BOSTON — Hardened vegetable oil, a main ingredient of margarine and shortening, raises cholesterol levels and may be even worse for people’s health than saturated fat, a study concludes.

The study raises health questions about trans fatty acids, the kind of fat that makes margarine and shortening hard so they can be used for baking, frying and spreading and won’t turn rancid.

About a quarter of the fat in a typical stick of margarine is trans fatty acid. This fat can occur naturally, but most is made when food companies add hydrogen to polyunsaturated and monounsaturated vegetable oils. Food labels list these as hydrogenated oils.

Because these fats make up only 2 percent to 4 percent of a typical American diet, they probably have a relatively small impact on most people’s cholesterol levels. However, one top cholesterol expert recommended that food companies work to remove them from their products.

Saturated fat is considered to be the primary villain in the war on cholesterol. This fat, found in meat, dairy products, tropical oils and some other foods, raises cholesterol levels in the bloodstream.

Polyunsaturated and monounsaturated fats, on the other hand, were thought to have no effect on cholesterol levels or even, in some cases, to lower them. But the new research suggests that hardening these fats makes them act more like saturated fat and perhaps even worse.

The study found that unlike saturated fat, trans fatty acids have a double-barreled harmful effect: They increase bad cholesterol while lowering good cholesterol.

The research, conducted in the Netherlands, showed that when people eat food with this kind of fat, they increase their bloodstream’s supply of low-density lipoprotein, or LDL, which contributes to fatty buildups in the arteries. But they lower levels of high-density lipoprotein, or HDL, which protects the body from cholesterol’s harmful effects.

Dr. Scott M. Grundy, a cholesterol expert at the University of Texas Southwestern Medical Center at Dallas, called this “a combination of detrimental effects that is particularly worrisome.”

However, Grundy cautioned that people should not switch to butter, which is still clearly more harmful.

The International Food Information Council, an industry group, raised technical questions about the study and said in a statement: “People should refrain from making any dietary changes based on the results of this one study in which subjects consumed extremely high levels of trans fatty acids for a relatively short time.”

Dr. Edward A. Emken of the U.S. Department of Agriculture research service in Peoria, Ill., said the study should be repeated to check the effects of lower doses of the fat. But he said the research “has implications in terms of the U.S. diet, and it certainly shouldn’t be taken lightly.”

The study was conducted by Drs. Ronald P. Mensink and Martijn B. Katan of the Agricultural University in Wageningen. It was published in Thursday’s New England Journal of Medicine.

The Dutch researchers estimated that if Americans stopped eating all trans fatty acid and instead used monounsaturated fats, such as olive oil, they would raise their HDL levels two points and lower their LDL levels four points.

Dr. Carl Orringer of the University of Michigan said this overall effect is so small that “I have a hard time getting very excited about this.”

The study was conducted on 59 men and women who were randomly assigned to eat three different diets for three weeks each. In each diet, 40 percent of the total calories came from fat. But they varied in their levels of monounsaturated fats, trans fatty acids and saturated fats.