Certain types of unsaturated vegetable fat, including a main ingredient of most stick margarines and shortening, appear to be at least as likely as saturated animal fat to increase the risk of heart disease, according to a report published today.

About one-fourth of the fat in stick margarine is of this type. It is used to make vegetable oil solid at room temperature.

The finding suggests that consumers worried about their blood cholesterol levels should switch to liquid vegetable oil rather than hard margarine for frying and use soft margarine rather than hard on their bread, the report's principal author, Ronald Mensink, said.

The findings do not mean that consumers may sensibly substitute butter for margarine, several experts said. Butter is probably worse for a person's cholesterol level because it has more saturated fat than margarine has of the special form of unsaturated fat. The fat in question is one of several usually labeled as hydrogenated vegetable oil.

Mensink warned, however, that more studies are needed before official recommendations are made.

But in an editorial accompanying the report in today's New England Journal of Medicine a leading nutrition researcher said the results call for a reevaluation of the way fats are labeled for consumers.

"It is no longer justifiable to identify 'saturated fatty acid' as the dietary culprit responsible for raising LDL cholesterol {the "bad" cholesterol} levels," wrote Scott M. Grundy of the University of Texas Southwestern Medical Center at Dallas. "Instead, the various culpable acids should be grouped together as cholesterol-raising fatty acids. This approach to labeling should greatly simplify the identification of the cholesterol-raising potential of various fats."

The study found that a diet high in a certain type of unsaturated fatty acid -- a fat with a chemical bond structure slightly different from that of other unsaturated fats and called trans fatty acid -- raised the ratio of "bad" to "good" cholesterol in the blood, an index that many consider to be a measure of heart disease risk.

It had previously been thought that all unsaturated fats were preferable to saturated fats, which are found in animal products, in coconut and palm oils and in some hydrogenated vegetable fats. Hydrogenation changes the chemical bonds in the fat molecule, making it stiffer and, therefore, solid at room temperature.

All unsaturated fats had been thought to lower LDL, the "bad" cholesterol, when substituted for saturated fats.

"It's obvious from this study that trans fatty acids are completely different from the others," said Mensink, a researcher at the Agricultural University in Wageningen in the Netherlands. "It cannot be labeled like an unsaturated fatty acid. The effects on cholesterol are more like that of a saturated fatty acid."

Many manufacturers label the fats in their products according to whether they are saturated or unsaturated. But Grundy said this distinction is not adequate, because some saturated fats do not affect blood cholesterol and because, as the study showed, trans unsaturated fats may worsen cholesterol levels.

"In essence there's no way for the public to go out and avoid trans {unsaturated fat} unless they avoid shortening and margarine altogether," he said. "FDA and industry should begin to take this into account."

However, Mensink warned that one study alone does not warrant a revision of recommendations. "It deserves more research," he said.

The study followed 59 men and women for three weeks. All were fed a similar diet, except that 10 percent of the subjects' total calories differed. A third were given diets high in trans unsaturated fat, another third were given diets high in a similar form of the fat lacking the trans bond and a third were given diets high in saturated fat.

The study found that people on trans unsaturated fat had higher levels of "bad" LDL and lower levels of "good" HDL than people on a diet high in an alternative version of the fat, called cis fat, which differs in shape. People on the saturated fat diet also had higher LDL levels, but their HDL levels were not lowered. This difference suggests saturated fats might be more healthful than trans unsaturated fats.

There is evidence that a high LDL-to-HDL ratio increases the risk of heart disease. It is thought that LDL molecules carry cholesterol from the liver and deposit it on artery walls. HDL molecules carry cholesterol back to the liver.