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By Sarah-Jane Reilly, RD
Breast cancer is the most commonly diagnosed type of cancer among women worldwide, regardless of age or ethnicity.¹ In the UK, one in seven women will be diagnosed with breast cancer during their lifetime.² It is important to note that less than 10% of breast cancer cases are associated with a recognised inherited genetic mutation.³ In fact, one in four breast cancers are preventable.² According to the World Cancer Research Fund (WCRF), roughly one third of the most common breast neoplasms could be prevented through lifestyle and behaviour modification, such as maintaining a healthy weight, engaging in regular physical activity, consuming a balanced and varied diet, limiting alcohol consumption and breastfeeding infants postpartum.⁴
Many epidemiological studies have examined the association between individual foods and breast cancer risk. Studies have found that high intakes of red meat, animal fats and refined grains are associated with an increased risk of breast cancer ⁵,⁶ while high intakes of wholegrains, fruits, vegetables, and dietary fibre are associated with a reduced risk.⁷-⁹
Several studies have explored the associations between dietary patterns and breast cancer, with a positive association between breast cancer risk and the Western diet having been reported.¹⁰, ¹¹ In addition, an inverse association between a healthy diet and the risk of developing breast cancer has been identified, with a healthy dietary pattern defined as a diet rich in fruits, vegetables, fish, wholegrains and low-fat dairy products.¹², ¹³
These findings are supported by a recent meta-analysis of 32 observational studies which included a total of 43,285 breast cancer cases.¹⁴ In the analysis, a Western diet was associated with a 14% increased risk for breast cancer (relative risk 1.14) while the ‘prudent’ healthy dietary pattern was associated with an 18% reduction in breast cancer risk (relative risk 0.82). Of note, the positive association between the Western diet and breast cancer risk was statistically significant among postmenopausal women who were at a 20% increased risk, but these results were not seen among premenopausal women.
This could be explained by the difference in the metabolic pathway of oestrogen following menopause. When the ovaries stop producing oestrogen, the main source of oestrogen for post-menopausal women becomes adipose tissue.¹⁴ There is strong and convincing evidence that obesity is a risk factor for post-menopausal breast cancer.⁴ Therefore, it is important for dietitians to be aware of the importance of encouraging post-menopausal women to follow a healthy diet and maintain a healthy body weight.
There appears to be a trend of increasing popularity of plant-based eating among breast cancer patients as well as those seeking to improve their health. The British Dietetic Association defines a plant-based diet as “a diet which is based on foods derived from plants, including vegetables, wholegrains, legumes, nuts, seeds and fruits, with few or no animal products.”¹⁵ It is important to highlight that plant-based diets encompass many ways of eating, including:
One study among oncology patients in New Mexico hospitals found that there was an increasing preference for plant-based meals among inpatients, with vegetarian options preferred over vegan choices mostly among female oncology patients.¹⁶ This highlights how these preventative messages are now starting to sway hospital-based food choices.
Whether the adoption of a plant-based diet can reduce additional breast cancer events or mortality remains controversial. The results of the Women’s Intervention Nutrition Study (WINS) and Women’s Healthy Eating and Living (WHEL) randomised control studies suggest that there is no strong evidence that changing dietary patterns following breast cancer diagnosis will improve the prognosis for patients with early-stage breast cancer.¹⁷
Recently, a meta-analysis revealed that individuals who follow a vegetarian diet have an 8% reduced risk of total cancer (relative risk 0.92) when compared to meat eaters.¹⁸ However, this study didn’t show a statistical significance when analysed by cancer type. The WCRF Continuous Update Project (2018) analyses global research to advise on how certain lifestyle factors can influence the risk of developing breast cancer.¹⁹ With regards to plant-based dietary patterns, the WCRF recommend the following.
Premenopausal breast cancer
There is some (suggestive) evidence that¹⁹:
Postmenopausal breast cancer
There is limited (non-conclusive) evidence that¹⁹:
Dietitians play a key role in educating patients about what constitutes a plant-based diet together with supporting patients with a breast cancer diagnosis who chose to follow this dietary pattern. Dietitians should be aware of the potential nutritional gaps in vitamins and minerals such as calcium, vitamin B12, iron and long chain n-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). New resources are now available to support patients with a cancer diagnosis who choose to follow a plant-based diet.
The WCRF Continuous Update project has shown that modifying dietary and lifestyle factors plays an important role in reducing risk of breast cancer. However, dietitians should be aware that it is not advisable to recommend a transition to plant-based eating to patients when they are diagnosed with breast cancer. Current evidence does not suggest that this improves the prognosis of the disease.¹⁷, ¹⁸ Quality of life is key and should be at the centre of clinical decision making among this population group.