Trans: the phantom fat.

It's not saturated, monounsaturated, or polyunsaturated, according to the "Nutrition Facts" label on food packages. Except for a few margarines that proudly declare "no trans fatty acids," it's invisible.

But not to your blood vessels. In half a dozen clinical studies, trans fat raised people's blood cholesterol about as much as saturated fat did.

"It's a secret killer," says Walter Willett, head of the nutrition department at the Harvard School of Public Health. "With saturated fat, at least food labels tell you how much you're eating. With trans, it's anybody's guess."

That's why the Center for Science in the Public Interest (CSPI)--the nonprofit consumer advocacy group that publishes Nutrition Action Healthletter--has petitioned the Food and Drug Administration (FDA) to require that trans fat be included not just in the Total Fat number on food labels (as it is now), but in the Saturated Fat number as well. That way, consumers would be able to see how much artery-clogging fat any food had.

That wouldn't be necessary if it were easy to figure out how much trans fat a food contains. But it's just about impossible.

Trans is created when oils are "partially hydrogenated." Hydrogenation is what turns liquid oil into Crisco or stick margarines. It also makes oils more stable (so they can be re-used more times in deep-frying) and makes pie crusts flakier and french fries crispier.

You could cut trans fat by avoiding the thousands of foods with "partially hydrogenated oil" in their ingredient lists. But that may not be practical. Besides, the oil in some foods is only slightly hydrogenated--which means there's just a little trans. Unfortunately, there's no simple way to distinguish them from foods whose oil is heavily hydrogenated--which means it has lots of trans.

It's not just the "Nutrition Facts" panels that will snooker you. You've got to watch out for the claims on the packages as well. The FDA limits the amount of saturated fat in foods that make a "no-cholesterol" or "low-cholesterol" claim. But it sets no limit on trans fat. If the agency counted trans along with saturated fat, it would be illegal for products like Nabisco Oreos or Wheat Thins to call themselves "no-cholesterol."

And how do you avoid trans fat when you eat out? There's no ingredient list to clue you in, and the companies' nutrition brochures ignore it.

That's why we conducted our own nationwide trans test. We analyzed 41 foods or meals that are made with partially hydrogenated oils or shortenings.

Our conclusion: Unsuspecting consumers--some under doctors' orders to cut artery-clogging fat to reduce their risk of heart disease--are being broadsided by foods that are far more damaging than they appear to be.

FRENCH FRY FRAUDS

It was a huge victory...or so we thought. In the late 1980s, pressure from CSPI and other consumer groups forced the major fast food hamburger chains to stop frying their potatoes, fish, and chicken in beef tallow.

"McDonald's French Fries to be Cooked in Cholesterol-Free, 100% Vegetable Oil," announced the company's press release in 1990.

It was only partially right. The switch was not to pure vegetable oil, but to partially hydrogenated vegetable shortening.

The distinction isn't trivial. Our tests show that, thanks to their trans fat, the french fries sold at McDonald's, Arby's, and Hardee's have roughly as much artery-clogging fat as if they were fried in lard. Burger King and Wendy's fries are even worse. They're a bigger threat to your arteries than potatoes cooked in beef tallow.

In each case, if you ask the order-taker for a nutrition brochure, you'll see only the amount of saturated fat the fries contain. Not a peep about trans. That means half the artery-clogging fat is invisible.

Until fast food chains make good on their promise to use 100% vegetable oil, keep in mind that, to your arteries, a large order of fries from McDonald's, Arby's, or Hardee's looks like a Quarter Pounder. And a large fries from Burger King or Wendy's looks like 1 1/2 Quarter Pounders. Some side dish.

What about frozen supermarket potatoes? Ounce for ounce, the two we tested, Ore-Ida Tater Tots and Ore-Ida Snackin' Fries, were no different than fries from McDonald's, Hardees, and Arby's. (Ore-Ida says that it has reformulated its Snackin' Fries to have less saturated fat, but the newer incarnation is still bad news.)

CHICKEN BIG MAC NUGGETS 

It doesn't take a rocket scientist to figure out that fried chicken is fatty. But it does take a chemist, test tubes, and a lab to ferret out how much your heart is threatened by fried chicken's fat. Thanks to the trans fat:

* An order of nine McDonald's Chicken McNuggets is no healthier than a Big Mac.

* A Burger King Chicken Sandwich is roughly the same as a corned beef sandwich plus a bag of potato chips.

* A KFC Original Recipe (thigh and drumstick) Dinner with mashed potatoes and gravy, cole slaw, and a biscuit harbors 52 grams of fat and a day's worth of artery-clogging fat wrapped up in 1,160 calories. (If you skip the skin, we figure you'll save 550 calories and 22 grams of fat, 14 of them trans or saturated.)

* Your chicken pot pie might as well be fried. The trans plus saturated fat--most of it in the crust--use up 85 percent of your day's artery-clogging fat if you eat at KFC, and 55 percent if you're a Boston Market fan.

FRIED FISH: WORSE THAN STEAK

Fettuccini Alfredo--one of the worst foods we've ever analyzed--has 1,500 calories, 97 grams of fat, and 48 grams (a two-day supply) of saturated fat.

