The Cancer Research
The link between cruciferous vegetables and cancer prevention is relatively well-studied.
Lab studies showed several ways that sulforaphane, formed from glucosinolates in broccoli, could thwart the development and progression of prostate cancer. Limited human intervention trials support this potential, however, population studies that followed men for 9 to 22 years showed no link with total – or any form – of prostate cancer so far.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how cruciferous vegetables and their nutrients affect the risk of developing cancer.
- Evidence categorized as “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A probable judgment is strong enough to justify recommendations.
- Genetic differences mean that cruciferous vegetables’ isothiocyanate compounds remain in the body longer—and provide more benefits —in some people than in others.
- There is probable evidence that foods containing dietary fiber DECREASE the risk of:
- colorectal cancer
- There is probable evidence that a diet high in non-starchy vegetables and fruits DECREASES the risk of:
- mouth, pharynx, nasopharynx, larynx, esophagus, lung, stomach and colorectal (aerodigestive) cancers
- Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce the risk of cancer.
- Limited evidence suggests that non-starchy vegetables may DECREASE the risk of:
- estrogen receptor-negative (ER-) breast cancer and bladder cancer.
- Limited evidence suggests that foods containing carotenoids may DECREASE the risk of:
- Lung and estrogen receptor-negative (ER-) breast cancers
- Limited evidence suggests that foods containing beta-carotene may DECREASE the risk of:
- Lung cancer
- Limited evidence suggests that foods containing vitamin C may DECREASE the risk of:
• Lung cancer (in people who smoke) and colon cancer
Ongoing Areas of Investigation
- Carotenoids act as antioxidants themselves and stimulate the body’s own antioxidant defenses, decreasing free radical damage to DNA that can lead to cancer. Very high levels in cell studies, however, can have an opposite effect, promoting damage from oxidation.
- Beta-carotene and lutein promote cell-to-cell communication that helps control cell growth. These carotenoids also increase carcinogen-metabolizing enzymes and stimulate self-destruction of abnormal cells. The body uses beta-carotene to form vitamin A, which helps protect against cancer through the immune system and expression of genes that regulate cell growth.
- Lutein and zeaxanthin are antioxidants that are especially concentrated in the eyes, brain and skin. Limited animal studies suggest that they may help protect against skin cancer related to sun exposure.
- Vitamin C is a powerful antioxidant. In lab studies, it protects cells’ DNA by trapping free radicals, and it helps renew the antioxidant ability of Vitamin E. In cell studies, vitamin C also inhibits the formation of carcinogens and supports the immune system.
- Glucosinolates in vegetables are broken down into isothiocyanates (such as sulforaphane) and In laboratory studies, these compounds decrease inflammation that could cause cell damage leading to cancer. They also inhibit enzymes that activate carcinogens and stimulate enzymes that de-activate carcinogens. These compounds turn on tumor suppressor genes, slow abnormal cell growth and stimulate self-destruction of abnormal cells.
Indole compounds decrease development of breast cancer in cell and animal studies. In some studies, indoles shift the active form of estrogen into a weaker form. (High amounts of estrogen are a risk factor for certain hormone-linked cancers).
- Folate helps maintain healthy DNA and keeps cancer-promoting genes “turned off”. Animal studies, however, suggest that exceptionally high amounts or intervention after cancer cells have formed might promote development of cancer.
- Flavonols like kaempferol and quercetin influence gene expression and cell signaling in ways that increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes in cell and animal studies. They inhibit cancer cells’ growth and ability to spread, and activate signaling that leads to self-destruction of abnormal cells.
They dial down the expression of oncogenes (genes that have the potential to cause increased cell growth that can lead to cancer) and increase expression of tumor suppressor genes.
People who eat more vegetables and fruits have lower risk of a wide range of cancers. This probably reflects combined protection from many different nutrients and compounds they contain.
Analysis from two large cohorts combined that was published after the AICR/WCRF Third Expert Report showed a lower risk of breast cancer with more than 5.5 servings/day total vegetable and fruit intake versus 2.5 or fewer servings (RR 0.89, CI 0.83-0.96).
