Not long after disclosing that its french fries contain more trans fat than thought, McDonald's Corp. said Monday that wheat and dairy ingredients are used to flavor the popular menu item — an acknowledgment it had not previously made.

The presence of those substances can cause allergic or other medical reactions in food-sensitive consumers.

McDonald's had said until recently that its fries were free of gluten and milk or wheat allergens and safe to eat for those with dietary issues related to the consumption of dairy items. But the fast-food company quietly added "Contains wheat and milk ingredients" this month to the french fries listing on its Web site.

The company said the move came in response to new rules by the U.S. Food and Drug Administration for the packaged foods industry, including one requiring that the presence of common allergens such as milk, eggs, wheat, fish or peanuts be reported. As a restaurant operator, Oak Brook, Ill.-based McDonald's does not have to comply but is doing so voluntarily.

McDonald's director of global nutrition, Cathy Kapica, said its potato suppliers remove all wheat and dairy proteins, such as gluten, which can cause allergic reactions. But the flavoring agent in the cooking oil is a derivative of wheat and dairy ingredients, and the company decided to note their presence because of the FDA's stipulation that potential allergens be disclosed.

"We knew there were always wheat and dairy derivatives in there, but they were not the protein component," she said. "Technically there are no allergens in there. What this is an example of is science evolving" and McDonald's responding as more is learned, she said.

While the company wanted to make consumers aware that fries were derived in part from wheat and dairy sources, she said, those who have eaten the product without problem should be able to continue to do so without incident.

Since it was posted on McDonald's Web site, the acknowledgment has stirred anger and some concern among consumers who are on gluten-free diets.

"If they're saying there's wheat and dairy derivatives in the oil, as far as anyone with this disease is concerned there's actually wheat in it," said New York resident Jillian Williams, one of more than 2 million Americans with celiac disease, an autoimmune disorder triggered by gluten.

"They should have disclosed that all along," she said. "They should never have been calling them gluten-free."

McDonald's has been reluctant to risk changing the taste of its heavy-selling fries. It pledged in September 2002 to switch to a new oil that would halve the level of harmful trans fatty acid in its fries. But it has delayed those plans, citing product quality and customer satisfaction as priorities while continuing testing.

Asked about the status of its efforts Monday, Kapica said: "It's a very high priority and we are very committed to continuing with testing and lowering the level of trans fat without raising the level of saturated fat. ... It's a lot harder than we originally thought but that is not stopping us."

McDonald's shares fell 8 cents to $36.25 in afternoon trading on the New York Stock Exchange — up 7 percent in 2006.