Now we've found a meal with that much fat plus 2,000 calories. A Red Lobster Admiral's Feast--that's fried seafood, french fries, cole slaw, and two pieces of garlic cheese bread--is a coronary from the sea, thanks, in part, to the trans-heavy shortening it's fried in.

True, the Feast is no worse than a 12-ounce sirloin steak, a baked potato with butter, green beans with more butter, and a slice of apple pie topped with half a cup of Haagen-Dazs premium chocolate ice cream. But most people already know that beef and butter make a beeline to their arteries. You don't expect that kind of behavior from a seafood dinner.

Long John Silver's Fish & More dinner (fried fish, french fries, cole slaw, and hushpuppies) is another heart attack with your name on it. It's got the calories (1,270), fat (58 grams), and artery-clogging fat (27 grams) of three Swanson frozen Meatloaf Dinners.

And Burger King's BK Big Fish Sandwich is big, all right. To your heart, it's two slices of Pizza Hut Meat Lover's hand-tossed pizza--just one more way to get almost half a day's artery-clogging fat. But the sandwich's 830 calories make the pizza's 640 look petite.

Did someone say fish is good for your heart?

MARGARINE: TRICKY STICKS

A glance at the label will tell you that a tablespoon of butter has seven grams of saturated fat--a third of a day's worth. Check the label on a full-fat margarine like Parkay stick--which lists only two grams of sat fat--and you'd think the margarine was far superior. That's because the Parkay label omits its trans fat. Count the phantom trans and Parkay's total "unhealthy" fat hits four grams.

Adding insult to injury, some margarine labels have the nerve to carry misleading claims about their saturated fat. When Parkay and Promise boast that their sticks have "70 percent less saturated fat than butter," for example, they're not counting trans fat. Neither does Crisco when it comes up with its "50 percent less" claim.

What to do? Leave the sticks on the shelf and pick up a tub. If you're trying to cut fat and calories, search for a tub with the least fat your tastebuds will tolerate. You may be surprised.

One of the winners of our informal taste test-Smart Beat Smarter than Fat Free Super Light Margarine--has only two grams of fat--and zero trans--per tablespoon. And its got only a fifth of the 100-something calories you'd get in that much regular margarine or butter.

Even our taste-test champion, Fleischmann's Lower Fat Margarine, has just five grams of fat--only one of them trans or saturated. And its calories are less than half of butter's.

If you prefer a full-fat margarine, try Promise tub, which has no trans. If you can't live without butter, try Land O' Lakes Light Whipped Butter. It tastes great, and its numbers beat Parkay, Promise, and other full-fat stick margarines pats down. Just don't assume that less fat means you can have twice as much.

BAKED GOODS: DOUBLE TROUBLE

Checking your labels? Whether it's danish or doughnuts, pies or biscuits, cookies or crackers, the "saturated fat" number listed on the package probably underestimates the mischief those foods will do to the arteries that nourish your heart.

Our tests showed that when you add the trans fat that's in these baked goods to their saturated fat, you roughly double the trouble they can cause your blood vessels.

Cakes aren't quite that uniform. While the trans doubles the artery-clogging fat in Little Debbie Swiss Cake Rolls, it "only" makes Entenmann's Chocolate Fudge Cake and The Cheesecake Factory Linda's Fudge Cake 25 percent worse. Of course, a single slice of Linda's still packs 1,470 calories and three-quarters of your day's quota of artery-clogging fat.

Doubling the bad fat is enough to push some baked goods into the danger zone. A Pillsbury Grands! Buttermilk Biscuit, for example, ends up with more than a third of your 20-gram daily limit of harmful fat. A McDonald's Iced Cheese Danish, an Entenmann's Rich Frosted Donut or a Dunkin' Donuts Old Fashioned Cake Donut uses up 50 percent. To your heart, each is like eating eight strips of bacon.

RELATED ARTICLE: How to Avoid Trans Fat

1. Look for foods that contain no "vegetable shortening" or "partially hydrogenated" oil. 2. Avoid deep-fried foods. The less fat, the less trans, so buy lower-fat margarines, chips, crackers, cookies, pastries, and other processed foods (and don't eat twice as much). 3. Use olive or canola oil instead of butter, margarine, or shortening whenever possible. 4. If you see margarine, buy tubs rather than sticks. To cut fat and calories, get "light," low-fat," or "fat-free brands. 5. Foods that are "cholesterol-free," "low-cholesterol," "low-saturated-fat," or "made with vegetable oil" aren't necessarily low in trans fat. "Saturated-fat-free" foods are.

RELATED ARTICLE: The Trans Schedule

Here are the foods we tested for trans fat, plus a selection of other foods for which we could obtain or calculate trans numbers. The list is not exhaustive. It represents just a sampling of what's available. Within each category, foods are ranked from least artery-clogging fat to most. "Artery-clogging" means "Saturated Fat" plus "Trans Fat" (the total may be off by one gram--a trivial amount--due to rounding). Foods that we tested are in italics. Serving sizes are in parentheses following each name.

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(1) American Journal of Clinical Nutrition 59: 861, 1994.

(2) Journal of Lipid Research 33: 399, 1992.

(3) New England Journal of Medicine 323: 439, 1990.