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.
"Unless prescribed by your doctor, avoid supplements with choline."
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.
“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.
"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."
“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.”
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.
Maggie Fox is a senior writer for NBC News and TODAY, covering health policy, science, medical treatments and disease.