Intake of trans fat in the diets of Americans has declined in the last three decades but still remains above what is recommended by the American Heart Association, a study has found.

Consumption dropped by around a third between 1980 and 2009, the study authors say.

"Other studies have previously shown a decline in trans fat intake over time, but our study is the first to look at such a long period of time," says lead author Mary Ann Honors of the University of Minnesota's School of Public.

The study is based on data collected in six surveys of more than 12,000 adults aged 25 to 74 in the Minneapolis-St. Paul region, conducted over a 30-year period as part of the Minnesota Heart Survey

"Trans fat intake declined by over one third, which was great to see," Honors says.

During the period there were declines in consumption of total fat, trans fat and saturated fat, the researchers reported in the Journal of the American Heart Association, although they noted all were still above the levels recommended by experts.

Total fat consumption dropped from an average 39 percent of daily caloric intake to 33 percent.

Consumption of saturated fats also declined, but were still accounting for 11.4 percent of daily calorie intake for both men and women, above the 5 to 6 percent recommended by the American Hearth Association.

 "There's a downward trend in trans and saturated fat intake levels, but it's clear that we still have room for improvement," Honors, an epidemiology researcher, says.

Saturated fats and trans fats, which can elevate levels of bad cholesterol and lower levels of good cholesterol, increase the risks of heart disease.

Products such as meat and full-fat dairy items can contain saturated fats, as can some tropical oils such as palm or coconut oil.

Trans fats are mostly contained in foods that are processed, commercially baked or fried, such cookies, crackers, pies, pastries or pizza.

While many food companies have taken trans fats out of their product lines, one in 10 packaged foods still contain it, researchers at the New York Department of Health and Mental Hygiene have reported.

"A lot of people think it's out of the food supply, but it's still in a lot of places," says Christine Johnson Curtis, an assistant commissioner at the department.

Honors says consumers can take steps to reduce the risks fat in their diet represents.

"To make your diet more in line with the recommendations, use the nutritional panel on food labels to choose foods with little or no trans fats," she says.