Researchers contend that trans fat -- coveted by cooks for boosting the shelflife and texture of foods -- increases the chance of heart disease. Now it is also giving the restaurant industry higher blood pressure.

The Food and Drug Administration (FDA), prompted by a consumer advocacy group, is considering whether to force restaurants to place notices in menus or on signs if they are using partially hydrogenated vegetable oil, the trans-fatty-acid-laden ingredient in many fried foods.

The restaurants are facing pressure on other fronts as well. Lawmakers in Congress and the California legislature have introduced bills requiring nutritional labeling on chain restaurant menus. Plus, McDonald's Corp. has been taken to court by another advocacy group that contends the fast-food chain has failed to make good on a promise that it would reduce its use of trans fat and saturated fat.

Trans fat, a natural component of animal and dairy fat, is created artificially when food makers add hydrogen to vegetable oil to make it more solid. Researchers contend that trans fat raises levels of low-density lipoprotein, or "bad" cholesterol, increasing the risk of heart disease.

Manufacturers of packaged foods, already facing a 2006 deadline to display trans fat content on the nutritional labels of their products, have unleashed a flurry of trans-fat-free offerings, including reformulated versions of Crisco shortening and Triscuit snack crackers.

The push for restaurants to disclose their use of trans fat comes as diners increasingly go out to eat. According to the National Restaurant Association, about 46% of the food dollar is now spent outside the home, compared with 25% in 1955.

"We buy a lot of food outside of grocery stores," said Michael Jacobson, executive director of the Center for Science in the Public Interest (CSPI), the group that petitioned the FDA for the rule change. "People should be informed about that food."

Mandatory labeling, however, faces stiff opposition from the restaurant industry.

Restaurateurs are concerned that because cooking is not always a precise science, labeling could expose them to litigation if trans fat and other nutritional content differ from amounts listed on menus.

"What goes on in kitchens is a creative endeavor that is not necessarily dictated by teaspoons and tablespoons," said Jot Condie, president of the California Restaurant Association.

Then, there are the customers themselves. More than 60% of them customize their orders, making it nearly impossible to calculate the trans fat content in every variation, said Stephenie Shah, a senior legislative director at the California Restaurant Association. For example, she said, there are 1.3 trillion combinations for a pizza when there are 15 toppings to choose from, all with varying fat content.

The restaurant industry also maintains that previous nutritional labeling efforts have failed to address health concerns, said Rick Berman, executive director of Center for Consumer Freedom, an advocacy group supported by restaurants and food companies. The main problem with nutritional labels, Berman said, is that they are aimed at people least likely to read them.

"The government shows that obesity is concentrated among people who have less than a high school diploma," Berman said. "Many of these people are functionally illiterate, yet all the bright-eyed bureaucrats and regulators think the answer is to provide fat grams and carbohydrate grams to people who don't read."

Jacobson said that sort of thinking was off-base.

"Trans fat has nothing to do with obesity," he said. "If all the trans fat were banned and replaced by other oils, there would be no effect on obesity. It has to do with heart disease.

"Nutritional labeling clearly hasn't been a panacea for America's health problems, but it has been a godsend for millions and millions of people who read labels carefully and are trying to protect their health."

Jacobson's group, which filed a separate petition with the FDA to prohibit partially hydrogenated oils as a food ingredient, contends that virtually all burger and fried chicken chains use the oils to fry foods, as do most casual dining establishments.

Some food suppliers also blanch certain foods, including French fries, in trans fat before freezing and shipping them to restaurants.

One anti-trans-fat group has taken the fight beyond petitions and legislation. BanTransFats.com, a California nonprofit, sued Oak Brook Ill.-based McDonald's this year, alleging false advertising over its 2002 announcement that it would cook its famous fries in an oil with lower levels of trans fat and saturated fat.

The company released a statement last year that said the change would be delayed.

Jacobson's group placed a full-page ad in the New York Times that chastised McDonald's for "A Broken McPromise" and depicted a heart attack victim being resuscitated.

"It's two years after McDonald's said it was going to phase out partially hydrogenated oil," Jacobson said. "We thought two years was a long-enough time, and the company hasn't done anything."

McDonald's declined to comment specifically about the lawsuit or the ad. In a statement e-mailed to The Times, the company said, "As we reported in February of 2003, the change has taken longer than anticipated, but we are continuing with ongoing tests. As previously stated, (trans fatty acid) information has been added to our nutrition information materials, including our website, in-store nutrition brochures and tray liners beginning next month."

Some casual dining chains already use trans-fat-free oils to fry foods, including Ruby's Diner. Ruby's also will begin offering trans-fat-free FitFries at all its outlets Tuesday. The fries cost about $0.60 more per order than their conventional counterparts and will automatically be included in kids meals.

Doug Cavanaugh, chief executive of Newport Beach-based Ruby Restaurant Group, said his company was responding to an industry trend of providing healthier fare. He predicted that more food suppliers would offer foods blanched in trans-free oil, allowing chains to list more trans-free items on menus.

"Onion ring manufacturers are going to do the same thing," Cavanaugh said. "I guarantee you they're probably in their laboratories as we speak. This is not going to go away."

Last month, a variety of vendors were peddling trans-fat-free oils at the Western Foodservice & Hospitality Expo in Los Angeles, and ads touting the oils have surfaced in trade publications.

The restaurant industry points to these voluntary efforts as evidence that mandatory labeling is not necessary.

Many restaurants have expanded their menus to include healthier fare or already provide nutritional information on company websites. Visitors to a website run by San Diego-based Jack in the Box Inc., for example, can calculate the amount of trans fat contained in any combination of menu items.

Ruby Tuesday Inc., a Maryville, Tenn.-based chain not related to Ruby Restaurant Group, also switched to trans-fat-free oil nearly a year ago. However, it abandoned its voluntary effort to provide nutritional information on menus this year, opting instead to print the information on tabletop guides. The guides allow the company to reflect ingredient changes without reprinting menus and permit customers to view the information throughout their meal.

"As is often the case in the restaurant industry, we make tweaks to the menu as needed," said Angie Heig, a Ruby Tuesday spokeswoman. "Making these minor adjustments has a direct impact on nutritional information."

Despite such voluntary efforts, proponents of mandatory menu labels plan to press ahead with legislation.

U.S. Sen. Tom Harkin (D-Iowa) plans to reintroduce his menu-labeling measure next year, said Maureen Knightly, Harkin's press secretary. His attempt for the current session was folded into a broader health bill that the senator's office believes will not clear Congress this year. Harkin wants restaurants with 20 or more locations to disclose calorie, saturated fat, trans fat and sodium amounts on menus.

State Sen. Deborah Ortiz (D-Sacramento) is weighing whether to reintroduce next year a similar menu-labeling measure in California. Her effort last year to require chain restaurants to post the nutritional content of standard menu items on a wall, or to customers upon request, failed after facing resistance from opponents including the California Restaurant Association. and the California Chamber of Commerce.

Ortiz said she offered to address opponents' litigation concerns, including inserting a disclaimer that there could be variations based on serving size and special orders.

"We took away that argument and they still opposed it," Ortiz said. "Ultimately, I concluded if the consumers had this information they may not buy ... the cheaper, high-fat products."

Ortiz said the measure failed to clear the Assembly Health Committee after a group of moderate Democrats withheld their votes. "It was an indication of the industry's influence into that committee," she said.

However, Berman contends it is not industry influence that is blocking such laws; it is the public's lack of interest: "There's no groundswell among consumers for information they're not particularly interested in."