By Amy Norton

NEW YORK (Reuters Health) - Atkins-style diets may help people shed pounds, but once the weight battle is won, diets low in saturated fat are the healthy choice, a new study suggests.

The study, published in the Journal of the American Dietetic Association, looked at three popular diets: Atkins, South Beach and the Ornish plan.

The Atkins diet slashes carbohydrates while allowing foods high in saturated fat, like butter and red meat, while South Beach emphasizes moderate amounts of unsaturated fat, like olive oil, and "good" carbohydrates like vegetables and beans. The Ornish plan is a vegetarian diet that is very low in fat overall and intended to prevent and treat heart disease.

Researchers had 26 healthy, non-obese adults follow each of the diets for one month apiece. The goal was not to have them lose weight, but to study the biological effects of each eating plan -- namely, the effects on cholesterol, blood vessel function and inflammation.

Each participant's diet was calculated to provide enough calories for weight maintenance.

After one month, the study found, the Atkins diet had caused participants' "bad" LDL cholesterol to tick upward, on average. In contrast, the South Beach and Ornish plans led to a nearly 12 percent and 17 percent reduction, respectively.

In addition, the Ornish diet seemed to have the best impact on blood vessel function. Blood vessel function also correlated with the amount of saturated fat participants had been eating -- improving as saturated fat intake declined.

In real life, an overweight person who successfully sheds pounds, on any diet, would likely see improvements in LDL and blood vessel function.

Lead researcher Dr. Michael Miller, of the University of Maryland Medical Center in Baltimore, told Reuters Health: "We were more interested in evaluating extreme diets after the acute phase -- reasoning that these dietary regimens often become habitual."

"The bottom line," Miller said, "is that once weight loss has been attained, a diet low in saturated fat represents an excellent prescription for a healthy heart."

He and his colleagues say that similar studies should be conducted in people who have risk factors for heart disease, to see how various popular diets may affect their cardiovascular health over the long term.

SOURCE: Journal of the American Dietetic Association, April 2009.

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