Breast Cancer Risk: Does eating your vegetables matter?

The latest analysis of overall research on breast cancer risk and consumption of vegetables and fruits links greater consumption of vegetables and fruits with a modest decrease in breast cancer risk. Yet when considered separately, greater fruit consumption was tied to lower breast cancer risk, but eating more vegetables was not.  What?!? Aren’t vegetables a major part of a diet to lower cancer risk?

Before you toss out the broccoli and carrot sticks and go load up on ice cream or fat-free cookies, let’s take a closer look at the details of this analysis and some other recent studies to see the big picture about what eating your vegetables may or may not have to do with your risk of breast cancer.

AICR’s CUP analysis of vegetables, fruits and breast cancer risk

A few months ago an updated analysis was released of prospective studies — a scientifically strong type of study in which people are first asked about their eating habits and then followed to see what health developments may be linked to those habits. This is part of the Continuous Update Project (CUP) of the American Institute for Cancer Research and World Cancer Research Fund, an effort to keep updating findings that went into a landmark international report on diet, physical activity and cancer risk.

The landmark report released in 2007 concluded that evidence was then too limited or inconsistent for conclusions about any link between vegetable and fruit consumption and breast cancer risk. Additional research has since been published, and the CUP report analyzed 15 prospective population studies. The results show, compared to women with lowest consumption:

• Highest vegetable and fruit consumption is linked with 11 percent lower risk of breast cancer
• Highest fruit consumption is linked with 6 percent lower risk of breast cancer
• Highest vegetable consumption is linked with 1 percent lower risk of breast cancer, which is so small an effect it may have occurred simply by chance.

Take-home points from this analysis

Amount matters: When researchers switched from comparing highest versus lowest consumption to analyzing risk based on specific amounts of vegetables and fruits consumed, each 200 grams of total vegetable and fruit consumption was linked to a four percent decreased breast cancer risk.  Although researchers could not identify any threshold amount you need to have an effect, that means it takes, on average, one to two cups of vegetables and fruits to get a four percent decrease in risk.  A half-cup portion at breakfast and again at dinner is better than none, but it may not be enough to make much impact on breast cancer risk.

Individual differences probably matter:  The studies in this analysis generally included statistical adjustments for factors such as weight, physical activity and alcohol consumption that influence breast cancer risk.

♦ Researchers wonder whether consumption of vegetables and fruits might have different degree of impact on breast cancer risk at different stages of life.  Some evidence suggests that during adolescence and early adulthood breast tissue is particularly vulnerable to positive and negative influence.  However, it’s also possible that produce components could have particular impact at later stages of breast cancer development.  None of the studies in this analysis looks at earlier life consumption.

♦ Genetic differences may also cause people to differ in how they absorb, metabolize or use protective compounds in vegetables; some people may benefit more than others.  When we bunch everyone together, we can miss relevant effects in some people.

♦ Vegetable and fruit consumption is generally measured in these studies by having people check off a list how frequently they eat various choices.  This method is not the most accurate way to identify how much people really eat.  And looking at total vegetable consumption may miss potential differences in impact of different vegetable choices.

AICR’s CUP analysis of carotenoids and breast cancer risk

Another analysis of multiple studies undertaken as part of the CUP for breast cancer risk looked at dietary and blood levels of carotenoids. Carotenoids are a family of natural antioxidant compounds. Beta-carotene is the most famous; others studied include alpha-carotene, beta-cryptoxanthin, lycopene, lutein and zeaxanthin.  All these carotenoids are supplied in large amount by the vegetables and fruits we eat. The results show that, compared to women with lowest levels:

• Highest dietary consumption of total carotenoids is not linked to breast cancer risk
• Highest dietary consumption of beta-carotene is linked to 7 percent lower breast cancer risk
• Consumption of beta-carotene from supplements, however, is not linked to lower breast cancer risk
• Highest blood level of total carotenoids is linked with 26 percent lower breast cancer risk.  Blood levels of beta-carotene, alpha-carotene and lutein were also linked with reduced risk.

Take-home points from this analysis

 Vegetables may help lower breast cancer risk after all:  Two studies that measured both dietary carotenoids and blood carotenoids both found blood levels much more strongly linked to reduced risk than estimates of dietary consumption. Researchers suggest that the lack of link between vegetable consumption and breast cancer in some studies could simply reflect the difficulty of getting an accurate report of people’s intake.  Research overall has previously found that blood levels of carotenoid are a reliable indicator of vegetable and fruit consumption.

