A nutrient found red meat may increase the risk of heart disease when it interacts with the bacteria in the human gut, according to a paper published this week in Nature Medicine.
The results point to the nutrient L-carnitine, rather than saturated fat and cholesterol, as the link between eating red meat and heart disease.
In a series of studies in humans and mice, the US researchers found that when L-carnitine is consumed, the bacteria in the human gut produces a compound called Trimethylamine-N-oxide, or TMAO.
TMAO slows the body’s ability to remove cholesterol from the walls of the arteries. The subsequent build up of cholesterol can clog arteries, leading to heart disease.
L-carnititine is also used in some dietary supplements.
In one of the studies, the researchers tested the blood of 77 participants, 26 of whom were vegetarian or vegan. They found that meat eaters who consumed L-carnitine produced higher levels of TMAO, suggesting a regular diet of meat encourages the growth of bacteria which turns L-carnitine into TMAO.
However, it is still unclear which species of gut bacteria contributed to TMAO formation.
In another study, the researchers reviewed the blood tests of 2,600 people having elective heart checks. They found that people with high levels of both L-carnitine and TMAO were prime targets for heart disease and stroke.
In yet another study, this time in animal models, the researchers found that mice who were fed L-carnitine were twice as likely to have have risky levels of arterial plaque.
But this only occurred when they had their usual gut bacteria: mice who were fed antibiotics to kill this bacteria experienced no such effect.
The researchers conclude that intestinal bacteria of meat eaters feed on L-carnitine to produce TMAO, which can clog arteries.
The findings raise the possibility that other dietary nutrients may also generate TMAO from gut microbiota and slow the removal of arterial plaque, the researchers say.
Professor John Funder, Executive Chairman of Obesity Australia and Senior Fellow at Prince Henry’s Institute said the findings were relevant to Australian meat eaters.
“Kangaroo meat - long considered very healthy, given its very low fat content - has more L-carnitine per gram than any other red meat; on the basis of the authors’ findings, it may not be such a healthy option after all,” Professor Funder said.
But he said more work needed to be done to confirm a causative link between L-carnitine and heart disease.
“As a scientist, one unanswered question is that of the particular microbes in the gut of meat eaters that are absent from that of vegans: do they require something in red meat for their ability to colonise the gut, and what are they like in species that are primarily carnivorous rather than omnivorous?”
Professor Gary Jennings, cardiologist and Director of the Baker IDI Heart and Diabetes Institute, said the study was impressive in the way it started with a novel hypothesis and used the latest scientific approaches to analyse the data.
“To date, any association with coronary heart disease was thought to be due to the saturated fat and cholesterol content of red meat. This possibility remains but the study adds another intriguing possibility to any link between red meat and coronary heart disease,” he said.
But the human component of the study was small, Professor Jennings said, and the findings should not prompt people to stop eating meat.
“The evidence is not sufficiently compelling to cause concern amongst the red meat industry,” he said, adding that eating a balanced diet remained the best recommendation for Australians.
“Red meat can be an important source of protein, iron and other nutrients in the diet,” he said.
With the help of the Australian Science Media Centre.