Researchers trying to figure out how meat causes heart disease came up with another possible explanation Monday: an essential nutrient found in meat and eggs might be a culprit.
They found evidence that choline may feed gut bacteria that in turn produce a compound that makes blood sticky and prone to form clots that can cause heart attacks and strokes.
It's the latest in their series of studies looking at what specific compounds in food raise people's risk of heart disease, the no. 1 cause of death in the U.S. and much of the world.
Choline is considered an essential nutrient and getting too little can, ironically, lead to heart disease, cancer and other conditions.
It's found in abundant quantities in egg yolks, liver, red meat, peanuts and wheat germ.
Dr. Stanley Hazen of the Cleveland Clinic and colleagues did a small but intense study in 18 volunteers - eight of them vegetarians or vegans, and 10 people who eat meat, eggs and dairy.
They gave all 18 choline supplements - 500 mg daily. The recommended adequate intake of choline from all sources is 425 mg a day for women and 550 mg a day for men.
After a month, their blood levels of a compound called trimethylamine N-oxide (TMAO) rose 10-fold. In tests, their blood became much more likely to form clots, the team reported in the journal Circulation.
"TMAO supercharges platelet function," Hazen said.
Platelets are are cell-like structures in the blood that help form blood clots. TMAO makes them sticky, Hazen said.
"What is clear from this study is if you increase the choline in your diet, the TMAO level goes up and that changes your platelet function," Hazen told NBC News.
The vegans and vegetarians had much lower levels of choline to start with than the meat-eaters did, Hazen's team reported. Even after taking choline, their blood levels were much lower.
"Foods that raise TMAO may increase your risk for clotting and thrombotic events. Unless prescribed by your doctor, avoid supplements with choline," Hazen said in a statement.
"A Mediterranean or vegetarian diet is reported to help reduce TMAO."
The research team did not find that people who took the supplements had an actual higher risk of heart disease - the study did not last long enough or include enough people to show that.
But they've also found that other compounds found in animal products also raise levels of TMAO. "We previously showed gut microbial production of trimethylamine N-oxide (TMAO) from dietary nutrients like choline, lecithin, and L-carnitine is linked to the development of cardiovascular diseases," they wrote.
"The new study provides the first direct evidence in humans that consuming excess choline, an essential nutrient plentiful in a Western diet, raises both levels of the bacteria-produced compound, called trimethylamine N-oxide (TMAO), and the tendency of platelets to clump together and form clots," the American Heart Association, which publishes Circulation, said in a statement.
"Numerous studies have shown that higher blood levels of TMAO are associated with a greater risk of heart disease, including heart attacks and strokes in humans, and recent studies showed that feeding animals choline-supplemented diets also raised their risk of clotting."
Taking a daily aspirin reduced the clotting effect, they added. "An unanticipated finding was that low-dose aspirin partially reduced choline supplement-dependent rise in TMAO," they wrote.
"Although the mechanism for this result is unknown, aspirin has been reported to alter the composition of the gut microbial community."
Choline is important for human development and health. It's used to make membranes and affects brain development. It also affects how the body clears out "bad" cholesterol.
Related: Do I Have to Stop Eating Meat?
A large egg delivers about 125 mg of choline.
Scientists know that gut bacteria are vital for digesting and metabolizing food. People could not survive without the trillions of bacteria, viruses and fungi species that live in and on our bodies.
But the balance of those germs can affect health greatly. Gut bacteria can protect against certain infections and play a role in obesity. Studies indicate they may affect the risk of cancer, heart disease and even mental illnesses such as schizophrenia.
Other teams have narrowed down the species of bacteria that metabolize choline, l-carnitine and other compounds to make TMAO. Hazen's hope is to develop ways to alter these bacteria or their products to lower the risk of heart disease.
Right now, products may promise to do so but none are proven, said Hazen, who helped found a company that licensed a test for TMAO to the Cleveland Clinic.
"Almost all of the probiotics that are commercially sold now have virtually no studies to demonstrate that they actually do anything, even survive in your stomach," Hazen said.