This is a shopping expedition with a cause. Cut down on dietary fat to reduce the risk of heart disease and cancer, say the experts. But where does it all begin? In the supermarket.

Clearly, there are products that send "fat" messages to the shopper: cream, lard, bacon. And there are those that signal "no fat" or "low fat," such as fruits, vegetables and fish.

But there is a large gray area, those aisles of packaged foods labeled with either too little information or lots of information -- but often not quite the right information -- that can make low-fat shopping confusing. Our knowledge is outpacing the marketplace. Product labels need to catch up.

As the label debate slowly continues at the Food and Drug Administration, senators Howard Metzenbaum (D-Ohio) and Paula Hawkins (R-Fla.) introduced a bill in Congress two weeks ago that would require all manufacturers to label their products with both the amount of fat and specific type of fat.

"Americans have both the need and the right to know exactly what is contained in the foods they buy -- especially when the ingredients can be harmful to their health," said Metzenbaum. He plans to ask for hearings on the bill, according to a staff spokesman.

In the meantime, there are two tools -- the ingredient label and the nutrition label -- that we will use to help find the fat. This is what you can and can't expect to find out from each:

* The ingredient label, required by law for all packaged and processed foods, lists what's in the food by descending order of weight. This will identify a fat ingredient -- at least part of the time.

An exemption granted to manufacturers in the late 1970s permits them to list oils in "and/or" fashion, making for shopping roulette when trying to avoid saturated fats. "Safflower oil and/or palm oil the saturated fat ," a label might read. It "really leaves consumers in the dark" says Bonnie Liebman, director of nutrition for the Center for Science in the Public Interest.

According to the FDA, the labeling flexibility was granted to manufacturers due to the fluctuations in price and availability of oils. James Summers of the agency's Division of Regulatory Guidance said that it "puts a disadvantage on the manufacturer, not the consumer" because even though the product may contain the ingredient, a consumer might decide not to take his chances and not buy it.

Another point that the ingredient label does not address is the relative quantities of ingredients to each other. You may know that oats are the most prevalent ingredient in a granola bar, followed by "partially hydrogenated soybean oil and/or cottonseed and/or palm oil." But are the oats 85 percent of the product, and the oil 10 percent? Or, are the oats 45 percent, and the oil 44 percent?

* The nutrition label, a voluntary disclosure (unless a manufacturer makes a nutrition claim and then it's mandatory), lists, among other pieces of information, the amount of fat per serving in grams.

Unless you're a nutritionist, however, fat in grams may give you little idea of just how fatty a food is. Campbell's Cream of Chicken Soup contains seven grams of fat. What does that tell you? If nothing else, it may enable you to compare products, to know that it contains more fat than the tomato soup, with two grams.

In addition, fat in grams can sometimes be misleading. Armour Classic Lite Seafood Natural Herbs Dinner and a serving of Kraft French Dressing each have six grams of fat. Yet the seafood dinner derives 20 percent of its calories from fat, the dressing 90 percent.

Many in the nutrition community have been pushing the idea of listing fat as a percentage of calories because it flags whether a food is high or low in fat. That french dressing may send you a "high" message, indicating that you might want to chose a lower-fat alternative, eat less of it, or perhaps that you need to make a trade-off and leave the shredded cheese off your salad this time.

Some manufacturers have already started using percentage of calories from fat on their labels, some even dividing that total percentage into saturated and unsaturated fats. (For how to calculate percentage of calories from fat in a product yourself, see page E6.)

Nutritionist Linda Smith of Public Voice for Food and Health Policy, a consumer organization on a fight fat campaign, will be our guide through a supermarket, in this case the Georgetown Safeway. With us, we'll take our guidebook, the American Heart Association's "Grocery Guide."

Enter and turn right, we'll go aisle by aisle. Obviously, we won't get near to tackling every product; this is a just a guide of how to look for fat in commercially processed food products.

Bulk nut mixes. These white plastic bulk bins list ingredients, but don't give nutrition information. But anytime you see a mix with nuts in it, says Smith, it's guaranteed to be more than 50 percent fat.

While they are good sources of protein and B vitamins, nuts are high-fat foods, most of them containing more than 150 calories per ounce with about 80 percent of the calories coming from fat. These should be considered "accessory" foods, says Smith.

