Researchers looked at how harder, processed fats, such as stick margarine, butter and lard, affect cholesterol levels when compared with softer products, like tub margarine and oil.
The softer products were found to be healthier because the harder ones have more of what are called trans fatty acids, which raise cholesterol levels and the risk of heart disease.
Researchers found semiliquid margarine does a better job of lowering cholesterol levels than soft or stick margarine, and stick margarine is worse than butter for people trying to control their cholesterol levels.
"Does it matter which fat you use? The bottom line is, yes, it matters a lot," said Dr. Alice H. Lichtenstein, who led the study at the U.S. Department of Agriculture Nutrition Center at Tufts University in Boston.
Researchers checked the participants' levels of two opposing cholesterol forces in the bloodstream: LDL, or bad, cholesterol, which can clog arteries, and HDL, or good cholesterol, which protects against heart disease.
Compared with people who ate butter, people who consumed soybean oil reduced their levels of LDL cholesterol by an average of 12 percent, while those given semiliquid margarine lowered their levels 11 percent.
Among those who ate soft margarine, the level dropped 9 percent, and for shortening, the level fell 7 percent. Those who ate stick margarine reduced their LDL cholesterol only 5 percent.
Conversely, those who ate the softer fats had smaller reductions in HDL, or "good" cholesterol.
"The problem is that we've been pushing margarine as better than butter, and this is not the case," says CBS This Morning Health Contributor Dr. Bernadine Healy.
Although margarine has less saturated fat than butter, the problem is in the trans fatty acids, Dr. Healy explains. An artificial fat, trans fatty acids are used to make items like margarine less soft, and prevent it from going bad - a process called hydrogenation. Groups like the American Heart Association have long advocated using margarine instead of butter.
"But the message now is, all margarines are not created equal. The harder the margarine is, the more you should stay away from it," Dr. Healy says.
But the National Association of Margarine Manufacturers criticized the research, saying that the full-fat stick margarine used in the study is not representative of the products now found in most supermarkets.
Manufacturers have come up with products significantly lower in total fat, saturated fat and trans fatty acids, the association noted.
In another journal article, Dr. Walter C. Willett of the Harvard School of Public Health and colleagues said the Food and Drug Administration should require that trans fatty acids be lbeled on products.
Willett also said the FDA should require fast-food restaurants to label menu items because fried fast foods and commercially baked goods often contain trans fatty acids.
"Trans fat is the largest amount of artificial chemical in our food supply," Dr. Walter Willett of the Harvard School of Public Health said. "I think a lot of parents are concerned about their kids' health, and they should have the information that their kids are eating a type of fat that can be extremely dangerous."
NAMM President Richard Cristol said the industry would not oppose mandatory labeling of trans fatty acids.