London - A nutrient abundant in red meat and also sold as a dietary supplement could raise the risk of heart disease.
The UK government already recommends individuals eat no more than 2.5oz (70g) of red meat daily, with increased consumption already linked to bowel cancer and cardiovascular disease.
Saturated fats, and added salt present in processed meats, have until now been considered the key reasons why eating too much can be bad for health.
But new research has found a link between the red meat compound L-carnitine and heart disease.
It is widely available in health food stores and online, as a weight-loss supplement or muscle growth promoters. In some instances, it has even been promoted as beneficial for people with heart conditions.
Scientists discovered that L-carnitine is broken down by gut bacteria to produce trimethylamine N-oxide (TMAO), which previous research has linked to heart and artery damage.
But meat-eaters were found to produce significantly higher levels of TMAO than vegetarians after consuming L-carnitine. This suggests that a red meat diet favours the growth of gut bacteria that digests the nutrient.
In mice, L-carnitine supplements markedly increased TMAO levels and artery damage, but not if their gut bacteria was suppressed.
The study concluded that high amounts of L-carnitine in the blood did cause heart disease, but only in individuals with raised levels of TMAO.
“The bacteria living in our digestive tracts are dictated by our long-term dietary patterns,” said lead researcher Dr Stanley Hazen from the Cleveland Clinic in Ohio.
“A diet high in carnitine actually shifts our gut microbe composition to those that like carnitine, making meat eaters even more susceptible to forming TMAO and its artery-clogging effects.
“Meanwhile, vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets.”
But others questioned the research, as TMAO is also found in foods which lower the risk of heart disease, such as seafood and soya beans.
The study of 2 595 patients undergoing heart check-ups, published in the journal Nature Medicine, found a “significant dose-dependent associations” between levels of the nutrient and the risk of heart disease.
Dr Hazen said: “Carnitine metabolism suggests a new way to help explain why a diet rich in red meat promotes atherosclerosis (narrowing of the arteries).”
The report added there is a “public health relevance” as the nutrient is a “common over-the-counter dietary supplement”.
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).
“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.”
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.” - Daily Mail