Washington ? Consumers are about to get a better idea of just how unhealthy some brands of potato chips, cookies, even margarines really are: Food labels will soon be required to reveal how much artery-clogging trans fat they contain.

Trans fat helps make such foods as doughnuts, french fries, crackers and fried chicken taste good. But it’s at least as dangerous to the heart as its better-known cousin, saturated fat — and many doctors consider it worse. Yet today, consumers have no way of knowing how much trans fat they eat.

Under regulations to be announced by the Food and Drug Administration today, that will change.

The nutrition label will get a new line listing the amount of trans fat in each food under the amount of saturated fat it contains, say consumer advocates and industry representatives familiar with FDA’s decision.

The addition of those two lines will show the total of heart-risky fats in every serving.

“It’s a good first step,” said Margo Wootan of the Center for Science in the Public Interest, which petitioned the FDA 10 years ago to make the change. “People will be able to compare different products and determine which ones are worse for their hearts.”

But Wootan said the comparisons won’t be easy. The labels won’t tell consumers how much a candy bar or doughnut counts against their daily allotment of unhealthy fat. Nor will they bear a message FDA debated this spring — that trans fat consumption should be as low as possible.

The FDA has estimated that merely revealing trans fat content on labels would save between 2,000 and 5,600 lives a year, as people either chose healthier foods or manufacturers changed their recipes to leave out the damaging ingredient.

Trans fat is in numerous products, from meats and dairy products to pastries.

The most common source is partially hydrogenated vegetable oil, where liquid oil is turned into a solid to protect against spoiling and maintain flavor.