Low-carb slimming diets may clog arteries and increase the risk of heart attacks and strokes, a study suggests.
Diets based on eating lots of meat, fish and cheese, while restricting carbohydrates have grown in popularity in recent years.
But the Beth Israel Deaconess Medical Center in the US found such habits caused artery damage in tests on mice.
The researchers and independent experts both agreed a balanced diet was the best option.
Low-carb diets have attracted a lot of attention and controversy after a surge in interest in them in the 1990s.
The researchers at the Beth Israel institute, which is part of Harvard Medical School, decided to investigate their impact on the cardiovascular system after hearing of reports of people on the diets suffering heart attacks.
They fed the mice three different diets - a standard mouse type, a western diet which was high in fat, and a low-carb, high-protein version, the journal Proceedings of the National Academy of Sciences reported.
The low-carb diet did not affect cholesterol levels, but there was a significant difference on the impact on atherosclerosis - the build-up of fatty plaque deposits in the arteries that can lead to heart attacks or strokes.
After 12 weeks, the mice eating the low-carb diet had gained less weight, but developed 15% more atherosclerosis than those on the standard mice food. For the western diet group there was 9% more atherosclerosis.
The team could not be certain why the effect was seen, but thought low-carb diets may affect the way bone marrow cells effectively clean arteries of fatty deposits.
Lead researcher Anthony Rosenzweig said the findings were so concerning to him that he decided to come off the low-carb diet he was following.
He added: "Our research suggests that, at least in animals, these diets could be having adverse cardiovascular effects.
"It appears that a moderate and balanced diet, coupled with regular exercise, is probably best for most people."
Joanne Murphy, from the Stroke Association, agreed following a balanced diet was the best advice.
"We know that foods such as red meat and dairy products, which are high in protein, also contain high levels of saturated fat. These fats then cause the build up in the arteries."
But she added the research was still at an early stage and she wanted to see more work done on the subject.
Ellen Mason, from the British Heart Foundation, said it was difficult to apply the findings to humans.
But she added: "Low-carb, high-protein diets are not considered as healthy as eating a balanced diet, which is good for health because we get the different nutrients our body needs by eating from the different food groups every day."
Professor Alan Maryon-Davis, president, UK Faculty of Public Health, said: "This research helps to back up the basic message that our diet should contain more starchy carbohydrate, not less.
"For long-term health at least one-third of what we eat should be bread, rice, potatoes, pasta or other starchy food."