Findings are mixed; crucifers may raise breast cancer survival rates 12/31/2018
We all know that our diets should be packed with vegetables.
And conventional wisdom holds that cruciferous vegetables like broccoli, kale, and cabbage help prevent common cancers.
That perception is based on lab research in animals and human cells, which suggests that cruciferous vegetables possess potent anti-cancer properties.
However, epidemiological (population) studies haven’t always detected links between cruciferous vegetables and reduced cancer rates — and as you’d expect, there are no controlled clinical studies on this subject.
Some epidemiological studies find substantial links between higher intakes of cruciferous veggies and reduced risks for lung, colorectal, stomach, breast, prostate, and other cancers — but others have either found weaker links or none.
We probed the anti-cancer potential of cruciferous vegetables in To Block Cancer, Favor the Crucifer Family.
And intriguing new findings from Harvard and the Dana Farber Cancer Institute mean that it’s time for a fresh look — especially regarding breast cancer.
What are cruciferous vegetables and what can they do?
Cruciferous vegetables are members of the larger brassica family.
The name for this group of brassica vegetables comes from the fact that their four-petal flowers form a head that resembles a cross or "crucifer”.
Broccoli is the best-known crucifer-type vegetable, but the family includes arugula, bok choy, Brussels sprouts, broccoli rabe, cabbage, cauliflower, collard greens, cress, daikon, horseradish, kale, kohlrabi, mustard greens, rutabaga, radish, turnips, and wasabi.
Generally speaking, cruciferous vegetables are rich in vitamins C, E, and K, folic acid, minerals, and carotenoid-class antioxidant compounds such as beta-carotene, lutein, and zeaxanthin.
And cruciferous vegetables contain compounds called glucosinolates, whose sulfur content accounts for the strong smells that some emit when they are cooked.
When we consume and metabolize cruciferous vegetables, their glucosinolates break down into two groups of chemicals — isothiocyanates and indoles — that display anti-cancer effects in test tube and animal experiments.
Animal research suggests that these two groups of glucosinolate-derived chemicals can help prevent cancers of the bladder, breast, colon, liver, lung, and stomach.
And many epidemiological (population-based) studies also link diets rich in cruciferous vegetables with lower rates of those malignancies.
Why would the metabolic byproducts of dietary glucosinolates help curb cancer risks?
First, the isothiocyanates and indoles created when we digest cruciferous vegetables exert antioxidant effects that can help protect cellular DNA from cancer-promoting damage.
In addition, these chemicals can neutralize some carcinogens, fight inflammation, and block the formation of blood vessels that would feed tumors: an effect called anti-angiogenesis.
New study supports cruciferous benefits versus breast cancer
Ten years ago, UC Santa Barbara researchers reported that one of the glucosinolate-derived compounds in broccoli (sulforaphane) undermines breast tumors in ways like those seen with two chemotherapy drugs (taxol and vincristine).
Four years later, we reported on a joint U.S.-Chinese study that linked diets higher in vegetables — especially cruciferous types such as broccoli and carotenoid-rich yellow/orange types to reduced breast cancer rates.
Specifically, the U.S.-Chinese team's analysis linked higher cruciferous vegetable intake during the first 36 months after breast cancer diagnosis to a reduced risk for death from any cause, breast cancer-specific death, and disease recurrence.
Importantly, the women’s survival rates varied in a persuasive “dose–response” pattern.
By about five years after diagnosis, the women whose self-reported intake of cruciferous vegetables ranked in the top one-fifth — an average of 150 grams (five ounces) a day — were 42 percent less likely to have died from breast cancer.
And the results of a new epidemiological study — which looked for links between diets rich in fruits and veggies and breast cancer — found especially strong links between higher intakes of cruciferous vegetables and reduced risks for aggressive breast tumors.
A Boston-based team from Harvard University and the Dana-Farber Cancer Institute analyzed diet questionnaires submitted every four years by participants in the huge Nurses' Health Studies I and II, which includes 182,145 women.
They found that women who ate more than 5½ servings of fruits and vegetables every day had an 11% lower risk of breast cancer than those who ate 2½ or fewer servings.
