In contrast, they say low-fat regimens such as the South Beach and Ornish diets lower cholesterol and appear to benefit artery function.
"It really is the Atkins diet that is the worst," said Dr. Michael Miller, director of preventive cardiology at the University of Maryland Medical Centre in Baltimore.
"The Atkins diet caused the LDL [bad cholesterol that clogs blood vessels] levels to go up about 7%, whereas in the Ornish and South Beach diets ... they went down 7% to 10%."
Various researchers have tested the benefits of the popular diets and reached wildly differing conclusions.
Miller designed what he said was a unique approach -- to see how people fared once they stopped losing weight on any of the diets.
Studies show that people usually lose weight rapidly on any diet if they follow it properly and the weight loss itself can cause cholesterol to plummet.
"When you lose weight, everything looks good, but after a while, you plateau and hit a maintenance stage," said Miller, who presented his findings to a meeting of the American Heart Association in Orlando, Florida.
His team studied 18 people, each of whom completed a full month on each of the three diets. They were carefully monitored to ensure that they did not lose weight.
The Atkins diet was set to deliver 50% of calories as fat; the South Beach was 30% fat, and the Ornish diet, designed by nutritionist Dr Dean Ornish, was 10% fat.
While on each diet, the volunteers were tested for levels of blood fats, including cholesterol and markers for inflammation.
The researchers used ultrasound scans to measure the flexibility and dilation of blood vessels and measured proteins in the blood that can indicate inflammation.
"Some markers of inflammation were increased by as much as 30% to 40% during the Atkins phase, whereas during the South Beach and Ornish phases the markers either were stable or went down, some by as much as 15% to 20%," Miller said.
Most studies have shown that diets that stress vegetables, low-fat sources of protein such as beans and legumes, and whole grains provide the best long-term weight loss. Many low-fat diets allow processed carbohydrates such as white flour, which have also been shown to be unhealthy, experts agree.
Said Miller, "Why not start with a diet that will be healthier for you in the long run after weight loss?"
From the November 19, 2007, Prepared Foods e-Flash