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Two new scientific studies are taking a little more of the sizzle out of steak.

Researchers at the Cleveland Clinic say they’ve found that a diet rich in red meat as the primary protein source increases people’s circulating levels of a substance known as TMAO, trimethlamine N-oxide. Previous research has shown TMAO can lead to the development of cardiovascular disease, including heart attacks and strokes.

The findings “provide further evidence for how dietary interventions may be an effective treatment strategy to reduce TMAO levels and lower subsequent risk of heart disease,” Dr. Alan Hazen, chairman of the department of cellular and molecular medicine in the Cleveland Clinic’s Lerner Research Institute, who has been studying TMAO, said in a statement.

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One study was published in the European Heart Journal; the other was published in the Journal of Clinical Investigation.

Researchers believe that TMAO is produced when bacteria in the gut digest choline, lecithin, and carnitine, nutrients that are abundant in animal products such as red meat and liver and other animal products, the statement said.

In the European Heart Journal study, researchers fed study participants diets that used for protein either red meat or white meat or were mostly vegetarian. They found that the vast majority of those fed the red meat diet saw an elevation of TMAO in their blood and urine. TMAO in the blood increased about threefold for those with a red meat diet, compared with the other diets, and some patients saw a 10-fold increase. Similar increases in TMAO were found in the urine.

The research found that the red meat consumption also reduced the kidneys’ efficiency of expelling TMAO, the statement said.

In the Journal of Clinical Investigation, researchers found that carnitine is converted to TMAO in a two-step process facilitated by distinct gut bacteria, the statement said.

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An October article in the journal Nutrients reviewed the research in recent years on TMAO, saying “there is convincing evidence suggesting an association between TMAO and inflammation. Furthermore, in the last decade the studies suggesting an association between high plasma TMAO levels and risk for developing atherosclerosis have increased markedly. However, the exact mechanism underlying this correlation is still unknown.”

Dr. Robert Eckel, a professor of medicine at the University of Colorado and past president of the American Heart Association, said Hazen has been a leading researcher on TMAO and it’s a “particularly exciting area” of reearch that has emerged over the past five years.

He also said it was “evolving science” that “needs to be validated by other labs and proven correct.”

Should people eat red meat? Eckel noted that the Heart Association already recommends a diet that is rich in fruits, vegetables, whole grains, lean poultry, and lean meat.

“Should people avoid red meat entirely? No,” he said. “But eat a minimal amount, compared to white meat or fish? I would say yes.”

Heart disease is the leading cause of death for both men and women in the United States. Every year, about 630,000 people die of heart disease in this country, or about 1 in 4 deaths, according to the US Centers for Disease Control and Prevention.