Another problem, however, with some of the bulk nut mixes is the added ingredients; additional oils and/or sugar. The "yogurt-coated maltballs" or the "yogurt-coated pineapple" lists "palm kernel oil" in the ingredient list; you don't find "yogurt" until the fifth or later ingredient. "You're no better" eating some of these products than you are eating candy, says Smith, and you pay more for them, she adds.

Some are better than others, though, says Smith. If you do want to buy a bulk mix, go with one higher in fruits, such as the "Raw Trail Mix," which lists raisins and date pieces as the two most prominant ingredients.

As for plain packaged nuts, many contain nutrition labeling, and Planter's even gives the P/S ratio (polyunsaturated to saturated fats) on its labels now.

Also, says the AHA Grocery Guide, avoid those nuts roasted in coconut and palm oils (cornnuts frequently are roasted in coconut oil). Dry roasted nuts are slightly lower in fat than roasted nuts, says Smith. And sunflower seeds contain a high proportion of polyunsaturated fats.

Breads and pastries. Breads aren't considered high-fat foods (it's often what you put on bread that adds fat), yet some contain higher proportions of fat than others.

Some general rules for this department: Breads higher in saturated fat include croissants, "butter-topped" breads, challah, crescent rolls, biscuits and toaster cakes.

Breads with zero fat include french bread, pita bread, water bagels and italian bread. English muffins, non-cheese bread sticks, whole grain or enriched breads and corn tortillas (not fried) all qualify as lowfat breads, says the AHA guide.

When it comes to pastries, most processed baked goods derive from 30 to 50 percent of their calories from fat, says Smith, poking among the cheese coffee cakes and chocolate doughnuts. "If we can satisfy ourselves with a bagel or an english muffin, we'll be better off," Smith says.

Oils. "No vegetable products contain cholesterol," says Smith, heading up the oil aisle and noticing the flag "no cholesterol" on several vegetable oil containers.

Most oils give plenty of nutrition information, indicating the ratio of polyunsaturated to saturated fats. The best oils to buy are those with a ratio of at least 2:1 (labels state P/S amounts in grams of fat and don't convert these amounts into a ratio, but obviously 4 grams of polunsaturated fats to 2 grams of saturated fats is the same as 2:1). Soybean, corn, sunflower and safflower generally have higher percentages of polyunsaturated to saturated fats than 2:1. And oils that are simply called "vegetable oils" are predominantly soybean oil, which has been partially hydrogenated.

The products labeled on these shelves with the highest percentage of polyunsaturated to saturated fats include Hollywood Safflower Oil (10:1), Wesson Sunflower Oil (10:2), NuMade Sunflower Oil (9:2) and Puritan Vegetable Oil (9:2). All of the corn oils have P/S ratios of 8:2. NuMade Vegetable Oil has a 7:2 ratio, but several of the other vegetable oils don't list this information. And Planters Peanut Oil, a monounsaturated fat, lists its P/S ratio at 4:2.

Next to the oils are solid shortenings such as Crisco. These shortenings have been highly saturated through a process called hydrogenation, which converts unsaturated fats to mono and saturated fats and turns the product into solid form. The brands on this shelf are made from hydrogenated palm (already saturated) and soybean oils.

Hot dogs and other processed meats. These are all considered in the upper ranges of fat foods, even though you'll never see "fat" listed in the ingredient list, says Smith. Hot dogs, whether they are beef or pork, draw around 80 percent of their calories from fat within the listed ingredients.

Processed meats that are lower in fat than bologna or salami include lean boiled ham, pressed wafer-thin meats such as chicken, turkey or lean beef, says the AHA guide.

Cake and cookie mixes. Fortunately, many of these boxes contain nutrition labeling. Unfortunately, many cake mixes contain beef fat, lard or hydrogenated vegetable oils, all saturated fats, says Smith, as she scans the ingredient labels. In addition, the product has more fat when the flavor is chocolate. Plus, many mixes require that you add eggs, stepping up the cholesterol count.

Although most of the cookie mixes are almost 50 percent fat, comments Smith, she wagers that figure is lower than those fancy cookies being sold in numerous Washington eating malls.

The alternatives for cakes beside fruit? You can always make your own cake using corn oil and egg whites, says Smith. Or, adds the AHA guide, if you must a buy a cake mix, choose angel food.

