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Broccoli Sprouts May Protect Against Bladder Cancer 2-29-08
BUFFALO, N.Y., Feb. 28 -- Even before broccoli matures, the much-maligned
vegetable shows promise in prevention of bladder cancer.
Rats given high doses of freeze-dried broccoli sprout extract were less than
half as likely to develop bladder cancer when exposed to a potent carcinogen as
rats not fed the extract, reported Yuesheng Zhang, M.D., Ph.D., of the Roswell
Park Cancer Institute here, and colleagues in the March 1 issue of the journal
Cancer Research.
Broccoli sprouts are a particularly good source of a class of phytochemicals
called isothiocyanates, which have been linked to reduced risk of bladder cancer
in observational studies.
The sprouts contain about 30 times more isothiocyanates than mature broccoli.
The broccoli sprout extract fed to the rats contained about 600 times more.
The daily intake of total isothiocyanate in humans through consumption of
cruciferous vegetables is estimated to be 0.14 to 1.43 µmol/kg. Although it's
probably not possible to eat enough broccoli sprouts to replicate the doses used
in the study (40 to 160 µmol/kg), Dr. Zhang acknowledged, he said it likely
wouldn't be necessary.
"The carcinogen exposure we as humans have is going to be much lower than those
animals had," he said. "I think if we just simply extrapolate these data to
humans, the dose of broccoli sprout extract required to have a similar impact on
carcinogenesis is going to be much, much lower."
Dr. Zhang's group previously showed that broccoli sprout extract induced
production of enzymes in the bladder that protect against carcinogens and
oxidants.
To see whether this would result in lower rates of bladder cancer, the
researchers randomly assigned 24 rats to each of five groups.
The control group got regular chow and water. Another group got regular chow but
received N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in their water as a
bladder-specific carcinogen representative of nitrosamines, an important class
of human carcinogens.
Another two groups got the carcinogen in their water starting two weeks after
initiation of the broccoli sprout extract in their diet at a dose of either 40
or 160 µmol isothiocyanate per kg body weight per day.
The final group received broccoli sprout extract without the carcinogen.
After 12 weeks of treatment, all rats were switched to regular diets and water
and were sacrificed 24 weeks later.
None of the rats in the control group or the group that received only the
extract developed bladder cancer.
All the rats in the group that received only the carcinogen developed dysplasia
(4.2%) or carcinoma (95.8%) of the bladder with an average of 1.96 tumors each.
Broccoli sprouts reduced the likelihood of bladder cancer in a dose-dependent
manner.
In the lower dose group, 73.9% of the rats developed cancer (a 22.9% reduction)
with an average of 1.39 tumors each a 29.1%).
In the higher dose group, only 37.5% developed cancer (60.9% inhibition) with an
average of 0.46 tumors per animal (76.5% inhibition).
Tumor size and progression were similarly lower in extract-treated rats.
Another group of rats sacrificed at various times after being given the broccoli
sprout extract showed that isothiocyanate equivalents were rapidly absorbed with
a peak at one hour after dosing and excreted in the urine over the next 12
hours.
The researchers said the most remarkable finding was that urinary concentrations
were two to three orders of magnitude higher in the urine than in the plasma.
Likewise, levels in the bladder were 2.7 to 3.6 times higher in the bladder than
in the liver in the first 12 hours after the extract was given and 19.7 times
higher at 24 hours.
Isothiocyanate levels in the bladder epithelium, which is directly exposed to
urine, were likely substantially higher than those measured in the bladder
tissue overall, Dr. Zhang and colleagues noted, suggesting the isothiocyanate
selectively reached the target tissue.
"Given that nearly all bladder cancers occur in the epithelium," the researchers
concluded, "isothiocyanate-enriched broccoli sprout extract and other
cruciferous vegetables rich in isothiocyanates may be particularly useful for
prevention of bladder cancer."