to allow in their diets, it's not enough to worry about limiting your intake of steaks and croissants, the Wall Street Journal reported recently.

Now you have to know about all the different kinds of fats and their values.

The fat, the newspaper said, hit the fire last year when Walter Willett of Harvard University did studies linking heavy margarine consumption to heart-attack risk.

The study questioned some old assumptions about margarine being much more healthful than butter, making discoveries that seemed unfair to consumers who had long forsaken the taste of real butter and establishing one of the reasons that we now need a scorecard to know the players in fat research.

The American diet is about 34 percent fat, although the American Heart Association and others think it should be below 30 percent. Fat is either saturated, which is bad, or unsaturated, which is less bad. Saturated fat raises the level of blood cholesterol, which leads to heart disease, and comes from animal sources like meat and butter, plus tropical palm and coconut oils.

Unsaturated fat affects cholesterol less; it comes mainly from vegetable sources.

Cholesterol sticks to proteins in the body that either prevent or promote heart disease. The proteins are high density lipoprotein (HDL), which clears cholesterol from arteries, and low-density lipoprotein (LDL), which contributes to heart disease. The ideal diet promotes low saturated fat intake, low cholesterol and low LDL, but simultaneously encourages high HDL.

Unfortunately, diet is only one factor that influences cholesterol; genetics also play a major role.

In the '60s and '70s, diets in the US starred polyunsaturated oils, like corn and safflower oil, which lower cholesterol and LDL. They had one drawback, however: they also lowered beneficial HDL.

Enter the new stars of Fat City: the monounsaturated oils -- olive oil and canola oil. Nutritionists like monounsaturates because they lower harmful LDL but maintain HDL levels. Olive oil is the cornerstone of the Mediterranean diet. Two studies linked low breast cancer rates to this diet, but skeptics reserve judgement.

To confuse us further, other potentially beneficial oils drawing interest were omega-3 fatty acids in fish and alpha linolenic acids found in canola and soybean oils, certain vegetables and nuts.

Then came the great margarine meltdown of 1994. Dr. Willett's concern about coronary risk stems from the practice of treating polyunsaturated vegetable oil to harden it and prevent spoilage. But this process -- hydrogenation -- makes vegetable oil more like saturated fat, thus boosting its risk to the heart.

Besides margarine, hydrogenated oils are used in many foods, from french fries to cookies and crackers. Such foods, while low in saturated fats, may be high in harmful "trans fatty acids.'' He worries a cupcake's long shelf life may shorten ours.

Recently, the US Centre for Science in the Public Interest asked the US Food and Drug Administration to include trans fats in the saturated fat on food labels. It also asked the FDA to bar food from boasting "low in saturated fat'' if it's high in trans fats. The bottom line, says CSPI scientist Margo Wootan, is that "margarine is still better than butter. But margarine isn't as good as we thought it was.'' The FDA, meanwhile, is still weighing the label changes, while the American Heart Association and the American Dietetic Association took a wait and see stance on the labelling issue. "It's premature,'' says Alice Lichtenstein of Tufts University. Still, she concedes, it's better to choose a soft margarine than a hard one.

Fat's arch-foe, Dean Ornish, a cardiac researcher known for treating heart disease with diet, thinks the margarine flap is beside the point. People should cease drizzling and spreading fat on food, he argues, to get total dietary fat down to 10 percent. Having proven his strict regimen can reverse heart disease, he now advocates it for maintaining heart health.

"Fat is an acquired taste,'' he said during a lunch interview, frowning at an olive oil-dressed salad.

He notes that olive oil, like margarine and butter, contains 120 calories a tablespoon. He doesn't consider it a health elixir or a treat.

"Nobody raids the olive oil jar at night,'' he says. "I'd rather have a truffle.'' The Ornish regimen isn't to everyone's taste.

"The idea that all fat is bad is plain wrong,'' counters researcher Frank Sacks of Harvard. He favours the Mediterranean diet based on grains, vegetables, fruits and olive oil or, alternatively, an Asian diet based on rice, vegetables and fish.

"I'd weight the diet toward monounsaturates like olive oil, but I wouldn't be dogmatic about it,'' says Dr. Sacks. "I want to give people flexibility.'' In this fractious field, one of the rare points of consensus is that saturated fat should be kept low. Most other issues -- from the ideal levels of fat to the risks of margarine or the virtues of the oil du jour -- are subject to scientific fashion.