Science News - Oct. 20, 2007
By Janet Raloff
Women take note. Researchers find that a chemical that forms in
overcooked meat, especially charred portions, is a potent mimic of
estrogen, the primary female sex hormone. That's anything but
appetizing, since studies have linked a higher lifetime cumulative
exposure to estrogen in women with an elevated risk of breast cancer.
Indeed, the new finding offers a "biologically plausible" explanation
for why diets rich in red meats might elevate breast-cancer risk,
notes Nigel J. Gooderham of Imperial College London.
At the very high temperatures reached during frying and charbroiling,
natural constituents of meats can undergo chemical reactions that
generate carcinogens known as heterocyclic amines (see Carcinogens in
the Diet http://www.sciencenews.org/articles/20050219/food.asp)
Because these compounds all have very long, unwieldy
chemical monikers, most scientists refer to them by their
abbreviations, such as IQ, MeIQ, MeIQx, and PhIP.
Of the nearly two dozen different heterocyclic amines that can form,
PhIP dominates. It sometimes accumulates in amounts 10 to 50 times
higher than that of any other member of this toxic chemical family,
Gooderham says. Moreover, he adds, although heterocyclic amines
normally cause liver tumors in exposed animals, PhIP is different: "It
causes breast cancer in female rats, prostate cancer in male rats, and
colon cancer in both." These are the same cancers that in people are
associated with eating a lot of cooked meats.
However, the means by which such foods might induce cancer has
remained somewhat elusive. So, building on his team's earlier work,
Gooderham decided to probe what the heterocyclic amine did in rat
pituitary cells. These cells make prolactin -- another female sex
hormone -- but only when triggered by the presence of estrogen.
Prolactin, like estrogen, fuels the growth of many breast cancers.
In their new test-tube study, Gooderham and coauthor Saundra N. Lauber
show that upon exposure to PhIP, pituitary cells not only make
progesterone, but also secrete it. If these cells do the same thing
when they're part of the body, those secretions would circulate to
other organs -- including the breast.
But "what was startling," Gooderham told Science News Online, is that
it took just trace quantities of the heterocyclic amine to spur
prolactin production. "PhIP was incredibly potent," he says, able to
trigger progesterone production at concentrations comparable to what
might be found circulating in the blood of people who had eaten a
couple of well-done burgers.
The toxicologist cautions that there's a big gap between observing an
effect in isolated cells growing in a test-tube and showing that the
same holds true in people.
However, even if PhIP does operate similarly in people, he says that's
no reason to give up grilled meat. Certain cooking techniques, such as
flipping hamburgers frequently, can limit the formation of
heterocyclic amines. Moreover, earlier work by the Imperial College
team showed that dining on certain members of the mustard family
appear to detoxify much of the PhIP that might have inadvertently been
consumed as part of a meal.
The human link
Three recent epidemiological studies support concerns about the
consumption of grilled meats.
In the first, Harvard Medical School researchers compared the diets of
more than 90,000 premenopausal U.S. nurses. Over a 12-year period,
1,021 of the relatively young women developed invasive breast cancers.
The more red meat a woman ate, the higher was her risk of developing
invasive breast cancer, Eunyoung Cho and her colleagues reported in
the Archives of Internal Medicine last November. The increased risk
was restricted, however, only to those types of breast cancers that
are fueled by estrogen or progesterone.
Overall, women who ate the most red meat -- typically 1.5 servings or
more per day -- faced nearly double the invasive breast-cancer risk of
those eating little red meat each week.
Related findings emerged in the April 10 British Journal of Cancer.
There, researchers at the University of Leeds reported data from a
long-running study of more than 35,000 women in the United Kingdom who
ranged in age from roughly 35 to 70. Regardless of the volunteers'
age, Janet E. Cade's team found, those who consumed the most meat had
the highest risk of breast cancer.
Shortly thereafter, Susan E. Steck of the University of South
Carolina's school of public health and her colleagues linked meat
consumption yet again with increased cancer risk, but only in the
older segment of the women they investigated. By comparing the diets
of 1,500 women with breast cancer to those of 1,550 cancerfree women,
the scientists showed that postmenopausal women consuming the most
grilled, barbecued, and smoked meats faced the highest breast-cancer
These data support accumulating evidence that a penchant for well-done
meats can hike a woman's breast-cancer risk, Steck and her colleagues
concluded in the May Epidemiology.
