Research links broccoli, cauliflower, and other cruciferous vegetables to protection from common cancers and from age-accelerating free radicals
by Craig Weatherby
More than most vegetables, a particular group of plants called “cruciferous vegetables” appears to reduce the risk of cancer.
This family includes broccoli, cauliflower, cabbage, bok choy, kale, kohlrabi, Brussels sprouts, horseradish, mustard, capers, cress, rutabaga, arugula, and turnips.
Its botanical name—Cruciferae—derives from the cross-like marking on the flower-buds of some family members.
Lab research on cruciferous (crew-sif-er-us) vegetables shows that they contain a group of sulfurous constituents—called glucosinolates (gloo-kah-suh-nates)—which, when consume, break down into byproducts that may help curb the growth of cancers in the breast, endometrium, lung, colon, liver, colon, and cervix.
And, in some epidemiological (diet-disease) studies, diets high in cruciferous vegetables are associated with reduced risk of lung and colorectal cancer.
While generally positive, the evidence from epidemiological studies is mixed, and much remains to be learned about the effects of various cruciferous vegetables with regard to specific cancers.
Today, let’s take a look at a several recently published studies on broccoli and its cruciferous companions, which shed more light on these vegetables’ potential to protect human health.
Study # 1 – Broccoli compound boosts antioxidant and immune response
The strength of our internal defenses against oxidative stress (free radicals) helps to determine the rate and manner in which people age.
In 1992, researchers from John Hopkins University School of Medicine discovered that broccoli contains a glucosinolate called SGS, which the body converts to a compound called sulforaphane.
The results of a mouse study by researchers at UCLA indicate that sulforaphane activates a set of antioxidant genes and enzymes that combat the damaging, disease-promoting effects of free radicals (Kim HJ et al. 2008).
The UCLA researchers administered sulforaphane to aged mice for five days before administering an infectious agent, and continued giving it to the mice for 11 more days.
The mice given sulforaphane enjoyed substantial reversals of the age-related declines in their immune function.
Similar results were also obtained when individual immune cells were taken from old mice, exposed to sulforaphane and then injected back into the animals.
The UCLA scientists knew that sulforaphane stimulates the “phase-2” enzyme antioxidant defenses in mice and humans, but their experiment revealed that sulforaphane also induces a free radical defense called the Nrf2 pathway.
The Nrf2 pathway acts as a master switch that controls much of the body's overall antioxidant response to free radicals, including hundreds of antioxidant and rejuvenating genes and enzymes.
As the authors, wrote, “This finding could be of major significance in preventing or reversing the effects of immune senescence in elderly human subjects. Dietary antioxidants have been shown to have important effects on immune function… to this list we can now add broccoli and other cruciferous vegetables that are deserving of a human trial” (Kim HJ et al. 2008).
Study #2 – Broccoli sprouts linked to bladder cancer protection
Bladder cancer is diagnosed in about 336,000 people every year worldwide, and it is three times more likely to affect men than women.
Reporting the results of a study in mice, researchers from the Roswell Park Cancer Institute in New York said that a concentrated extract from broccoli sprouts cut the animals’ risk of developing bladder cancer by more than 50 percent (Munday R et al. 2008).
Female rats were assigned to one of five groups, three of which were given a chemical, (BBN) that induces bladder cancer.
Two of the groups given the carcinogen were also given the broccoli extract in their food (low or high dose), starting two weeks before the carcinogen was delivered.
At the end of the study, about 96 percent of the animals given BBN but no broccoli extract developed an average of almost two tumors each of varying sizes.
By comparison, 22 percent fewer of the animals given the low dose broccoli extract developed tumors, and the number of tumors per animal (1.39) was less as well.
Better yet, the animals given the high-dose broccoli extract developed about 58 percent fewer tumors, and the average number of tumors per animal was only 0.46.
The bladder is particularly responsive to sulforaphane, the main isothiocyanate the body makes from broccoli. The new experiment showed that the urine of animals fed broccoli sprouts delivered sulforaphane selectively to their bladder tissues.
Even though the high-dose extract contained about 600 times more isothiocyanate precursor than mature broccoli, the study’s authors believe that people at increased risk for bladder cancer probably do not need to eat huge amounts of broccoli sprouts in order to gain some protection.
Here’s how they expressed their findings and interpretation (Munday R et al. 2008):
- “These findings show for the first time that broccoli sprout extract strongly inhibits carcinogenesis in the bladder. The results are consistent with epidemiologic studies showing that increased consumption of broccoli and other cruciferous vegetables is associated with reduced bladder cancer risk.”