Importantly, these risk-reductions applied most strongly to estrogen-receptor-negative tumors, which are generally more aggressive and harder to treat. (Likewise, lab tests on human breast cancer cells suggest that seafood-source omega-3 fatty acids (DHA and EPA) may help discourage ER-negative breast tumors.)
This is how lead author Maryam Farvid from the Harvard School of Public Health characterized the significance of their findings:
“Although prior studies have suggested an association, they have been limited in power, particularly for specific fruits and vegetables and aggressive subtypes of breast cancer. This research provides the most complete picture of the importance of consuming high amounts of fruit and vegetables for breast cancer prevention.”
Current evidence is solid but needs more research
The American Institute for Cancer Research (AICR) periodically publishes summaries of the research concerning specific food groups and their effects on the risk of various cancers.
Their current review for cruciferous vegetables sees convincing evidence that they can help prevent colon cancer, and some evidence linking them to reduced risks for mouth, throat, and lung cancers.
These are the details of the 2016 AICR review:
- Prostate cancer: Studies in the United States and Europe have found little or no links between cruciferous vegetable intakes and prostate cancer risk. However, some case-control studies have found that people who ate more cruciferous vegetables than average were less likely to develop prostate cancer.
- Colorectal cancer: Studies in the United States and Europe have generally found no links. However, one Dutch study found that women (but not men) who reported eating crucifer-rich diets were less likely to develop colon cancer, but no less likely to develop rectal cancer.
- Lung cancer: Most studies found either no links are weak ones, but an analysis of data from the Nurses’ Health Study and the Health Professionals’ Follow-up Study showed that women who ate more than five servings per week had a lower risk of lung cancer.
- Breast cancer: One case-control study found that women who ate more cruciferous vegetables and average had a lower risk of breast cancer. But a meta-analysis of studies from the United States, Canada, Sweden, and the Netherlands found no links between cruciferous vegetables and breast cancer risk. Likewise, a study of American women found only a weak link to reduced breast cancer risk. [Editor’s note: The AICR summary seems to have overlooked the results of the 2008 and 2012 studies mentioned and linked to above.]
Unsurprisingly, the AICR review highlights the role that personal genetics play in the degree of cancer-preventive benefits (if any) that a given person would gain from cruciferous vegetables — and the review noted that genetic influences likely explain inconsistent findings concerning cruciferous vegetables and cancer.
Confirming that suspicion, researchers recently discovered that nearly half of all people lack the specific gene needed to retain and use certain of the compounds in cruciferous vegetables.
- American Institute for Cancer Research (AICR). AICR's Foods That Fight Cancer. Accessed at www.aicr.org/foods-that-fight-cancer/broccoli-cruciferous.html .
- Chen YJ, Wallig MA, Jeffery EH. Dietary Broccoli Lessens Development of Fatty Liver and Liver Cancer in Mice Given Diethylnitrosamine and Fed a Western or Control Diet. J Nutr. 2016 Mar;146(3):542-50. doi: 10.3945/jn.115.228148. Epub 2016 Feb 10.
- Farvid MS, Chen WY, Rosner BA, Tamimi RM, Willett WC, Eliassen AH. Fruit and vegetable consumption and breast cancer incidence: Repeated measures over 30 years of follow-up. Int J Cancer. 2018 Jul 6. doi: 10.1002/ijc.31653. [Epub ahead of print] PubMed PMID: 29978479.
- National Cancer Institute. Cruciferous Vegetables and Cancer Prevention. June 7, 2012. Accessed at www.cancer.gov/about-cancer/causes-prevention/risk/diet/cruciferous-vegetables-fact-sheet.
- Terry P, Wolk A, Persson I, Magnusson C. Brassica vegetables and breast cancer risk. JAMA 2001;285(23):2975-2977.
- Voorrips LE, Goldbohm RA, van Poppel G, et al. Vegetable and fruit consumption and risks of colon and rectal cancer in a prospective cohort study: The Netherlands Cohort Study on Diet and Cancer. American Journal of Epidemiology 2000;152(11):1081-1092.