Can eating your greens, reds and oranges reduce your risk of breast cancer?

A  healthy diet, rich in fruit and vegetables, can help to prevent many diseases, including cancers such as breast cancer. But what is the connection between a healthy diet and breast cancer?

This was the question asked recently by a team of scientists from the Harvard T.H. Chan School of Public Health who investigated the relationship between consumption of certain fruit and vegetables and the incidence of breast cancer . Our new science officer discusses the results of their study and explains how it might help us with dietary advice on reducing our risk of breast cancer.

A new study by researchers at “Harvard T.H. Chan School of Public Health” in Boston found that the consumption of more than 5.5 portions of fruit and vegetables per day was associated with an 11% lower risk of breast cancer compared to the risk for women consuming 2.5 portions or less. The team also found that eating cruciferous vegetables (such as cauliflower, broccoli and cabbage) and orange and red coloured vegetables (such as carrots and sweet potatoes) were especially beneficial.

How does the study differ from other research?

Findings of previous studies that investigated a possible link between breast cancer risk and fruit and vegetable intake have been inconsistent. Some studies found no protective effect from fruit or vegetables (e.g. 3, 4); others identified a link between fruit consumption, but not vegetable consumption, and reduced breast cancer risk, whilst others have found consumption of both fruit and vegetables reduces breast cancer risk.

The study focused on consumption of different varieties of fruit and vegetables and the risk of different subtypes of breast cancer, using data from a large-scale long term study over several decades. The participants filled out questionnaires on their diet and other variables related to breast cancer, such as age, weight and smoking. A limitation of previous studies was that diet was assessed with a single questionnaire, and in many studies, the follow-up was short. In this investigation dietary intake was measured with food frequency questionnaires in the beginning and every 4 years thereafter. The average follow-up time was more than 23 years.

When distinguishing between different types of vegetables, yellow/orange and cruciferous vegetables appeared to be particularly beneficial. The intake of more than five servings of yellow/orange vegetables a week lowered the risk of incident invasive breast cancer by 9% and the intake of more than five servings of cruciferous vegetables a week lowered the risk by 10%.

Do fruit and vegetables protect against all types of breast cancer?

The researchers wanted to find out whether the benefits of fruit and vegetable consumption in reducing breast cancer risk differed according to breast cancer type. While most of us think of breast cancer as a single disease, evidence shows there are multiple subtypes of breast cancer with distinct morphologies and clinical implications. They included in their analysis oestrogen receptor (ER) status, progesterone receptor status and HER2 receptor status. A cancer is called oestrogen-receptor (ER)-positive if it has receptors for oestrogen on its cell surface. Cancer cells, like normal breast cells, may receive signals from oestrogen that could promote their growth. Similarly, a cancer is progesterone-receptor-positive if it has progesterone receptors which may receive signals from the hormone progesterone, and if it is HER2 receptor positive it has additional receptors for the HER2 protein; these cell surface receptors may also promote cancer growth.

The results of the study indicate the strongest associations were between higher consumption of total vegetables and prevention of ER-negative tumours and higher consumption of fruit and vegetables and prevention of HER2-enriched, and basal-like tumours (those in the basal layer of breast cells), which are aggressive tumours that tend to grow and spread quickly.

Did the study find specific components of fruit and vegetables were protective?

The new study not only found that dietary fibre, but also micronutrients in fruit and vegetables, help to reduce the risk of breast cancer. For example, carotenoids (yellow, red, or orange substances found in plants, such as carrots and sweet potatoes) have been hypothesized to reduce cancer risk by protecting cells from damage or suppressing cell growth. Vitamin C may also act as a cell protector in reducing breast cancer risk.

The results of the study are also consistent with those of the WCRF (World Cancer Research Fund), which concluded in its latest report on breast cancer that consuming non-starchy vegetables and foods containing carotenoids might decrease the risk of breast cancer .

One serving in this study was defined as, for example, half a cup of grapes, one apple, one small banana, half a cup of cooked broccoli or one large carrot (2). For more advice on portion sizes the NHS has created a website with helpful tips on portion sizes.

It is important to eat the whole fruit or vegetable, as the researchers did not find an association between consumption of fruit juice and breast cancer risk. This is thought to be because juicing processes lead to lower contents of beneficial phytochemicals and dietary fibre.

Some examples of fruit and vegetables that may help reduce risk

Cancer cannot always be avoided with a healthy diet. However, the results of this study show us that a diet rich in fruit and vegetables can have a preventive effect against breast cancer. Especially by incorporating orange, yellow and cruciferous vegetables into our meal plan we may reduce our risk. The risk reducing amount of about 300 to 400 g of fruit and vegetables per day can be easily achieved with a large banana and blueberries in a bowl of porridge for breakfast, a dish for lunch rich in vegetables such as carrots, broccoli and peppers and eating fruit or vegetables instead of sweets as a snack.

References:

1. Locke A. (2018). Diets for Health: Goals and Guidelines: American family physician 97(11):721–728. https://www.ncbi.nlm.nih.gov/pubmed/30215930
2. Farvid, M. S. et al. (2019). Fruit and vegetable consumption and breast cancer incidence: Repeated measures over 30 years of follow-up: International journal of cancer 144(7):1496–1510. https://onlinelibrary.wiley.com/doi/full/10.1002/ijc.31653
3. Jung, S. et al. (2013). Fruit and vegetable intake and risk of breast cancer by hormone receptor status. Journal of the National Cancer Institute 105(3):219–236. https://www.ncbi.nlm.nih.gov/pubmed/23349252
4. van Gils, C. H. et al. (2005). Consumption of vegetables and fruits and risk of breast cancer. Journal of the American Medical Association 293(2):183–193. https://www.ncbi.nlm.nih.gov/pubmed/15644545
5. Aune, D. C. et al. (2012). Fruits, vegetables and breast cancer risk: a systematic review and meta-analysis of prospective studies. Breast cancer research and treatment 134(2):479–493. https://www.ncbi.nlm.nih.gov/pubmed/22706630
6. AICR/WCRF. Diet, nutrition, physical activity and breast cancer 2018: https://www.aicr.org/continuous-update-project/reports/breast-cancer-report-2017.pdf .(Accessed July 09 2019)
7. Canadian Cancer Society. Triple-negative and basal-like breast cancers – Canadian Cancer Society. Available from: https://www.cancer.ca/en/cancer-information/cancer-type/breast/breast-cancer/cancerous-tumours/triple-negative-breast-cancer/?region=on (Accessed July 09 2019)
8. Muraki, I. et al. (2013). Fruit consumption and risk of type 2 diabetes: results from three prospective longitudinal cohort studies: British medical journal (Clinical research ed.) 347:f5001. https://www.ncbi.nlm.nih.gov/pubmed/23990623