Researchers have more evidence about how eating red meat can raise heart disease risk.

Two studies published Monday show that people who eat red meat — but not vegetarians or people who eat only white meat such as chicken — produce more of a chemical called TMAO, which has been shown to raise heart disease risk. And they stop making so much of the compound a month after they stop eating red meat.

They also showed that a diet heavy in red meat can change kidney function, a surprising finding that raises more questions about red meat’s effects on the body.

“These studies really show what a large impact a diet that is rich in red meat can have on your metabolism,” said Dr. Stanley Hazen of the Cleveland Clinic, who has been studying the effects of TMAO on heart disease.

“One of our most surprising findings was that a diet rich in red meat actually changed kidney function. We saw that the kidneys were being regulated by a chronic diet. This is something that, as far as I am aware, hasn’t been shown before,” Hazen told NBC News.

TMAO, short for trimethylamine N-oxide, is made by gut bacteria as they digest food. Red meat, especially, causes these gut germs to make a lot of a precursor, which the human body metabolizes into TMAO. Certain species of bacteria cause production of TMAO when they metabolize choline, lecithin and carnitine — all of which are abundant in red meat, full-fat dairy products and eggs.

Hazen’s team has shown that people who have more TMAO in their blood also have a higher risk of heart disease. And people with higher TMAO levels also have a higher risk of dying earlier. Vegetarians and vegans have lower levels of TMAO — unless they take certain supplements — and usually also have a lower risk of heart disease than meat-eaters.

One study piggybacked on a federally funded study of diet and showed people’s TMAO levels went up significantly when they were fed a controlled diet rich in red meat. Some people had a 10-fold rise in TMAO levels after a month of eating red meat, Hazen’s team reported in the European Heart Journal.

That didn’t happen in people fed poultry and fish, or vegetarian diets. And other, non-meat sources of saturated fat didn’t affect TMAO.

“Interestingly, discontinuation of dietary red meat reduced plasma TMAO within four weeks,” the team wrote.

They did not test intake of dairy products or eggs in this study, but other work has shown that eating eggs and dairy can raise TMAO levels, also.

In a second study, Hazen’s team showed gut bacteria convert an amino acid called carnitine into TMAO. When they gave carnitine supplements to meat-eaters, their gut bacteria quickly started making TMAO. At first the bodies of vegetarians and vegans did not produce much TMAO even when they took supplements, but after a few weeks they did, Hazen’s team reported in the Journal of Clinical Investigation.

“Omnivores generate significantly more TMAO than vegans/vegetarians from oral carnitine,” they wrote.

Other studies have shown that TMAO may make blood more likely to clot, which would help explain why it raises the risk of heart attacks and stroke. But Hazen said it’s clear that red meat, perhaps because of TMAO, has other bad effects on health.

“Red meat raises risk not only of heart disease but of colorectal cancer,” he said.

Hazen is working on a drug that would lower TMAO levels. He hopes his work might lead to a pill that could lower the risk of heart disease.

“These findings reinforce current dietary recommendations that encourage all ages to follow a heart-healthy eating plan that limits red meat,” said Charlotte Pratt of the National Heart, Lung and Blood Institute, which helped fund the work. “This means eating a variety of foods, including more vegetables, fruits, whole grains, low-fat dairy foods, and plant-based protein sources such as beans and peas."

Hazen himself has changed his diet since he started studying TMAO. “I have to admit I used to be a really big red meat eater,” he said. Now he doesn’t eat much at all.

He also counsels his patients to watch not only the food they eat, but the supplements they take. “Few studies show effects of taking supplements long-term,” he said. Choline, carnitine and lecithin supplements have the potential to affect heart risk and kidney function, he said. People should try to get the nutrients they need in their diets.

“I tell them to try and get it from real foods, from fruits, from vegetables and from real foods,” Hazen said.