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The finding explains why even lean meat poses a heart risk
Chris Woolston and
From Nature magazine.
Lean steak is low in fat and
cholesterol and high in protein -- qualities normally considered
healthy. But eating a lot of it can still cause
disease. Researchers have now laid the blame on bacteria in the human
gut that convert a common nutrient found in beef into a compound that may
speed up the build-up of plaques in the arteries.
The results are
published in Nature Medicine today. Co-author Stanley Hazen, head of
cardiovascular medicine at the Cleveland Clinic in Ohio, says that the study
could signal a new approach to diet and health. In some cases, an
individual's collection of intestinal microbes may be as important to their
diet as anything on a nutrition label, he says. "Bacteria make a whole slew
of molecules from food," he says, "and those molecules can have a huge
effect on our metabolic processes."
CoConsumption of red meat has been
found to increase the risk of death from heart disease, even when
controlling for levels of fat and cholesterol. To find out why, Hazen and
his colleagues gave the nutrient l-carnitine -- found in red meat and dairy
products -- to 77 volunteers, including 26 who were vegans or vegetarians.
One committed vegan even agreed to eat a 200-gram sirloin steak.
showed that consuming l-rnitine increased blood levels of trimethylamine-N-oxide
(TMAO), a compound that, evidence suggests, can alter the metabolism of
cholesterol and slow the removal of cholesterol that accumulates on
But even when they took l-carnitine supplements, vegans
and vegetarians made far less TMAO than meat eaters. Fecal studies showed
that meat eaters and non-meat eaters also had very different types of
bacteria in their guts. Hazen says that a regular diet of meat probably
encourages the growth of bacteria that can turn l-carnitine into TMAO.
ToTo further make the case, researchers checked the
levels of l-carnitine in the blood of nearly 2,600 people who were having
elective heart check-ups. By itself, the nutrient didn't seem to make a
difference. However, people who had high levels of both l-carnitine and TMAO
were prime targets for heart disease, further evidence that it's the
bacterial alchemy -- not the l-carnitine alone -- that poses the real
Finally, the researchers found that feeding l-rnitine to mice doubled the
risk of developing arterial plaques, but only when the mice had their usual
gut bacteria. When the animals were treated with gut-clearing antibiotics,
l-carnitine in the diet did not encourage plaques.
director of preventive cardiovascular medicine at the University of
Pennsylvania in Philadelphia, says that the study makes a "fairly
compelling" case that intestinal bacteria feeding onl-carnitine increase the risk of heart
The finding should give pause not only to meat lovers, but
also to people who take l-carnitine supplements, which are marketed with the
promise that they promote energy, weight loss and athletic performance, says
Hazen. "None of those claims have been proven," he says. "I see no reason
why anyone needs to take it."