Frostings. Most contain hydrogenated vegetable fats or animal fats, some as the third ingredient, notices Smith. It's easy to make your own cake frosting, Smith adds. Although high in sugar, cookbooks have no-fat frosting recipes made with such ingredients as confectioners' sugar, egg whites, lemon juice and water.

Tunafish. Tunafish can turn into a high-fat food when packed in oil. A comparison, using the gram information on the nutrition label, of two cans of Fancy Albacore Bumble Bee tuna demonstrates this. Tuna packed in water: 2 grams fat (26 percent calories from fat). Tuna packed in oil: 11 grams fat (56 percent of calories from fat).

But, Smith suggests, since tuna packed in water can be substantially more expensive than that packed in oil, you can always buy the one packed in oil, place it in a strainer and rinse thoroughly with water to remove most of the oil.

Snack foods. Potato chips and corn chips are high sources of fat, says Smith. And many commercially made popcorns are prepared in coconut oil. Pretzels, although topped with salt, which is another health matter, are lower in fat than chips. Smith does a nutrition-labeling comparison between pretzels (1 gram of fat per serving) and a corn chip product (10 grams per serving).

Passing comments up the aisles . . . Avoid those packaged vegetable mixes, such as potato, with added cheese, and add skim milk instead of whole milk . . . Watch out for frozen whipped toppings; they contain high amounts of saturated fats, as do non-dairy coffee creamers. . .

Frozen dinners. Frozen dinners that contain cream sauces, breading or butter are no fat bargain. The new "upscale" frozen dinners, while certainly not price bargains, are lower in fat than some of the traditional TV dinners.

A comparison in percentage of fat/calories: Lean Cuisine's Glazed Chicken with Vegetable Rice at 27 percent fat or Dinner Classics Chicken Fricasse at 33 percent fat as compared to Swanson's Fried Chicken Dinner at 51 percent of its calories coming from fat.

Cold cereal and granola bars. Cold cereals don't contain fat, with a small number of exceptions such as Cocoa Puffs, which contain coconut oil. Many commercial granola cereals, however, do contain vegetable or animal shortening, says the AHA guide, and often have three times as many calories per cup as plain cereals. (Although most cereals don't contain fat, they vary widely in their sugar content, warns the AHA guide. If sugar is listed among the first three ingredients, chances are the cereal is high in sugar.)

As for granola bars, they "don't belong on a low-fat diet," says Smith. Some granola bars weigh in at about 40 percent of their calories coming from fat, the fat coming from the oils, the peanuts or chocolate. Yet some crackers have more fat.

Crackers. Crackers are a tough area for a shopper to judge fat, says Smith. Many are rich and buttery, she says, containing some type of oil, often saturated. But as with other products, it's difficult to determine how crackers differ in fat from the ingredient labeling. Very few cracker packages contain nutrition information.

Here are at least some fat percentages for crackers, according to the Center for the Science in the Public Interest's poster "Fat & Calorie Guide": Ritz crackers (48 percent fat), Wheatsworth (42 percent fat), Triscuits (32 percent fat).

The AHA guide lists these low-fat suggestions for the cracker dilemma: melba toast, oyster crackers, saltines, soda crackers, matzo, swedish flatbreads.

Some other tips provided by the AHA Grocery Guide and not covered in these aisles:

* Many commercial soups are low in fat (watch out for the sodium, however) when prepared with skim milk. Among the "acceptable soups:" lentil, minestrone, turkey noodle, chicken rice, Manhattan-style clam chowder, bean.

* While they are concentrated sources of sugar, low-fat dessert substitutions for ice cream include sherbet, fruit bars and Popsicles, low-fat frozen yogurt.

* Most commercial cookies are made with saturated fats. For moderate use, consider those lower in fat -- animal crackers, ginger snaps, graham crackers (not chocolate coated) and molasses cookies.

As part of the AHA's "Eat to Compete" heart month campaign, the grocery guide, plus the pamphlets "Guide to Restaurant Dining," "Exercise and Your Heart" and a tape measure featuring monthly exercise tips are available free. Write or call the American Heart Association, Nation's Capital Affiliate, 2233 Wisconsin Ave. NW, Suite 200, Washington D.C. 20007. 337-6400.