April 1 (HealthDay News) -- In the "maintenance" phase that occurs after initial weight loss, the popular Ornish and South Beach diets seem to be easier on the heart than the high-fat, low-carbohydrate Atkins regimen, a new study finds.
Unlike numerous studies that have evaluated diets to see which might be better at achieving weight loss, this study, funded by the U.S. National Institutes of Health, looked at what happens to cholesterol levels and other cardiac risk factors when dieters reach their goal weight and remain on the diet.
The study involved 18 healthy people, with an average body-mass index (BMI) of 22.6 (18.5 to 24.9 is considered normal weight). Participants completed four weeks each on the Atkins (50 percent fat), South Beach (30 percent fat) and Ornish (10 percent fat) diets, in random order and with a four-week "washout" period between each diet. The study was done from January to December 2006.
The switch between diets meant that "each person served as his own control," explained principal investigator Dr. Michael Miller, director of the Center for Preventive Cardiology at the University of Maryland Medical Center in Baltimore.
At the start and after each four-week diet, the researchers evaluated cholesterol levels and other cardiac risk factors. They also looked at three-day food records at the end of each diet phase. And they checked blood vessel functioning by measuring blood vessel dilation in the arm.
They found that "as you increase the amount of saturated fat [in the diet], blood vessel dilation is reduced," Miller said. Healthy vessel dilation is important to proper blood flow.
"The diet that performed the worst [on the blood vessel test] was the Atkins diet," Miller said. "It contains more saturated fat."
Participants ate about 30 grams of saturated fat a day while on the Atkins diet, compared to about 14 grams on South Beach and about 3 grams while on Ornish.
"We like to say saturated fat should be below 7 percent of total calories," Miller said. "So, if your caloric intake is 2,000, saturated fat should be about 14 grams [or less] daily."
The researchers also measured cholesterol levels, including total cholesterol, "good" cholesterol (HDL) and "bad" cholesterol (LDL). "On Atkins, cholesterol levels tended to go up," Miller said. "LDL increased by 8 percent [which was not enough to be statistically significant]."
On the South Beach diet, LDL decreased by about 12 percent, and on Ornish it declined by about 17 percent, the study showed.
The findings are published in the April issue of the Journal of the American Dietetic Association. An early look at the study was first discussed at the American Heart Association's 2007 annual meeting.
Connie Diekman, a registered dietitian and director of university nutrition at Washington University in St. Louis, said that although the study was small, the findings are notable.
Representatives from Atkins Nutritionals took exception with the study, however. In a statement, Colette Heimowitz, vice president of nutrition and education at Atkins Nutritionals, noted that, "given the short duration of his study, the very small sample size and the weak correlations, drawing conclusions about possible long-term health risks tied to fat consumption in the maintenance phase of any weight control program is not good science." She also questioned whether the participants were actually following a true Atkins diet.
But Diekman saw the results as reason to worry.
"The outcome does show an indication for concern about the impact of a diet like the Atkins diet that relies on a large amount of saturated fat and the effect on heart health," she said.
"More studies are needed in both healthy and overweight individuals, but the early indication of this study is that high-saturated-fat diets are connected to heart disease risk, a fact that has been long known but not always seen when diets high in saturated fat are used for weight loss," Diekman noted.
SOURCES: Michael Miller, M.D., associate professor, medicine, University of Maryland School of Medicine, Baltimore, and director, Center for Preventive Cardiology, University of Maryland Medical Center, Baltimore; Connie Diekman, R.D., L.D., director, university nutrition, Washington University, St. Louis; Colette Heimowitz, vice president, nutrition and education, Atkins Nutritionals, Denver; April 2009, Journal of the American Dietetic Association