors, including diet, may exert a relatively larger influence and be more easily detectable," they write.

Dr van den Brandt also explained that older women, who were the subjects of the new study, are more likely to derive benefit than younger women.

"Generally speaking, postmenopausal breast cancer seems somewhat more influenced by environmental factors, such as lifestyle and diet, than premenopausal breast cancer, where genetic factors seem to play a more prominent role," he told Medscape Medical News.

Hanna E. Bloomfield, MD, MPH, an internist at the University of Minnesota and the Minneapolis VA Healthcare System, currently recommends the Mediterranean diet to her female patients.

"I can't see any downside, and there is some evidence of benefit," she told Medscape Medical News.

Dr Bloomfield, who was not involved in the Dutch study, was the lead author a 2016 meta-analysis of studies of the diet and concluded that "limited evidence" indicates it "may" reduce the incidence of breast cancer.

The new study is "well-conducted," said Estefania Toledo, PhD, of the Department of Preventive Medicine and Public Health at the Universidad de Navarra in Spain, who was asked for comment.

It is also "important" because few prospective cohort studies have addressed an association between the Mediterranean diet and breast cancer, she said.

Dr Toledo was the senior author of the only large, randomized trial to date in which postmenopausal women were assigned to a dietary intervention to promote their adherence to the traditional Mediterranean diet (JAMA Intern Med. 2015;175:1752–60). The study found that women with a higher adherence to the diet (supplemented with extra-virgin olive oil) showed a substantial reduction of their risk for breast cancer compared to a control group, as reported by Medscape Medical News.

Dr Toledo highlighted the fact that the Dutch authors, in their new article, also conducted a meta-analysis of cohort studies that confirmed their own cohort study's finding.

Indeed, the Dutch authors report that, in the meta-analysis, summary hazard ratios for high vs low Mediterranean diet adherence were not significant for total postmenopausal breast cancer cases (0.94) and cases of ER+ disease (0.98) but were statistically significant for ER- (0.73) and ER-/PR- (0.77) breast cancer cases.

Other major cohort studies with similar findings to the Dutch study include the Nurses' Health Study and the European Prospective Investigation into Cancer (EPIC), say Dr van den Brandt and his coauthors.

The mechanisms of action behind the potential beneficial effects of the Mediterranean diet on cancer risk are uncertain, say the authors.

However, they say that the effects may be attributable to high amounts of fiber, antioxidants, and vitamins; they may be mediated through biological mechanisms, such as chronic inflammation and oxidative stress and through the regulation of body weight; and they may be associated DNA oxidative damage.

The study was funded by the Wereld Kanker Onderzoek Fonds Nederland as part of the World Cancer Research Fund International grant program. The authors, Dr Toledo, and Dr Bloomfield have disclosed no relevant financial relationships.

Int J Cancer. Published online March 5, 2017. Abstract

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