Such findings have been percolating out of the epidemiology community
for years. Nearly a decade ago, for instance, National Cancer
Institute scientists reported finding that women who consistently ate
their meat very well done -- with a crispy, blackened crust -- faced a
substantially elevated breast-cancer risk when compared to those who
routinely ate rare- or medium-cooked meats.
However, even well-done meats without char can contain heterocyclic
amines, chemical analyses by others later showed. The compounds'
presence appears to correlate best with how meat is cooked, not merely
with how brown its interior ended up (SN: 11/28/98, p. 341).
At high temperatures, the simple sugar glucose, together with
creatinine -- a muscle-breakdown product, and additional free amino
acids, can all interact within beef, chicken, and other meats to form
heterocyclic amines. In contrast, low-temperature cooking or a quick
searing may generate none of the carcinogens.
Because there's no way to tell visually, by taste, or by smell whether
PhIP and its toxic kin lace cooked meat, food chemists have been
lobbying commercial and home chefs to reduce the heat they use to cook
meats -- or to turn meats frequently to keep the surfaces closest to
the heat source from getting too hot.
The significance of this was driven home to Gooderham several years
ago when just such tactics spoiled an experiment he was launching to
test whether Brussels sprouts and broccoli could help detoxify PhIP.
"I bought 30 kilograms of prime Aberdeen angus lean beef," he recalls.
"Then we ground it up and I gave it to a professional cook to turn
into burgers and cook." Professional cooks tend to move meats around
quite a bit, he found. The result: His expensive, chef-prepared meat
contained almost no PhIP.
In the end, he says, "I sacked the cook, bought another 30 kilos of
meat and prepared the burgers myself. It was a costly lesson."
Once restarted, however, that study yielded encouraging data.
One way the body detoxifies and sheds toxic chemicals is to link them
to what amounts to a sugar molecule. Consumption of certain members of
the mustard (Brassica) family, such as broccoli and Brussels sprouts
(both members of the B. oleracea species) -- can encourage this
process. So Gooderham's team fed 250 grams (roughly half a pound) each
of broccoli and Brussels sprouts each day to 20 men for almost 2
weeks. On the 12th day, the men each got a cooked-meat meal containing
4.9 micrograms of PhIP.
Compared to similar trial periods when their diets had been Brassica-
free, the volunteers excreted up to 40 percent more PhIP in urine, the
researchers reported in Carcinogenesis.
Experimental data suggest that two brews may also help detoxify
heterocyclic amines. In test-tube studies, white tea largely prevented
DNA damage from the heterocyclic amine IQ (SN: 4/15/00, p. 251), and
in mice, extracts of beer tackled MeIQx and Trp-P-2 (see Beer's Well
Done Benefit http://www.sciencenews.org/articles/20050305/food.asp)
The best strategy of all, most toxicologists say, is to prevent
formation of heterocyclic amines in the first place. In addition to
frequently turning meat on the grill or fry pan, partially cooking
meats in a microwave prior to grilling will limit the toxic chemicals'
formation. So will mixing in a little potato starch to ground beef
before grilling (see How Carbs Can Make Burgers Safer http://www.sciencenews.org/articles/20041204/food.asp)
marinating meats with a heavily sugared oil-and-vinegar sauce (SN: 4/24/99, p.
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Janet E. Cade
UK Women's Cohort Study Centre for Epidemiology and Biostatistics
30/32 Hyde Terrace
The University of Leeds
Leeds LS2 9LN
Department of Medicine
Harvard Medical School
181 Longwood Avenue
Boston, MA 02115
Nigel J. Gooderham
Imperial College London
Sir Alexander Fleming Building
London SW7 2AZ
Susan Elizabeth Steck
Department of Epidemiology and Biostatistics
Statewide Cancer Prevention and Control Program
Arnold School of Public Health
University of South Carolina
2221 Devine Street, Room 231
Columbia, SC 29208
Copyright 2007 Science Service
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