Find out what we know about diet and how it might affect breast cancer risk.
Researchers estimate that we might be able to prevent as many as 9 out of every 100 cancer cases (9%) by changing our diets. But it is difficult to be exact about this. Research also suggests that about 5 out of 100 cancers (5%) could be avoided by maintaining a healthy body weight.
There has been a lot of research into the effect of dietary factors on breast cancer risk and so far most findings have been inconclusive and inconsistent. But there is some evidence on the following factors.
Fats include oils, butter and margarine as well as the fat in meats, fish and nuts. Remember there are also hidden fats in sweets, biscuits, cakes and other foods that you buy ready made.
An overview study has reported that women had an increased risk of breast cancer if they had more fats in their diet after the menopause. Another study has shown that women who ate higher levels of saturated fats had double the risk of breast cancer compared to those eating the least.
Foods high in saturated fat include:
- fatty cuts of meat
- meat products, including sausages and pies
- butter, ghee and lard
- cheese, especially hard cheese
- cream, soured cream and ice cream
- some savoury snacks and chocolate products
- biscuits, cakes and pastries
Based on the evidence we have, it appears that saturated fats play a role in increasing breast cancer risk. But it is probably a combination of this as well as other things that causes breast cancer.
People who eat a lot of foods containing fish oils (marine omega-3 polyunsaturated fatty acids) seem to have a lower breast cancer risk than people who only eat small amounts.
There is no strong evidence of a direct link between sugars and carbohydrates and breast cancer. But a large study of Chinese women in the USA reported in 2009 that for women younger than 50 a high carbohydrate diet slightly increased the risk of developing breast cancer. High carbohydrate diets are linked to an increased risk of a type of breast cancer called oestrogen receptor negative breast cancer.
Eating too much sugar can make you put on weight and we know that being overweight increases the risk of breast cancer in post menopausal women.
Dairy products have been studied for their effect on breast cancer risk. Some recent studies have shown that women with a high intake of dairy products have a lower risk of breast cancer, but we need results from more studies before we can be sure about this.
Dairy products are high in calcium, and several studies show a lower risk of breast cancer for women with high calcium intakes or calcium blood levels.
Fibre is found mostly in fruit, vegetables and whole meal cereals (including flour and all kinds of bread, particularly whole grain). There is some evidence that diets containing more than 25g of fibre per day reduce the risk of breast cancer in pre menopausal women.
Eating wheat bran fibre has been found to lower the levels of oestrogen in the blood in women who have not yet had their menopause. Lower levels of oestrogen may help to reduce the risk of breast cancer. Researchers aren't quite clear about why wheat fibre reduces oestrogen levels. It may not be an effect of the fibre itself. Instead it may be that high fibre diets contain less fat and more antioxidants than low fibre diets.
An overview study found that women who ate more fruit had a lower risk of breast cancer. This may be due to the fibre and antioxidants that they contain. Anti oxidants are molecules that prevent a chemical process called oxidation, which occurs when oxygen molecules join with another chemical. Oxidation can cause gene damage in cells that may lead to cancer. Antioxidants include vitamins A, C and E and selenium.
If you change your diet to include more fruit and vegetables and more starchy carbohydrates, you will almost certainly eat less fat. So you will be more likely to keep your weight within a healthy range. This helps to reduce your risk of breast cancer.
Scientists have been looking into whether eating phyto oestrogens, could affect the risk of getting breast cancer.
Phyto oestrogens are chemicals found in plants (phyto means plant). So they can also be called plant oestrogens. They have a similar structure to the female sex hormone oestrogen.
There are different types of phyto oestrogens. Some are found in soya bean products (isoflavones). Others are found in the fibre of whole grains, fruit, vegetables and flax seed (lignans). Milk may also contain phyto oestrogens, but this depends on what the cows have been eating.
High soya diet
A joint study reported in July 2002, produced by Cancer Research UK, the National Cancer Institute of the USA, and the National University of Singapore. It found that women with a diet high in soya had less dense breast tissue than women with low soya diets. Higher density of breast tissue has been linked to a higher risk of breast cancer. This is the first study to directly link eating soya with an effect on breast tissue.
Combining the results of lots of separate studies (a meta analysis) showed that Asian women who eat the highest amounts of soy foods had a lower risk of breast cancer. In other parts of the world, most women don't eat enough soy to reduce their risk of breast cancer.
In some studies, eating phyto oestrogens (soya flour and linseed supplements) regularly over several weeks reduced oestrogen levels. One of the active ingredients in soya is isoflavone. This chemical is similar to oestrogen and reduces the effect of human oestrogen in the body. High levels of human oestrogen can increase breast cancer risk.
Lignans are the main type of phyto oestrogens in the Western diet. Research into the effect of lignans on breast cancer risk was conflicting. So in 2009 researchers looked at all the studies that had been done. They found that in women who had had their menopause, high levels of lignin in the diet slightly reduced their breast cancer risk. It had no effect for premenopausal women though.
Carotenoids are organic colourings (pigments) found in some plants. Foods that are good sources of carotenoids include carrots, sweet potatoes, spinach, kale, greens, papaya, bell peppers, and tomatoes. A summary of published studies has shown that women with higher levels of carotenoids in blood samples may have a lower risk of breast cancer.
Flavonols and flavones are substance found in plants and are also called flavonoids or bioflavonoids. Flavonols are found in high levels in:
- black tea, green tea and oolong tea
Flavones are found in:
- aromatic herbs (such as parsley)
- camomile tea
Studies have shown that people who have high levels of flavonoids in their diet have a lower risk of breast cancer than people with lower levels.
A lot of research has looked into coffee drinking and cancer risk. Breast cancer is one of the cancers investigated. But there is no research evidence to show that coffee increases breast cancer risk.
Preventing breast cancer with diet
It might help to prevent breast cancer if you:
- replace animal fats with polyunsaturated fats (in many vegetable oils and margarines) and monounsaturated fats (such as olive oil)
- eat more isoflavones (found in soy, peas and beans) and lignans (found in vegetables, fruits, grains, tea and coffee)
We are not sure, but it may help if you:
- eat more fibre from wheat bran, cereals, beans, fruit and vegetables
- make sure you have enough calcium in your diet – from milk and other dairy foods, green leafy vegetables (such as broccoli, cabbage and okra, but not spinach), soya beans, tofu, nuts, bread, and fish where you eat the bones, such as sardines and pilchards
- eat plenty of fruit and vegetables
If you follow these guidelines, you will be eating a healthier diet. This helps you keep your weight within normal limits and can help protect against a variety of chronic health conditions.
Researching diet and breast cancer
Researching diet and breast cancer is very difficult because we all eat such a range of different foods in such differing amounts. A large study called EPIC (the European Prospective Investigation into Cancer) is looking at the links between lifestyle and cancer. It involves around 520,000 people in 10 European countries.
EPIC is producing reports on diet and a variety of cancers and will continue over the next 10 to 20 years, including breast cancer. If you would like to keep up to date with the findings from the EPIC study you can look online at the EPIC website.