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Dr. Dean Ornish, an advocate of very low fat diets, said the findings send a misleading message that a dozen previous studies contradict.
The study measured risk factors for heart disease -- not heart disease itself, said Ornish, a faculty member at the University of California at San Francisco.
He noted that in his own research, a vegetarian diet limited to 10 percent of calories from fat reversed the clogging of arteries in patients with heart disease, as evidenced on heart scans.
Knopp's study involved 444 men with elevated "bad" cholesterol levels in their blood. Some men also had elevated triglycerides.
The men were divided into groups that ate varying levels of fat: level 1, 30 percent of calories from fat; level 2, 26 percent; level 3, 22 percent; level 4, 18 percent.
"The surprising thing was that total fat restriction below the level 2 diet had no further benefit and had an adverse effect on triglycerides and HDL," Knopp said.
Before the one-year study, the men were eating a more typical American diet -- about 35 percent of total calories from fat and up to 13 percent of total calories from saturated fats, believed to be the most harmful kind.
Men assigned to level 4 who were supposed to eat only 18 percent of calories from fat but actually averaged 22 percent HDL is called "good" cholesterol because higher levels of it are associated with less risk of heart disease. With LDL, the reverse is true -- higher levels are linked to a higher risk of heart disease.
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