Outlandish portions. Sky-high salt content. Employees who don’t wash their hands. There are hazards aplenty at your favorite restaurant, but now health officials in New York and Chicago believe that they have cornered a real killer in the kitchen: the trans fats in partially hydrogenated vegetable oils used for frying and baking.

Citing scientific evidence that these oils are contributing to heart attacks, officials in both cities want restaurants to take trans fats off their menus.

“New Yorkers are consuming a hazardous, artificial substance without their knowledge or consent,” said Dr. Thomas R. Frieden, the city health commissioner.

Trans fats aren’t good for you, that much most experts agree on. But whether banning them is a necessary — or even highly beneficial — solution is a subject of some debate. Some scientists say it might not save as many lives as one would expect. And for preparing certain kinds of foods, there are few alternatives besides the saturated fats that have long been high on the list of artery-clogging foods.

Saturated fats like those in butter and lard raise serum levels of both L.D.L., the so-called bad cholesterol, and H.D.L., or “good” cholesterol. The increase in good cholesterol somewhat eases the increased risk of heart disease from higher levels of bad cholesterol.

But trans fats, experts have found, skew the ratio in the worst possible way by increasing the bad and lowering the good.

This has made partially hydrogenated oils a piñata for public health officials worldwide. Trans fats have essentially been banned in Denmark. Authorities in Canada, the first country to require food labeling of trans fats, are taking steps to follow suit.

In the United States, the National Academy of Sciences’ Institute of Medicine has concluded that the optimal intake of trans fats is zero. Earlier this year, Dr. Walter Willett, a leading nutrition researcher, and his colleagues at Harvard University estimated that if artificially produced trans fats were removed from the American diet, up to 228,000 heart attacks could be prevented each year.

By Dr. Willett’s calculation, diners are consuming one-third to one-half of their trans fats in restaurants. New York’s proposed ban, he argues, could prevent 500 deaths a year.

“It would be like putting the whole population on a mild cholesterol-lowering statin,” Dr. Willett said. “Probably nothing else you could do would have such a great impact on mortality.”

But other researchers, if not enthusiastic about trans fats, are more sanguine about their use in restaurants.

For one thing, they say, it’s likely that Americans are eating less already. One widely used estimate — that the average American takes in 2.6 percent of his or her calories in trans fats — is “old data,” said Dr. Alice Lichtenstein, a nutritionist at Tufts University.

Because trans fats are already disappearing from foods on grocery store shelves, Dr. Lichtenstein says, the current average intake is probably lower — more like 1.5 percent to 2 percent. Much of the interventional research into dietary trans fats has been done with subjects who consumed much greater amounts, often as much as 10 percent of their calories. At much lower levels, experts say, it’s hard to gauge with accuracy what further benefits can be gained by eliminating trans fats altogether.

The most vocal critics of trans fats believe that the relationship between their intake and heart disease is linear. Even tiny amounts pose some threat, they say. But an interesting study by Dr. Lichtenstein suggests that it’s more complicated than that.

She and her colleagues put 36 volunteers on diets with various amounts of trans fats, then measured blood levels of L.D.L. and H.D.L. cholesterol.

Increased trans fats were associated with increased blood levels of bad cholesterol in a linear fashion, she found. But good cholesterol was significantly diminished only in subjects who consumed trans fats in the greatest amounts — nearly 7 percent of their daily calories — and even then just barely. H.D.L. was not affected in subjects consuming less.

This finding and others like it suggest that for consumers eating modest amounts of trans fat, the gain from reduced intake may not be as great as some might hope. In any event, the benefit is likely to accrue mostly for people who have elevated cholesterol to begin with. That’s one in four New Yorkers, according to the city’s health department.

“Cumulatively, this small step could have a beneficial effect,” Dr. Lichtenstein said. “But it’s not going to be a panacea.”

Starting in January, the Food and Drug Administration began requiring food labels to list quantities of trans fats per serving. But citing a lack of scientific data, the agency has declined to establish a daily value for trans fats, as it has for saturated fats and dietary cholesterol. Indeed, the agency still classifies partially hydrogenated oils as G.R.A.S., or generally regarded as safe.

Still, even at low levels, trans fats do raise bad cholesterol, and no matter how you slice it, that’s bad news. There are other concerns, too. Though the research is still evolving, some studies suggest that trans fats increase levels of certain other blood fats and promote inflammation throughout the body, another risk for heart trouble.

Banning them may not save 500 lives a year in New York, but it may well save some. Whether you turn out to be one of the lucky ones would depend not just on the heart risks you bring to the table, but also on what replacements the cooks use.

For many foods requiring hard fats, particularly baked goods, the tastiest alternatives to partially hydrogenated oils are tropical oils, like palm oil and coconut oil, or butter. Loaded with artery-blocking saturated fat, they are the very ingredients health advocates shooed us away from not so long ago.

Saturated fat is at least a natural constituent of our diets, according to officials, whereas trans fats are essentially chemical abominations that can no longer be countenanced.

For restaurant patrons who long ago learned to avoid butter and skip the bacon, all this amounts to a Hobbesian dilemma. Better saturated fat, the heart-stopping devil you know? Or trans fat, the heart-stopping devil you’ve just been introduced to?

No surprise, then, that many people are cynical about the proposed ban. Grand public health gestures make for perilous public relations. Not so long ago, remember, eggs were causing heart attacks. Vegetable oils were causing cancer.

And the saturated fats that we’re supposed to be using again — whose idea was it to avoid those? Oh. Right.