The study, published in the journal Nature Medicine on April 7, showed that a substance in red meat called carnitine is broken down by certain bacteria in the stomach into a gas, which is converted in the liver to a chemical called TMAO. The scientists found that TMAO was strongly linked with the buildup of fatty deposits in blood vessels, which can lead to heart disease and death.
So, the saturated fat and preservatives like nitrates in red meat are only part of the problem.
"The cholesterol and saturated fat content of lean red meat is not that high -- there's something else contributing to increases in cardiovascular risk," lead researcher Dr Stanley Hazen of Ohio's Cleveland Clinic told BBC News. He also said that TMAO's significance has been overlooked: "It may be a waste product, but it is significantly influencing cholesterol metabolism and the net effect leads to an accumulation of cholesterol."
Dr. Hazen said the findings have lead him to personally cut back his red meat consumption from five to seven servings a week to one serving every couple of weeks.
He also suggests the possibility of using yogurt containing probiotics to reduce the number of bacteria that feed on carnitine in the gut, which would in theory reduce the health risks of red meat.
Vegetarians naturally have fewer bacteria that are able to break down carnitine than meat-eaters.
The study also has implications for people who take supplements such as L-carnitine, lecithin, choline or betaine in an attempt to ward off cognitive decline or improve fat metabolism. "I would strongly recommend that unless you're a vegetarian or vegan, there is a potential risk" to taking these supplements, dietician Catherine Collins told BBC News.