Carotenoids: In population studies, higher blood levels of total carotenoids and beta-carotene are linked with a lower risk of overall cancer. Blood levels may more accurately reflect the consumption of carotenoid-rich foods than diet questionnaires, and they include differences in how much is absorbed from food. However, it may be that the lower cancer risk is seen because blood levels of these compounds are recognized as signals of greater overall vegetable and fruit consumption.
- Lung cancer: Population studies link higher dietary and blood levels of beta-carotene or total carotenoids with a lower risk of lung cancer. Larger studies now show protection less clearly than earlier studies, and the AICR/WCRF Third Expert Report categorizes this link as Limited Suggestive. Additional research is needed. Despite these findings, beta-carotene in high-dose supplements increases lung cancer risk when taken by people who smoke or used to smoke tobacco, according to evidence rated convincing in the AICR/WCRF Third Expert Report.
- Breast cancer: Some population studies also link higher blood levels of carotenoids (including beta-carotene) to lower risk of breast cancer, mainly for estrogen receptor-negative (ER-) forms. The AICR/WCRF Third Expert Report categorized this link as Limited Suggestive. It is possible that an effect of carotenoids on ER-positive (ER+) tumors is simply masked by the hormonal influences that dominate risk of ER+ breast cancer.
Vitamin C: Population studies comparing people with higher and lower levels of vitamin C in their diets, and especially levels circulating in their blood, link higher amounts with lower overall risk of cancer. This effect is larger comparing people with very low levels to moderately increased levels than comparing people with moderate and much higher levels.
Higher levels of vitamin C from foods are linked with lower risk of lung cancer among people who smoke tobacco, although not in those who used to smoke or who have never smoked. People with more vitamin C in their diet are also less likely to develop colon cancer. That’s even after adjusting for other risk factors for colon cancer, such as alcohol, red meat and tobacco. Evidence for both lung and colon cancer is rated as Limited Suggestive in the AICR/WCRF Third Expert Report, and more research is needed.
Cruciferous vegetables: An analysis that combined several population studies showed an association of cruciferous vegetables with reduced overall cancer risk, although there are inconsistencies between studies. Part of the inconsistent findings may stem from genetic differences that allow isothiocyanate compounds to remain in the body longer in some people than in others, as well as how differences in microbes that make up the gut microbiota can lead to differences compounds people absorb. Human studies are also complicated to interpret because people who eat the same amount of cruciferous vegetables may get different amounts of protective nutrients and compounds depending on how the vegetables are prepared.
- Prostate cancer: An analysis of 13 observational population studies reported higher cruciferous vegetable consumption associated with lower prostate cancer incidence. However, when looking only at studies less likely to be affected by bias (prospective cohort studies), no association was seen. In analysis for the AICR/WCRF Continuous Update Project, no significant association was seen for total prostate cancer or for advanced forms specifically.
- Breast cancer: Analysis from two large cohorts combined that was published after the WCRF/AICR report publication found cruciferous vegetables associated with lower risk of breast cancer, which is consistent with the lower risk of post-menopausal breast cancer (but not pre-menopausal cancer) with high cruciferous vegetables in an earlier analysis that included some prospective cohort studies. More research is needed.
Folate: Research on folate and cancer is challenging to interpret, since effects may differ based on time in the cancer process, amount consumed and individual genetic differences. Levels of folate from food and in the blood that are too low are linked with greater risk of several forms of cancer in some population studies. However, excess folic acid from randomized controlled trials with supplements has increased risk of some cancers. (Folic acid is the form of folate in supplements and fortified foods.) More research is needed to understand the amounts that are high enough to pose risk.
Tips for Selection, Storage and Preparation
- Choose compact, firm heads heavy for their size with no soft spots and no off-odors.
- Green leaves should be fresh and firm with no yellowing.
- Turnips are sweetest when small.
- Wait to wash until just before use.
- Cook just until tender-crisp, with greens still bright. Overcooking leads to strong odors and a less attractive color.
- Add to soup, or make them the star on their own.
- Broccoli and other cruciferous veggies can also be delicious served raw. Try cauliflower or broccoli dipped in hummus or a spicy peanut butter sauce. If the flavor of Brussels sprouts, for example, is too strong to enjoy raw, steam or blanch briefly, cool quickly in ice water and serve cold.
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