McDonald's said on Wednesday that its fleet of 14,000 U.S. restaurants would stop serving chicken raised with antibiotics that are used in human medicine, a significant change in food policy for the world's largest fast-food chain.

After one of the worst years in company history, the Golden Arches is eager to ditch its image as a haven for greasy fries and mystery meat. In an era when consumers are focused on healthy eating and eager to know where their food comes from, McDonald's has lost ground to fast-casual competitors such as Chipotle that have long emphasized a commitment to serving only ethically raised meat.

"Our customers want food that they feel great about eating — all the way from the farm to the restaurant — and these moves take a step toward better delivering on those expectations," McDonald's U.S. President Mike Andres said in a statement.

The company said the new policy will be implemented across its U.S. supply chain within two years. It also announced that McDonald's would begin offering milk jugs in its Happy Meals that contain milk from cows that have not been treated with the growth hormone rBST.

Public health advocates were hopeful after the announcement that McDonald's move would be a catalyst for other major restaurant chains and food processors to consider similar changes in policy.

"This is a super-sized change for McDonald's, and we're lovin' it," said Pam Clough, organizer of US PIRG's Stop Overuse of Antibiotics Campaign, in a news release. "They will signal to the marketplace a huge and growing demand for chicken raised without the routine use of antibiotics."

Advocates say that a drawdown in use of antibiotics in poultry and livestock will help protect the effectiveness ofantibiotics in humans. The Centers for Disease Control and Prevention reported in 2013 that antibiotic-resistant infections kill 23,000 Americans every year, warning that the nation faces "potentially catastrophic consequences" if it doesn't take action against the threat.

The overuse of the drugs is the strongest factor contributing to antibiotic resistance around the globe, the CDC said. The more a particular germ is exposed to antibiotics, the more rapidly it can develop resistance. The CDC said as many as half of all antibiotics prescribed are either unnecessary or used inappropriately.

But the vast majority of drugs — by some estimates 80 percent — are used in livestock, to promote growth and treat disease. Consumer groups have pushed Congress and the Food and Drug Administration to tighten regulations on antibiotics given to animals, though the more urgent threat comes from medical treatment, the CDC said.

Some advocates encouraged McDonald's and the rest of the industry to do even more.

"I have called on McDonald's to also phase out beef raised on medically important antibiotics, and I will continue to press them on that front," said Rep. Louise Slaughter, D-N.Y., who has supported policies that would help curb antibiotic resistance.

McDonald's said it will eliminate chicken raised with antibiotics that are "important to human medicine." It will continue to work with suppliers who use ionophores, a kind of antibiotic that is used only for chickens, not humans.

McDonald's change in food policy is the latest in a string of recent attempts to bring more transparency to how its food is made. McDonald's has launched a series of digital films hosted by former "Mythbusters" co-host Grant Imahara to answer consumers' questions about its menu items: How do you make the egg in the Egg McMuffin perfectly round? What exactly is in a Chicken McNugget?

It's not clear whether these videos did much to bolster the company's image. For example, one showed that there were 19 ingredients in McDonald's fries, a long list that included a kind of "anti-foaming agent." Critics said that might not be reassuring to diners looking for simple, natural foods.

Even though the Big Mac and other burgers are its marquee menu items, McDonald's reportedly serves more chicken than beef, so the policy affects a large share of its orders. McDonald's has not said why it implemented this policy only for poultry orders.

Erik Olson, the public health director at the Natural Resources Defense Council, said it may be because of greater complexity in beef supply chains. While chickens typically stay at the same farm from the time they hatch to the time they are slaughtered, Olson said, cattle can bounce around through their lifespan among various grazing areas and feedlots.

"They have much more control over the chicken up and down the supply chain, from farm to fork," Olson said.

The fast-food industry overall has been challenged by perceptions that its food is not healthy. According to research firm Technomic, sales were up just 1.2 percent at quick-service restaurants open more than a year in the third quarter, compared with a 7.9 percent increase in the fast-casual category and a 2 percent increase in the restaurant industry overall.

In its most recent quarter, McDonald's saw traffic fall in all of its major markets across the globe, and its profit nose-dived 21 percent.

The company's recent business woes are about more than the perception that it does not serve healthy food. For example, McDonald's executives have said its menu had become so bloated and complicated that it was slowing down wait times at drive-through windows, and in turn alienating customers.

After the rough year, chief executive Don Thompson announced in January that he would retire. The new leader, Steve Easterbrook, took the helm of the company this week.

McDonald's follows other chain restaurants in making a commitment to serve ethically sourced food in response to customer demand, including Chick-fil-A and Panera Bread.