ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

Chicago is a city known for its food, from deep-dish pizza to Polish sausage, laden with sauerkraut. Lately, the city's also getting the reputation as a place where the long arm of the law reaches into the restaurant kitchen. Last month, the city banned restaurants from selling fois gras, fattened goose livers. Now, if one of the city's most powerful aldermen has his way, the Windy City would also become the first in the nation to ban restaurants from using oils that contain artificial trans fats.

Alderman Edward Burke wants trans fats banned because they're linked to heart disease, increased cholesterol and obesity. Kim Severson is a reporter for the New York Times and she's written a book called The Trans Fat Solution: Cooking and Shopping to Eliminate the Deadliest Fat from Your Diet. She says trans fat is the one fat we shouldn't be eating at all.

Ms. KIM SEVERSON (Author, The Trans Fat Solution: Cooking and Shopping to Eliminate the Deadliest Fat From Your Diet): The federal dietary guidelines that came out last year recommended that people consume less than one percent of their calories, or about two grams of trans fat a day. And that might translate to, oh, a glazed donut or maybe a small handful of French fries from many of the big fast food companies that are still using trans fat in the deep fat fryers.

The Institutes of Medicine, which is part of the National Academies, said the only safe level of trans fat is zero.

NORRIS: Now Kim, we're hoping that you can give us the skinny on trans fat. First, what is trans fat?

Ms. SEVERSON: Well, trans fat's the only artificial fat in our diet. So, if you really want to get a dinner party going, bring up the subject of dietary fats. You'll clear the table quickly. But it's just, in a nutshell, there's three kinds of naturally occurring fats. We know saturated fat and polyunsaturated and olive oil, which is monounsaturated fat. Trans fat is a fat in transition; it's going from a polyunsaturated fat into a saturated fat. So that transition happens, it's stopped partially through, which they call partial hydrogenation, and you have partially hydrogenated oil, or an oil that's artificial. So when it goes into your body, your body doesn't quite know how to process it. It's sort of like putting a plastic fat into your body.

NORRIS: So it's sort of a semi-solid state and never really breaks down?

Ms. SEVERSON: Right. Which, if you think about Crisco, certainly the old-fashioned kind of Crisco, that's trans fat.

NORRIS: Kim, when were trans fats first developed and when did they start to enter the American diet?

Ms. SEVERSON: Well, trans fats go back almost 100 years, and they were invented when people started to be able to ship food across the country. Trans fats allowed that food to stay fresher longer so, as the food industry grew, so did the use of trans fats. But it was really the growth of the fast food industry in the 70s, that put a lot of trans fat in our diet because it was in the fast food deep fryers, you know, it's in the coating on the fillets of fish and chicken nuggets that we eat.

NORRIS: So if you're using trans fat in, say, French fries or donuts, or a cookie, what does that do to the end product? How much does that boost the fat content in that food item?

Ms. SEVERSON: Well, first of all, anybody who's made a pie crust knows that Crisco is a great thing to use because it stays solid, it makes a really flaky pie crust. It can really take a beating, which is why fast food restaurants love it so much. You can put it in the deep fat fryer, heat it repeatedly, and it's going to really do the job for a long period of time. And it's also very inexpensive and it can keep packaged good fresh for a long time on the shelf.

The reason that it's bad is that it raises your bad cholesterol, but unlike saturated fat, which also does that, it lowers your good cholesterol.

NORRIS: Where do we find trans fats?

Ms. SEVERSON: Well you can find it anywhere and at one point it was in about 40 percent of the packaged goods you'd buy; all the potato chip and the snack food companies, most of the major ones have now gone to sunflower oil, safflower oil, oils that don't contain trans fats, and you'll see zero trans fat on many labels. There's a little bit of a trick when you look on a food label and it says zero trans fat. If a serving contains a half a gram of trans fat or less, they can claim that it's zero trans fat. But if you're like me, one serving of chips, you know, I'm just kind of getting warmed up, right? So we all usually eat beyond what the recommended serving is on packages.

NORRIS: It's usually about five chips.

Ms. SEVERSON: Exactly. So you can get, you know, easily a gram or two of trans fat just by a snack of chips that say zero trans fat.

NORRIS: So if Alderman Burke has his way and Chicago bans the use of trans fat in restaurant kitchens, it would join some other countries that have done this, Denmark, for instance. What's happened in those countries where they've banned trans fat? What do they reach for in the restaurant instead?

Ms. SEVERSON: Well there are certainly a lot of new oils and very traditional oils, that you can use that take the place of trans fat. The problem is they're more expensive. Olive oil, butter, all of those very traditional kinds of fats that we used to eat, can do pretty well in industrial kitchens. And also the big seed oil manufacturers, Archer Daniel Midland and a few others, are coming up with alternatives and growing oil seeds that can perform the way that a trans fat-based oil does. It's just more expensive. So some of those European countries are using these new breeds of oil. Some of them are going back to more traditional oil, Canola oils, oils that break down a little more quickly and then therefore cost a little more money, but are better for you.

NORRIS: Kim Severson, thanks so much for talking to us.

Ms. SEVERSON: Thanks. Happy to be here.

NORRIS: Kim Severson. She's the author of the book, The Trans Fat Solution: Cooking and Shopping to Eliminate the Deadliest Fat from Your Diet. Transcript provided by NPR, Copyright NPR.