The cholesterol levels of U.S. adults have been dropping since the late 1980s, a new study suggests -- and not just because of the increased popularity of lipid-lowering drugs.

Researchers said that cholesterol, which is closely tied to heart disease risk, may be looking better because of improvements in diet, including the substitution of vegetable oils for less-healthy trans fats.

They found average total cholesterol dropped from 206 milligrams per deciliter in 1988-1994 to 196 in 2007-2010, with a similar decrease in "bad" LDL cholesterol.

"It's important and significant, the reduction that we see here, but it's not unbelievable," said Dr. Goodarz Danaei, from the Harvard School of Public Health in Boston. "I don't think we needed a huge change in diet... to produce this change."

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