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 reason 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.
A study of 2595 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."