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A nutrient found in red meat and added to energy drinks and supplements may crank up people’s risk of heart disease, a new study suggests. Bacteria in the gut digest the nutrient, L-carnitine, and help turn it into an artery-hardening chemical — particularly in meat eaters, researchers report April 7 in Nature Medicine.
The intestinal microbes of vegetarians and vegans didn’t make much of the chemical, even when researchers fed them an 8-ounce sirloin steak.
“I always thought that what I ate mattered, but I never realized that my gut bacteria might matter more,” says biochemist Harry Ischiropoulos of the University of Pennsylvania in Philadelphia, who was not involved with the study.
What’s more, high blood levels of the bacterial by-product of L-carnitine, called trimethylamine N-oxide or TMAO, were an “astoundingly good” warning sign of impending heart attack, stroke and death, says study coauthor Stanley Hazen of Cleveland Clinic. A test for TMAO, which will become commercially available this year, could give physicians a new tool for gauging heart disease risk.
Scientists have long known that eating red meat jacks up a person’s chances of developing heart disease, but reliable biomarkers — blood-borne indicators of disease or health — have been hard to find. One way physicians gauge risk is with blood tests for cholesterol, a greasy molecule in meat and other foods, which gums up arteries. But tests for cholesterol and other molecules don’t wholly explain meat’s link to heart disease, Hazen says. “Cholesterol, saturated fat and salt only account for a tiny little piece of the risk.”
Gut bacteria might account for a bit more. Hazen’s team first linked intestinal microbes to heart disease in 2011, when they spotted TMAO in blood collected from people who later suffered heart attacks, had strokes or died (SN Online: 4/7/11).
For the new study, Hazen zeroed in on L-carnitine because the nutrient is structurally similar to a compound that gut microbes can convert to TMAO.
Volunteers — a mix of omnivores, vegetarians and vegans — ate steak and L-carnitine capsules, and then researchers measured TMAO levels in the blood. Only meat eaters could make TMAO from L-carnitine, Hazen’s team found, and they needed their gut bacteria to do it. TMAO production shut down when researchers wiped out volunteers’ intestinal microbes with antibiotics.
L-carnitine passed right through the guts of long-term vegans and vegetarians, leaving their blood practically TMAO-free. When researchers examined volunteers’ stool, they found different groups of bacteria in people who did and didn’t eat meat.
Hazen’s group also found that blood levels of TMAO and L-carnitine could predict heart disease risk, which they learned by collecting blood samples from 2,595 patients and tracking their health for three years.
Carnitine is a substance that helps the body turn fat into energy. Your body makes it in the liver and kidneys and stores it in the skeletal muscles, heart, brain, and sperm.
Usually, your body can make all the carnitine it needs. Some people, however, may not have enough carnitine because their bodies cannot make enough or can’t transport it into tissues so it can be used. Some other conditions, such as angina or intermittent claudication, can also cause low levels of carnitine in the body, as can some medications
Carnitine has been proposed as a treatment for many conditions because it acts as an antioxidant. Antioxidants fight harmful particles in the body known as free radicals, which damage cells and tamper with DNA. Antioxidants can neutralize free radicals and may reduce or help prevent some of the damage they cause.
Some of the conditions carnitine may help treat are serious. Serious diseases and conditions require conventional medical treatment, and you should talk to your health care provider before taking carnitine. For other conditions, such as fatigue or improving athletic performance, carnitine seems safe but may not help much.
Angina -- Some good evidence shows that carnitine can be used along with conventional treatment for stable angina. Several clinical trials show that L-carnitine and propionyl-L-carnitine can help reduce symptoms of angina and improve the ability of people with angina to exercise without chest pain. Do not self-treat chest pain with carnitine, however. See your health care provider for diagnosis and conventional treatment, and take carnitine only under your health care provider's supervision.
Heart attack -- A few studies have found that carnitine may help when used with conventional medicines after a heart attack, but not all studies agree. Some small studies suggest that people who take L-carnitine supplements soon after a heart attack may be less likely to have another heart attack, die of heart disease, have chest pain and abnormal heart rhythms, or develop heart failure. However, other studies have shown no benefit. Treatment with oral carnitine may also improve muscle weakness. Carnitine should be used along with conventional medication under your health care provider supervision.
Heart failure -- A few small studies have suggested that carnitine (usually propionyl-L-carnitine) can help reduce symptoms of heart failure and improve exercise capacity in people with heart failure. However, more and larger studies are needed to know for sure.