NEW YORK (CNN/Money) - McDonald's Corp. is cutting the amount of "bad" fat in its french fries nearly in half, the fast-food chain said Tuesday as it moves to make all its fried menu items healthier.

But does that mean the popular shoestring fries won't taste the same? The company says no.

"It's a win-win for our customers because they are getting the same great french-fry taste along with an even healthier nutrition profile," said Mike Roberts, president of McDonald's USA.

But others are not so sure. McDonald's will not specifically discuss the kind of oil it plans to use, but at least one nutrition expert says playing with the formula could mean a different taste.

Shares of Oak Brook, Ill.-based McDonald's (MCD: down $0.54 to $23.22, Research, Estimates) were lower Tuesday afternoon.

It was unclear Tuesday whether competitors Burger King and Wendy's International (WEN: down $0.80 to $34.91, Research, Estimates) would follow suit. Neither company could immediately be reached for comment.

The new oil, to be introduced on Oct. 1 and completely phased in by February, leaves the total amount of fat in the fries unchanged. It just rejuggles the types of fat. The changes come just weeks after a New York man sued McDonald's and three other fast food chains claiming they caused his obesity.

"They are taking the right step along with public health recommendations," said Jule Anne Henstenburg, director of the nutrition program at LaSalle University in Philadelphia. "But you still can't eat a ton of it. It'll make you fat."

McDonald's said the new oil will reduce the amount of trans-fatty acids in its french fries by 48 percent and cut saturated fat by 16 percent, the company said. Trans-fatty acids, which are chemically produced from vegetable oil, and saturated fats, which occur naturally, are what significantly contribute to high cholesterol and heart disease, Henstenburg said.

At the same time, McDonald's is "dramatically" increasing polyunsaturated fat in its fries by 167 percent. U.S. dietary guidelines were recently changed to say that moderate amounts of polyunsaturated fats are OK, Henstenburg said.

In fact polyunsaturated fat in moderate amounts can help reduce "bad" cholesterol, said James Cleeman, coordinator of the National Cholesterol Education Program at the U.S. Department of Health.

The new cooking oil will also be used to prepare Chicken McNuggets, Filet-O-Fish, Hash Browns and crispy chicken sandwiches.

"In essence, we're saying that McDonald's is doing the right thing," Henstenburg said.

But if the new formula alters the taste, that could have an impact on the company, said Britt Beemer, a consumer pollster and brand expert.

"Their secret weapon has always been their french fries. I would say that if they do change the oils, I'll bet they did a million taste tests to make sure nothing has changed," Beemer said. "But there's always that risk when you're tampering with a product."

A McDonald's spokesman could not be reached for additional comment Tuesday.

McDonald's said in a press release Tuesday that the switch is part of its new worldwide nutrition initiative in which the company plans to reduce the amount of trans-fatty acids in foods prepared at all of its 13,000 restaurants.

The move comes amid a slew of research showing that obesity is on the rise among Americans, so much so that many clothing retailers have been expanding their plus-size offerings to meet rising demand.

In March, a Harris Poll showed about 80 percent of U.S. adults over the age of 25 are overweight. That's a big jump from 71 percent in 1995.

One consumer advocate said that although the move by McDonald's will reduce "artery-clogging" fat, it won't reduce obesity.

"This is a good step toward reducing artery-clogging fat in french fries, but it doesn't make french fries a health food," said Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest, a consumer group.

The change "probably won't have any impact on obesity," she said, since changing one type of fat for another doesn't affect the calorie content. A headline that said McDonald's fries "are better for you" would be misleading, she added. "We hope that doesn't lead people to eat more."

In addition to battling nutrition concerns, McDonald's has been saddled with several issues in the past few years, including fears of tainted beef in Asia and Europe and increasing competition from slightly more upscale restaurants that offer fewer fried foods.

The maturing company is also experimenting with new formats, including a new McDonald's with a diner inside, and is also considering a new retail initiative in which it would sell a full offering of merchandise.