Broccoli's Got the Right Stuff
Recent research in Finley's lab is demonstrating that high-selenium broccoli
may be the best source of an anticancer agent. Other researchers discovered
that garlic stores selenium in a form that appears to be most active against
cancer. And broccoli and brussels sprouts also store selenium in this form,
known as selenium methyl selenocysteine, or SeMSC. The body simply snips the
end off this amino acid to produce the anticancer agentmethyl selenol.
Though garlic is higher in SeMSC, most Americans are not likely to eat
enough of it to produce the desired effect, Finley notes. So his group has
focused on testing selenium-enriched broccoli as a way to get effective levels
of SeMSC into the body.
Along the way, however, he learned how animals and people metabolize other
food forms of the mineral. "It's a long and tortuous path for the form of
selenium prevalent in grains and some meatsselenomethionineto get
converted to methyl selenol. It's easier for selenium saltsthe forms used
in some supplementsto get there. And it's only one step for the form in
broccoli," Finley says.
In a series of rat studies, Finley, Davis, and former colleague Yi Feng, now
with the University of Louisville's medical school, confirmed that differences
in selenium metabolism translated to differences in the risk of colon cancer.
First, they demonstrated that selenium saltsboth selenate and
selenitecan prevent the first of several steps that can lead to cancer,
whereas the grain formselenomethioninewas ineffective.
Selenium salts reduced the number of adducts in the rats' colons by 53 to 70
percent. Adducts are formed when a carcinogen binds to DNA, explains Davis.
"If the damage isn't repaired, it can lead to tumor formation."
The researchers had beefed up the rats' selenium levels through their diets
for several weeks. Then they injected the animals with a potent carcinogen
called DMABP, for short. Their findings support those of others showing that
selenite protects against adduct formation in rats' mammary cells.
The group got similar results when they looked for a later stage of colon
tumor formation called aberrant crypts. These are immature colon cells that
have gone awry. "Not all aberrant crypts develop into cancer," says
Davis, "but all colon cancers begin as aberrant crypts." Feng
painstakingly counted the crypts and found more in the animals fed
selenomethionine than in those getting selenium salts.
Beefed-Up Broccoli Works Best
The most exciting phase of this work started in a Grand Forks greenhouse.
The researchers grew ordinary broccoli in soilless media with added selenium to
observe uptake of the metal. Finley says that studies show that broccoli grown
in the presence of selenium can accumulate substantial amounts. Some commercial
broccoli grown in California has up to 50 times more selenium than normal, he
notes, because the irrigation water is naturally high in the mineral. When
Finley analyzed his broccoli, however, he found it had 100 to 200 times more
selenium than the California heads.
When the researchers pitted the high-selenium broccoli against the salt form
selenate in rat studies, they made sure to control for any beneficial effects
of broccoli itself. The vegetable is high in antioxidants and contains other
substances shown to be active against cancer. So animals in each test group got
ordinary broccoli as well as the treatment.
Treatments consisted of daily doses of either 0.1 or 1.0 mg of selenium per
kilogram of the rats' body weight, either in the form of enriched broccoli or
selenate. The higher dose is representative of the selenium level that reduced
cancer risk in a human trial, Finley says.
After giving the animals DMABP, Feng again looked for precancerous aberrant
crypts and for collections of these cells, known as aberrant crypt foci.
High-selenium broccoli always resulted in fewer precancerous lesions than
selenate did, says Finleyabout one-third fewer at the 1.0 mg/kg dose. And
the number of lesions decreased as the dose increased.
The results were so promising that Finley and Davis decided to repeat the
experiment. And they confirmed the findings using a different
saltselenite instead of selenateand a single but higher dose of
selenium2.0 mg/kg. They also gave the animals a much more potent
carcinogendimethyl hydrazine (DMH). Although it produced many more
lesions, the rats fed high-selenium broccoli had half as many aberrant crypts
as the animals getting selenite.
"If there's a call to increase selenium intake, we currently have few
choices other than high-selenium yeast or selenium salts," says Finley.
"Selenium-enriched broccoli is a potential source of the mineral in a
highly effective form." His group is looking for other potential benefits
of the enriched vegetable.By Judy
McBride, Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National Program (#107)
described on the World Wide Web at
John W. Finley and
Cindy D. Davis are at the
USDA-ARS Grand Forks Human Nutrition
Research Center, P.O. Box 9034, University Station, Grand Forks, ND
58202-9034; phone (701) 795-8353, fax (701) 795-8395.