Nutrition and cancer; controversial, an oncology outlier. But it is important enough for the National Cancer Institute (NCI), part of the National Institutes of Health, think it’s important. Very important.
NCI has published a number of cancer related nutritional articles. Here’s an example of a specific one, focusing on cruciferous vegetables:
Cruciferous Vegetables and Cancer Prevention
- What are cruciferous vegetables?
Cruciferous vegetables are part of the Brassica genus of plants. They include the following vegetables, among others:
- Bok choy
- Brussels sprouts
- Collard greens
- Why are cancer researchers studying cruciferous vegetables?
In addition, cruciferous vegetables contain a group of substances known as glucosinolates, which are sulfur-containing chemicals. These chemicals are responsible for the pungent aroma and bitter flavor of cruciferous vegetables.
During food preparation, chewing, and digestion, the glucosinolates in cruciferous vegetables are broken down to form biologically active compounds such as indoles, nitriles, thiocyanates, and isothiocyanates (1). Indole-3-carbinol (an indole) and sulforaphane (an isothiocyanate) have been most frequently examined for their anticancer effects.
Indoles and isothiocyanates have been found to inhibit the development of cancer in several organs in rats and mice, including the bladder, breast, colon, liver, lung, and stomach (2, 3). Studies in animals and experiments with cells grown in the laboratory have identified several potential ways in which these compounds may help prevent cancer:
- They help protect cells from DNA damage.
- They help inactivate carcinogens.
- They have antiviral and antibacterial effects.
- They have anti-inflammatory effects.
- They induce cell death (apoptosis).
- They inhibit tumor blood vessel formation (angiogenesis) and tumor cell migration (needed for metastasis).
Studies in humans, however, have shown mixed results, as described in Question 3.
- Is there evidence that cruciferous vegetables can help reduce cancer risk in people?
Researchers have investigated possible associations between intake of cruciferous vegetables and the risk of cancer. The evidence has been reviewed by various experts. Key studies regarding four common forms of cancer are described briefly below.
- Prostate cancer: Cohort studies in the Netherlands (4), United States (5), and Europe (6) have examined a wide range of daily cruciferous vegetable intakes and found little or no association with prostate cancer risk. However, some case-control studies have found that people who ate greater amounts of cruciferous vegetables had a lower risk of prostate cancer (7, 8).
- Colorectal cancer: Cohort studies in the United States and the Netherlands have generally found no association between cruciferous vegetable intake and colorectal cancer risk (9-11). The exception is one study in the Netherlands—the Netherlands Cohort Study on Diet and Cancer—in which women (but not men) who had a high intake of cruciferous vegetables had a reduced risk of colon (but not rectal) cancer (12).
- Lung cancer: Cohort studies in Europe, the Netherlands, and the United States have had varying results (13-15). Most studies have reported little association, but one U.S. analysis—using data from the Nurses’ Health Study and the Health Professionals’ Follow-up Study—showed that women who ate more than 5 servings of cruciferous vegetables per week had a lower risk of lung cancer (16).
- Breast cancer: One case-control study found that women who ate greater amounts of cruciferous vegetables had a lower risk of breast cancer (17). A meta-analysis of studies conducted in the United States, Canada, Sweden, and the Netherlands found no association between cruciferous vegetable intake and breast cancer risk (18). An additional cohort study of women in the United States similarly showed only a weak association with breast cancer risk (19).
A few studies have shown that the bioactive components of cruciferous vegetables can have beneficial effects on biomarkers of cancer-related processes in people. For example, one study found that indole-3-carbinol was more effective than placebo in reducing the growth of abnormal cells on the surface of the cervix (20).
In addition, several case-control studies have shown that specific forms of the gene that encodes glutathione S-transferase, which is the enzyme that metabolizes and helps eliminate isothiocyanates from the body, may influence the association between cruciferous vegetable intake and human lung and colorectal cancer risk
- Are cruciferous vegetables part of a healthy diet?
The federal government’s Dietary Guidelines for Americans 2010 recommend consuming a variety of vegetables each day. Different vegetables are rich in different nutrients.
Vegetables are categorized into five subgroups: dark-green, red and orange, beans and peas (legumes), starchy, and other vegetables. Cruciferous vegetables fall into the “dark-green vegetables” category and the “other vegetables” category. More information about vegetables and diet, including how much of these foods should be eaten daily or weekly, is available from the U.S. Department of Agriculture website. Choose My Plate.
Higher consumption of vegetables in general may protect against some diseases, including some types of cancer. However, when researchers try to distinguish cruciferous vegetables from other foods in the diet, it can be challenging to get clear results because study participants may have trouble remembering precisely what they ate. Also, people who eat cruciferous vegetables may be more likely than people who don’t to have other healthy behaviors that reduce disease risk. It is also possible that some people, because of their genetic background, metabolize dietary isothiocyanates differently. However, research has not yet revealed a specific group of people who, because of their genetics, benefit more than other people from eating cruciferous vegetables.
