Besides American fast food invading Europe, she said, other features of American cuisine, such as getting foods out of season, also have contributed to a decline in ingredient quality.
"The basic foods still have vitamins and minerals," Nestle said. "It's very difficult to prove that it's less healthy, unless you're talking about the influx of fast food."
Other American habits also may be infiltrating Europe, she said.
For instance, portion sizes at American restaurants tend to be larger, though European portion sizes seem to be growing.
Europeans also are eating more on the go, as Americans often do, rather than sitting down for a meal, she said.
"They're doing things that didn't used to be culturally acceptable," Nestle said. "If you want to watch globalization in action, you can go to practically any European city and watch it."
Katz explained that fast food chains cannot be held solely responsible for not making certain changes to their menus.
"They're just trying to keep the customer satisfied," he said. "It may be that McDonald's customers in the U.S. are lagging behind customers in the U.K. on these issues."
In the U.S., he said, McDonald's would make the same changes if customers were willing to pay more for them, because they would come at a cost.
For that reason, Katz focuses more on keeping consumers informed to make better decisions, rather than looking to the distributors. In other words, he's trying to change demand rather than supply.
He said McDonald's could try healthier versions of its burger -- for instance, using lean turkey, or a combination of turkey and a vegetable, in place of ground beef, or serving the burger on a whole grain bun.
Restaurants, Katz said, are involved in creating the food trends, and so a fast food chain would have to step up to create something filling and nutritious that also satisfies a customer's taste buds.
"I could envision a day, 10, 20 years from now, where McDonald's is a health food restaurant," he said.
But customers would have to want and expect to get that type of food when they go out.
"There is one thing I can guarantee you will change the food supply," Katz said, "and that's the [customers'] food demand."