Differences in vegetable selection and preparation may matter: Higher blood carotenoids could reflect not just higher vegetable consumption, but greater selection of deep orange and dark green leafy vegetables and other nutrient-rich choices. Carotenoids are antioxidants that may protect DNA from damage that can lead to cancer. Carotenoids and other flavonoid compounds may act throughout the cancer process to decrease cell growth and support self-destruction of abnormal cells.  We absorb more of the carotenoids we consume when the vegetables have been cooked or served with a modest amount of fat.  People consuming equal amounts of vegetables may not absorb equal amounts of carotenoids, obscuring researchers’ ability to detect the protection they provide.

Analysis of cruciferous vegetable consumption and breast cancer risk

If vegetables differ in their effects on breast cancer risk, carotenoid-rich choices are not necessarily the only ones that offer protection.  Some cruciferous vegetables, such as broccoli, Brussels sprouts and kale, are rich in carotenoids.  All, however, provide compounds called glucosinolates that can be broken down to isothiocyanates and indole-3-carbinol, which may play a unique role in reducing cancer risk. For example, laboratory studies suggest these compounds may:

♦ inhibit enzymes that activate carcinogens, and stimulate enzymes that detoxify carcinogens

♦ turn on expression of tumor suppressor genes

♦ shift estrogen metabolism to favor a weaker, less cancer-promoting form

An analysis grouping together results of 13 population studies shows that compared to women who consume the least, highest consumption of cruciferous vegetables is linked with 15 percent lower risk of breast cancer.  This effect was even stronger in Chinese populations, where highest consumption was linked to 22 percent lower breast cancer risk. This could reflect different types of cruciferous vegetables, providing different types of amounts of glucosinolates or nutrients.  This could also reflect that, again, amount matters. “High consumers” of cruciferous vegetables in China eat considerably more than “high consumers” in the U.S. and Europe.

Because most studies conducted so far looking at cruciferous vegetable consumption ask people with and without breast cancer to look back and identify earlier dietary habits, this study is not as scientifically strong as the two CUP analyses above.  The researchers emphasize that no firm conclusions can be drawn at this time about cruciferous vegetables and breast cancer risk.

Bottom line take-home message:  For lower breast cancer risk, research does not suggest that eating more vegetables is likely to make up for an unhealthy weight or lifestyle.  As you look for steps you can take to reduce your risk of breast cancer, by far the strongest effects seem to come from reaching and maintaining a healthy weight, creating a lifestyle with frequent physical activity and minimizing alcohol consumption.

However, the lower risk seen related to blood carotenoid levels and cruciferous vegetable consumption add to a picture suggesting that we should not write off the multiple benefits of including plenty of vegetables and fruits in the predominantly plant-based diet recommended for lower overall cancer risk.  Whether or not vegetables offer direct protection, with weight control a major target to lower breast cancer risk, research shows supports substituting low-calorie vegetables for foods high in calories.

Go for a wide variety of vegetables, including deep orange, dark green and cruciferous choices. Boost their proportion in your meals and snacks by using them to replace all or part of the refined grains, meats, and high-calorie snack foods you currently eat.

Resources

Check the AICR website’s Foods that Fight Cancer™ for tips and recipe links for cruciferous vegetables.   You can find tips and recipes for winter squash there, too.

The National Cancer Institute (NCI) offers a fact sheet that lists many cruciferous vegetables.  It also discusses research regarding cruciferous vegetables and overall cancer risk, but note that it has not been updated since the analyses discussed here in Smart Bytes® were published.

References

Aune D, et al.  Fruits, vegetables and breast cancer risk: a systematic review and meta-analysis of prospective studies. Breast Cancer Res Treat, 2012. 134: 479-493.

Aune D, et al. Dietary compared with blood concentrations of carotenoids and breast cancer risk: a systematic review and meta-analysis of prospective studies. Am J Clin Nutr, 2012. 96: 356-373.

Liu X, Lv K, Cruciferous vegetables intake is inversely associated with risk of breast cancer: A meta-analysis.  The Breast (2012). http://dx.doi.org/10.1016/j.breast.2012.07.013  (electronically published version; not yet in print)