Mediterranean diet may reduce the incidence of breast cancer

1. This secondary analysis of the data from the PREDIMED trial found that there was a 70% decrease in breast cancer incidence for women randomized to Mediterranean diet supplemented with extra-virgin olive oil.

2. There was a non-significant reduction in breast cancer incidence in women who were randomized to the Mediterranean diet supplemented with nuts.

Evidence Rating Level: 1 (Excellent)

Study Rundown: The incidence of breast cancer is increasing worldwide: from 2008 to 2012 there was a 20% increase in incidence, accompanied by a 14% increase in mortality. Previous epidemiological studies have identified diet as a potential modifiable risk factor in breast cancer development however no controlled studies have been performed. The Mediterranean diet (MeDiet) in particular has garnered attention, as breast cancer incidence has historically been lower in Mediterranean countries as compared to North America and Central Europe. This secondary analysis of a randomized controlled trial, which was originally conducted to examine cardiovascular disease, was aimed to examine the effects of the MeDiet supplemented with extra-virgin olive oil or nuts on the incidence of breast cancer in women.

Mediterranean diet supplemented with extra-virgin olive oil was statistically associated with an approximately 70% reduction in breast cancer incidence. There was a non-significant reduction in breast cancer incidence in women who were randomized to the Mediterranean diet supplemented with mixed nuts. The strengths of this study include its randomized design and adjustment of hazard ratios with many relevant potential confounders. However, the number of observed breast cancers was small and thus may impact the power of the study to detect a reduction in breast cancer incidence. Moreover, this was a secondary analysis of data collected for the PREDIMED trial, which was not powered to detect breast cancer, but rather cardiovascular disease.

Click to read the study in JAMA Internal Medicine

Relevant Reading: Moderate alcohol consumption during adult life, drinking patterns and breast cancer risk

In-Depth [randomized controlled trial]: This 1:1:1 randomized, single-blind controlled trial was conducted at primary health care centres across Spain from 2003 to 2009, originally designed to examine cardiovascular disease. It included 4,282 women aged 60 to 80 years old at high risk for cardiovascular disease. These women were randomized to either a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with mixed nuts, or a control diet with advice to reduce dietary fat. The outcome of interest was breast cancer incidence in women without a prior history of breast cancer. Intention-to-treat analysis using multivariable Cox regression adjusted for many variables (including age, recruitment centre, baseline BMI, use of hormone therapy, family history of cancer and physical activity) was used.

In total, 4152 women were analyzed for breast cancer incidence. After a median of 4.8 years of follow-up, 35 new cases of breast cancer were identified. The Mediterranean diet supplemented with olive oil had an almost 70% decrease in breast cancer incidence: multivariable-adjusted hazard ratio vs. control group was 0.32 (95% CI 0.13-0.79). There was a non-significant relationship between the Mediterranean diet supplemented with mixed nuts group as compared to control (adjusted HR 0.59, 95% CI 0.26-1.35).

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