- “Because epidemiologic studies have shown that dietary isothiocyanates and cruciferous vegetable intake are inversely associated with bladder cancer risk in humans, it is possible that isothiocyanate doses much lower than those given to the rats in the present study may be adequate for bladder cancer prevention.”
It remains unclear how the broccoli compound protected against the formation of tumors in the animals’ bladders.
Study #3 – Broccoli and cauliflower may slash prostate cancer risk
More than 500,000 new cases of prostate cancer are diagnosed every year worldwide, and the cancer is the direct cause of over 200,000 deaths.
And the incidence of the disease has risen by 1.7 percent since 1983.
The results of a study in which researchers followed 1,338 men for an average of 4.2 years suggest that eating more than one serving of broccoli and/or cauliflower a week may reduce the risk of prostate cancer by up to 45 percent (Kirsh VA et al. 2007).
The researchers were affiliated with the National Cancer Institute, Yale University, Fred Hutchinson Cancer Research Center in Seattle, the University of Washington, and the Josephine Ford Cancer Center in Detroit.
After analyzing the men’s reported diets in comparison with diagnoses of prostate cancer after the four-year study, the researchers did not find a link between higher vegetable consumption and reduced prostate cancer risk, although men who ate higher amounts enjoyed a significant reduction in the risk of advanced stage III and IV tumors.
But the picture was much more positive when it came to intake of broccoli, cauliflower, and other cruciferous vegetables.
These were the results when they compared men who ate cruciferous vegetables less than once a month to those who ate at least one serving per week:
- Weekly enjoyment of any cruciferous vegetables was associated with a 40 percent reduction in prostate cancer risk.
- Weekly enjoyment of broccoli was associated with a 45 percent reduction in prostate cancer risk.
- Weekly enjoyment of cauliflower was associated with a 52 percent reduction in prostate cancer risk.
Spinach also showed some protective potential, but the effects were not statistically significant.
The reliability of the study was limited by the fact that people who report higher intake of fruit and vegetables tend to lead healthier lifestyles.
How cruciferous compounds curb carcinogens
Laboratory studies indicate that glucosinolates—and related compounds produced when we digest them—enhance the efficiency of immune-system agents called “phase II detoxification enzymes”, which defend against cancer: particularly the enzymes called glutathione S-transferases or GSTs.
These beneficial byproducts of consuming vegetables high in glucosinolates include indole-3-carbinol (I3C) and isothiocyanates (eye-so-thigh-oh-suh-nates).
Isothiocyanates facilitate elimination of various bodily, dietary, and environmental chemicals associated with cancer promotion, and they seem to do it by stimulating or facilitating production of detoxifying GSTs.
(The main isothiocyanate that our bodies produce from the glucosinolates specific to broccoli is called sulforaphane.)
However, variations in people’s GST-related genes—called “functional polymorphisms”—affect the ability of dietary glucosinolates from cruciferous vegetables to boost GST levels and thereby reduce cancer risk.
And—like oranges, apples, and grapefruit—cruciferous vegetables are rich in a promising compound called calcium-d-glucarate.
Oral supplements of calcium-D-glucarate appear to inhibit an enzyme called beta-glucuronidase, elevated activity of which is associated with an increased risk for various hormone-dependent cancers such as breast, prostate, and colon cancers (Hanausek M et al. 2003).
Cruciferous compounds' cancer-curbing effects on sex hormones
When we digest glucosinolate-rich foods, certain resulting metabolic byproducts – specifically, indole-3-carbinol – alter the metabolism or activity of sex hormones in ways that could inhibit the development of hormone-sensitive cancers.
In addition, indole-3-carbinol can reduce activation of carcinogens by certain bodily enzymes (e.g., P450).
But evidence that diets high in cruciferous vegetables reduce the risk of breast or prostate cancer is limited and inconsistent.
And animal tests show that under some circumstances—namely, when they are fed indole-3-carbinol immediately after administration of certain carcinogenic chemicals—this glucosinolate byproduct actually promotes cancer development.
This is why, even though the National Cancer Institute recommends eating five to nine servings of fruits and vegetables daily, they’ve not seen fit to set specific intake recommendations for cruciferous vegetables.
This also explains why most researchers advise against taking concentrated amounts of glucosinolates in supplement form.
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