- Hayes JD, Kelleher MO, Eggleston IM. The cancer chemopreventive actions of phytochemicals derived from glucosinolates. European Journal of Nutrition 2008;47 Suppl 2:73-88.
- Hecht SS. Inhibition of carcinogenesis by isothiocyanates. Drug Metabolism Reviews 2000;32(3-4):395-411.
- Murillo G, Mehta RG. Cruciferous vegetables and cancer prevention. Nutrition and Cancer 2001;41(1-2):17-28.
- Schuurman AG, Goldbohm RA, Dorant E, van den Brandt PA. Vegetable and fruit consumption and prostate cancer risk: a cohort study in The Netherlands. Cancer Epidemiology, Biomarkers & Prevention 1998;7(8):673-680.
- Giovannucci E, Rimm EB, Liu Y, Stampfer MJ, Willett WC. A prospective study of cruciferous vegetables and prostate cancer. Cancer Epidemiology, Biomarkers & Prevention 2003;12(12):1403-1409.
- Key TJ, Allen N, Appleby P, et al. Fruits and vegetables and prostate cancer: no association among 1104 cases in a prospective study of 130544 men in the European Prospective Investigation into Cancer and Nutrition (EPIC). International Journal of Cancer 2004;109(1):119-124.
- Kolonel LN, Hankin JH, Whittemore AS, et al. Vegetables, fruits, legumes and prostate cancer: a multiethnic case-control study. Cancer Epidemiology, Biomarkers & Prevention 2000;9(8):795-804.
- Jain MG, Hislop GT, Howe GR, Ghadirian P. Plant foods, antioxidants, and prostate cancer risk: findings from case-control studies in Canada. Nutrition and Cancer 1999;34(2):173-184.
- McCullough ML, Robertson AS, Chao A, et al. A prospective study of whole grains, fruits, vegetables and colon cancer risk. Cancer Causes & Control 2003;14(10):959-970.
- Flood A, Velie EM, Chaterjee N, et al. Fruit and vegetable intakes and the risk of colorectal cancer in the Breast Cancer Detection Demonstration Project follow-up cohort. The American Journal of Clinical Nutrition 2002;75(5):936-943.
- Michels KB, Edward Giovannucci, Joshipura KJ, et al. Prospective study of fruit and vegetable consumption and incidence of colon and rectal cancers. Journal of the National Cancer Institute 2000;92(21):1740-1752.
- 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.
- Neuhouser ML, Patterson RE, Thornquist MD, et al. Fruits and vegetables are associated with lower lung cancer risk only in the placebo arm of the beta-carotene and retinol efficacy trial (CARET). Cancer Epidemiology, Biomarkers & Prevention 2003;12(4):350-358.
- Voorrips LE, Goldbohm RA, Verhoeven DT, et al. Vegetable and fruit consumption and lung cancer risk in the Netherlands Cohort Study on diet and cancer. Cancer Causes and Control 2000;11(2):101-115.
- Chow WH, Schuman LM, McLaughlin JK, et al. A cohort study of tobacco use, diet, occupation, and lung cancer mortality. Cancer Causes and Control 1992;3(3):247-254.
- Feskanich D, Ziegler RG, Michaud DS, et al. Prospective study of fruit and vegetable consumption and risk of lung cancer among men and women. Journal of the National Cancer Institute 2000;92(22):1812-1823.
- Terry P, Wolk A, Persson I, Magnusson C. Brassica vegetables and breast cancer risk. JAMA 2001;285(23):2975-2977.
- Smith-Warner SA, Spiegelman D, Yaun SS, et al. Intake of fruits and vegetables and risk of breast cancer: a pooled analysis of cohort studies. JAMA 2001;285(6):769-776.
- Zhang S, Hunter DJ, Forman MR, et al. Dietary carotenoids and vitamins A, C, and E and risk of breast cancer. Journal of the National Cancer Institute 1999;91(6):547-556.
- Bell MC, Crowley-Nowick P, Bradlow HL, et al. Placebo-controlled trial of indole-3-carbinol in the treatment of CIN. Gynecologic Oncology 2000;78(2):123-129.
- Epplein M, Wilkens LR, Tiirikainen M, et al. Urinary isothiocyanates; glutathione S-transferase M1, T1, and P1 polymorphisms; and risk of colorectal cancer: the Multiethnic Cohort Study. Cancer Epidemiology, Biomarkers & Prevention 2009;18(1):314-320.
- London SJ, Yuan JM, Chung FL, et al. Isothiocyanates, glutathione S-transferase M1 and T1 polymorphisms, and lung-cancer risk: a prospective study of men in Shanghai, China. Lancet 2000;356(9231):724-729.
- Yang G, Gao YT, Shu XO, et al. Isothiocyanate exposure, glutathione S-transferase polymorphisms, and colorectal cancer risk. American Journal of Clinical Nutrition 2010;91(3):704-711.