A red meat nutrient sold as a supplement for weight loss and muscle growth may damage the heart and arteries, new research suggests.
Capsules of L-carnitine are widely available in health food stores and online.
They are advertised as a fat-burning slimming aid and powerful muscle builder - and are also said to help people with heart conditions.
But new research indicates a link between L-carnitine and heart disease. It may be a key reason why eating too much red meat can damage the heart, separate from the effects of saturated fat or cholesterol, say experts.
The studies show that L-carnitine is broken down by certain gut bacteria to produce a potentially harmful compound, trimethylamine N-oxide (TMAO).
Scientists found that high levels of L-carnitine in the blood were associated with heart disease, but only in individuals with raised TMAO.
Omnivorous individuals were found to produce more TMAO than vegetarians and vegans after consuming L-carnitine.
This suggests that, as well as containing L-carnitine, red meat favours the growth of gut bacteria that use the nutrient as an energy source, said the researchers.
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A study of 2,595 patients undergoing heart check-ups showed "significant dose-dependent associations" between L-carnitine levels and the risk of coronary artery disease.
Links were also seen between L-carnitine and major events such as heart attacks, strokes and death.
In mice, L-carnitine supplements markedly increased TMAO levels and artery damage, but not if their gut bacteria was suppressed.
The scientists, led by Dr Stanley Hazen from the Cleveland Clinic in Ohio, US, wrote in the journal Nature Medicine: "Discovery of a link between L-carnitine ingestion, gut microbiota metabolism and CVD (cardiovascular disease) risk has broad health-related implications.
"Our studies reveal a new pathway potentially linking dietary red meat ingestion with atherosclerosis (narrowing of the arteries)."
They added: "Our studies have public health relevance as L-carnitine is a common over-the-counter dietary supplement. Our results suggest that the safety of chronic L-carnitine supplementation should be examined, as high amounts of orally ingested L-carnitine may under some conditions foster growth of gut microbiota with an enhanced capacity to produce TMAO and potentially advance atherosclerosis."
Victoria Taylor, senior dietitian at the British Heart Foundation, said: "This is certainly an interesting discovery and sheds some light on why red meat might have an impact on heart health.
"While the findings won't necessarily mean a change to existing recommendations, these scientists have served up a good reminder for us to think about alternative sources of protein if we regularly eat a lot of red or processed meats.
"The odd meat-free day isn't such a bad thing and eating less meat automatically leaves room in your diet for other foods high in protein like fish, pulses, nuts and eggs, all of which should be part of a nutritious and varied diet.
"Unless told otherwise by a doctor or qualified health professional, we should be able to get all the nutrients we need from a healthy, balanced diet without additional supplements."
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Nutrition expert Professor Brian Ratcliffe, from Robert Gordon University in Scotland, said: "Dietary intakes of saturated fatty acids do not explain all the variation in blood cholesterol levels and these in turn do not explain all the variation in the occurrence of cardiovascular diseases (CVD).
"These diseases are complex and multi-factorial and this study provides another piece in the jigsaw puzzle showing the links between atherosclerosis and diet and lifestyle. The study is comprehensive and demonstrates a mechanism that may help to explain the observed associations between the consumption of red meat and the risk of CVD.
"This does not mean that we need to change current dietary recommendations because the advice is to limit the intake of red meat anyway. However, people who take supplements of L-carnitine for non-medical reasons may need to have some second thoughts."