Cholesterol levels in adults are falling, and changes in the amount of trans fats in the American diet may be part of the reason, new research suggests.

The findings, published Tuesday in The Journal of the American Medical Association, were celebrated as something of a triumph by health authorities, who said the data showed that the nation had reached its 2010 goal of getting the average total cholesterol level in adults below 200 milligrams per deciliter. Researchers examined a nationally representative sample of tens of thousands of Americans over the last two decades and recorded a decline of 10 points in average total cholesterol - to 196 mg/dL from 206 mg/dL.

While the so-called bad cholesterol decreased, there was a slight uptick in HDL cholesterol, higher levels of which are associated with a reduced risk of heart disease. Triglycerides, which are also linked to heart disease, initially rose 5 points to 123 mg/dL from 1994 to 2002, then dropped to 110 mg/dL by the end of 2010.

The study's authors said they were buoyed by their observations, but could not provide a solid explanation for them.

The popularity of cholesterol-lowering drugs like statins was only part of the explanation, they said. Their use more than quadrupled among adults during the study period, to 15.5 percent from 3.4 percent. As many as 35 percent of men and women over 50 took them, the study found.

But the same improvements in cholesterol profiles were also seen in adults who were not taking them, said Margaret D. Carroll, the study's lead author and a statistician with the National Health and Nutrition Examination Survey at the Centers for Disease Control and Prevention.

"It was somewhat of a surprise to us to see these favorable trends in people who were not on lipid-lowering medications," she said.

The study was observational, which means it could not prove what caused the decline in cholesterol levels. Dr. Carroll and her colleagues said, however, that exercise, obesity and saturated fat intake were unlikely to have been significant factors. Average physical activity levels have not increased, more than a third of the adult population is obese, and the intake of saturated fat as a percentage of calories in the American diet is the same as it was over a decade ago.

Instead they suggested widespread public health campaigns to rid trans fats from foods might have had an impact.

Once widely found in fried and packaged foods, trans fats, which can raise LDL and lower HDL cholesterol, have been slowly stripped from the food supply as cities have banned their use in restaurants and pressed companies to remove them from cookies, soups, crackers and other processed foods. One C.D.C. study this year found that since 2000, levels of trans fats in Americans' bloodstreams had fallen nearly 60 percent.

Two other trends in the last decade may have also been factors, the researchers said: declines in smoking and a drop in carbohydrate consumption.

Dr. David J. Frid, a cardiologist at the Cleveland Clinic, said the findings were unexpected given the high rates of obesity and Type II diabetes. He pointed to research showing a recent 30 percent drop in deaths from heart disease nationwide, and said the cholesterol data might be related.

The notion of trans fat consumption contributing to the fall in cholesterol numbers, while intriguing, has not been proven, said Dr. James A. Underberg of the NYU Center for the Prevention of Cardiovascular Disease.

Dr. Underberg said that standard measures of cholesterol could be deceiving. People who are obese or have Type II diabetes can have seemingly normal levels of LDL, but also have high amounts of the small LDL particles that drive into artery walls and cause heart disease.