Background 

The goal of this systematic review was to determine whether dietary patterns are associated with risk of breast cancer. Dietary patterns were defined as the quantities; proportions; variety or combination of different foods, drinks and nutrients in diets; and the frequency with which they are habitually consumed.

Conclusion Statement 

Moderate evidence indicates that dietary patterns rich in vegetables, fruit and whole grains, and lower in animal products and refined carbohydrate, are associated with reduced risk of postmenopausal breast cancer. The data regarding this dietary pattern and premenopausal breast cancer risk point in the same direction, but the evidence is limited due to fewer studies. 

2015 DGAC Grade:

• Postmenopausal breast cancer risk: Moderate
• Premenopausal breast cancer risk: Limited

Methods 

Literature searches were conducted using PubMed, Embase, Navigator (BIOSIS, CAB Abstracts and Food Science and Technology Abstracts) and Cochrane databases to identify studies that evaluated the association between dietary patterns and risk of breast cancer. Studies that met the following criteria were included in the review: randomized controlled trials (RCTs), non-randomized controlled trials, prospective cohort studies or nested case-control studies; human subjects aged two years and older who were healthy or at elevated chronic disease risk; subjects from countries with high or very high human development (2012 Human Development Index); and published in English in peer-reviewed journals. The date range was from January 2000 to January 2014. The intervention or exposure was adherence to a dietary pattern (e.g., a priori patterns, data-driven patterns, reduced rank regression, or patterns derived from other methods and a description of the dietary pattern(s) (i.e., foods and beverages) consumed by subjects was provided. The outcome was incidence of breast cancer.

Data from each included article were extracted, and risk of bias was assessed. The evidence was qualitatively synthesized, a conclusion statement was developed and the strength of the evidence (grade) was assessed using pre-established criteria including evaluation of the quality and risk of bias, quantity, consistency, magnitude of effect and generalizability of available evidence.

Findings

• This systematic review includes 26 articles, 25 prospective cohort studies and one RCT that examined the relationship between dietary patterns and risk of breast cancer
• The studies used multiple approaches to assess dietary patterns and cancer risk. Eight studies used indices and scores to assess dietary patterns, 13 studies used factor or principal components analysis, two used reduced rank regression (RRR), two made comparisons on the basis of animal product consumption and one conducted an RCT of a low-fat dietary pattern
• This moderate body of evidence encompassed a large diversity in methods to assess or determine dietary patterns, making comparison across studies challenging. Despite this variability, 17 of the included studies found statistically significant relationships between dietary patterns and breast cancer risk, particularly among certain groups of women: 
• Because a variety of different methodologies were employed to derive dietary patterns, and these patterns, while similar in many respects, were composed of different combinations of foods and beverages, it was difficult to determine which patterns had the greatest impact on breast cancer risk reduction
• The relationship between dietary patterns and breast cancer risk may be more consistent among postmenopausal women, but additional research is needed to explore the relationships for both premenopausal and postmenopausal cancer.

Limitations

The ability to draw strong conclusions was limited by the following issues: 

• Certain histopathologic and molecular phenotypes of breast cancer may be affected more by certain dietary patterns, but this has not yet been explored sufficiently. For example, limited studies to date suggest that estrogen or progesterone receptor status of breast cancers may define subgroups with unique dietary risk profiles, but no conclusions can be drawn at this time.
• More research is needed to explore other factors that may influence the relationship between dietary patterns during various stages of life and breast cancer risk, such as anthropometrics, body mass index (BMI) (including weight change over adulthood), physical activity, sedentary behavior and reproductive history, including ages of menarche, age of menopause, parity and breastfeeding.